remote sensing

Article Optimization of UAV Flight Missions in Steep Terrain

Klemen Kozmus Trajkovski *, Dejan Grigillo and Dušan Petroviˇc

Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova 2, SI-1000 Ljubljana, Slovenia; [email protected] (D.G.); [email protected] (D.P.) * Correspondence: [email protected]; Tel.: +386-1-4768-648



Received: 1 March 2020; Accepted: 17 April 2020; Published: 19 April 2020


Abstract: Unmanned aerial vehicle (UAV) photogrammetry is one of the most effective methods for capturing a terrain in smaller areas. Capturing a steep terrain is more complex than capturing a flat terrain. To fly a mission in steep rugged terrain, a ground control station with a terrain following mode is required, and a quality digital elevation model (DEM) of the terrain is needed. The methods and results of capturing such terrain were analyzed as part of the Belca rockfall surveys. In addition to the national digital terrain model (NDTM), two customized DEMs were developed to optimize the photogrammetric survey of the steep terrain with oblique images. Flight heights and slant distances between camera projection centers and terrain are analyzed in the article. Some issues were identified and discussed, namely the vertical images in steep slopes and the steady decrease of UAV heights above ground level (AGL) with the increase of height above take-off (ATO) at 6%-8% rate. To compensate for the latter issue, the custom DEMs and NDTM were tilted. Based on our experience, the proposed optimal method for capturing the steep terrain is a combination of vertical and oblique UAV images.

Keywords: UAV; vertical images; oblique images; steep terrain; rockfall; DTM; custom DEM; tilted DEM; flight mission

1. Introduction Many mountains in the world consist of sedimentary rocks such as limestone, dolomite, flysch, conglomerates, sandstone or even combinations of these rocks and molds. Their characteristic are the layers of coatings, which can be strongly differentiated due to later earth folds. Due to their lower compactness and porosity, some of them are also cut through by water currents, which further reduce their stability when large amounts of water and dirt are present. In the steep slopes of this type of rock, rockfalls and landslides are a common phenomenon. The consequences are scree, decay, debris flows and terraces of deposited material. Most of these areas are uninhabited and rarely visited, so that the consequences do not cause any significant damage to people and their possessions. However, these events damage vegetation and alter natural habitats and ecosystems. On the other hand, if potentially unstable areas are sufficiently close to populated areas or if the amount of broken or cracked material is large enough to reach populated areas either in the form of debris flows, rockfalls, landslides or mudflows, they may cause significant damage to facilities, infrastructure, forests or agricultural crops. Therefore, all potentially dangerous hinterland areas should be regularly monitored and timely anticipated for possible events and appropriate measures should be taken to prevent or at least mitigate possible consequences [1]. Surface monitoring is usually carried out using either point-based techniques or surface-based techniques. Point-based techniques, such as global navigation satellite systems (GNSS), extensometers, total stations, laser and radar distance meters, generally offer better precision, but they only provide information on some selected monitoring points [2,3]. Surface-based techniques (photogrammetry,

Remote Sens. 2020, 12, 1293; doi:10.3390/rs12081293 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 1293 2 of 20 satellite-based or ground-based radar interferometry and terrestrial or airborne laser scanning) mainly belong to the field of remote sensing [4] and are capable of monitoring the entire surface. However, most of these methods are associated with enormous costs and a lack of spatial and temporal resolution for monitoring most slope deformations [5]. Unmanned aerial vehicle (UAV) based remote sensing belongs to the domain of surface-based techniques and is a cost-effective alternative to data acquisition with high spatio-temporal resolution [6]. Although different sensors can be mounted on UAVs, mapping is generally based on images taken by digital cameras [7]. Together with the development of image processing techniques such as multiview stereo (MVS) and especially structure from motion (SfM), UAVs offer effective and cost-efficient photogrammetric techniques to obtain high-resolution data sets [8]. Several researchers used images from UAVs as a data source to measure changes in steep terrain. To obtain reliable results from images taken in steep terrain, flight planning of the UAV is a very important step. The main parameters in flight planning for UAVs are the definition of the area of interest, selection of the flight altitude above ground level (AGL), flight speed, forward and side overlap of successive images and parameters of the digital camera (sensor dimension, pixel resolution and focal length). All these parameters influence the ground sampling distance (GSD) of the images as well as the accuracy of the final results [7]. A comprehensive overview of mission planning techniques using passive optical sensors (cameras) is given in [9]. Manconi et al. [7] developed a routine that divides the area of interest into several flight lines and each of which has a certain height, so that the average distance to the surface is maintained. The optical axis is perpendicular to the ground. Similar was done in [10]. De Beni et al. [11] surveyed volcano Mt. Etna with a modified even-height mission, whereby the elevations of the upper waypoints were raised. The result is that the entire flight plan is on an inclined plane. Niethammer et al. [12] observed a landslide in France. The average inclination of the area was 25 degrees. They used a manual control of the flight heights. Möllney and Kremer [13] dealt with the so-called contour flying, but for manned aircrafts. However, a clear advantage of the less modulated resolution is emphasized. The authors of [14] used the mission planning for the UAV- based landslide monitoring in Canada, but at a constant height. Some papers contain only partial details of the UAV flight. Rossini et al. [15] surveyed a glacier retreat in the Alps. The adaptation of flight heights is mentioned, but no further specifics about flight altitudes are given. Similarly, [16] documented a geomorphic change detection of a gorge in Taiwan with images at different heights, but no flight specifications are revealed. Valkaniotis et al. [17] mapped an earthquake-related landslide in Greece. Of the actual UAV survey, only vertical and oblique views are mentioned. Agüera-Vega et al. [18] dealt with an extreme topography in an almost vertical road cut-slope in Spain using UAV. They took horizontal images in four flight lines on a vertical plane and oblique images taken at 45 degrees downwards in two passes. Some authors do not mention any specifics of the UAV survey. UAV was used by [19–21] to survey landslides and rockfalls. Yang et al. [22] conducted a study on the optimization of flight routes for the reconstruction of digital terrain models (DTM). The procedure assumes a constant UAV height. Pepe et al. [9] gave an overview of the airborne mission planning of the current platforms and sensors. The terrain-following planning mode is only mentioned. Some of the methods described above improve the suitability of UAV surveys in sloped terrain, either by tilting the flight plane, defining different heights of the flight lines or by flying the UAV manually. These solutions are of limited use in very uneven terrain where the elevations of the relief shapes vary greatly in all directions. The solution would be to adjust the UAV height according to the terrain elevation each time an image is captured. In our opinion, the terrain-following flight mission is the only reasonable solution. Our goal was to design customized digital elevation models (DEMs) that can optimize mission planning in steep terrain. The custom DEMs modify the actual DTM in a way that allows more consistent UAVdistances from the terrain when used for mission planning. This ensures more consistent Remote Sens. 2020, 12, 1293 3 of 20

Remote Sens. 2020, 12, x FOR PEER REVIEW 3 of 20 image overlap, more consistent GSD of images and reduces blind spots. These facts contribute to the contribute to the efficiency of the dense image matching algorithms used to calculate dense point efficiency of the dense image matching algorithms used to calculate dense point clouds. The custom clouds. The custom DEM also increases flight safety, as uniform distances of the UAV to the terrain DEM also increases flight safety, as uniform distances of the UAV to the terrain prevent the vehicle prevent the vehicle from colliding with exposed terrain points. To support the UAV mission flying, from colliding with exposed terrain points. To support the UAV mission flying, two custom terrain two custom terrain models and their alternatives have been developed. Additionally, the capturing models and their alternatives have been developed. Additionally, the capturing of oblique images is of oblique images is recommended either instead of or in addition to the conventionally used vertical recommended either instead of or in addition to the conventionally used vertical images. images. 2. Methodology 2. Methodology 2.1. UAV Height, Elevation and Altitude 2.1. UAV Height, Elevation and Altitude Basic vertical distances of UAV flying are shown in Figure1. Elevation is the vertical distance of a groundBasic vertical point abovedistances mean of UAV sea level flying (AMSL). are shown Altitude in Figure is the 1. vertical Elevation distance is the ofvertical the current distance UAV of positiona ground AMSL. point Theabove height mean above sea level take-o (AMSL).ff (ATO) Altitude is a difference is the betweenvertical thedistance altitude of andthe current the elevation UAV ofposition the take-o AMSL.ff point The AMSL. height The above height take AGL-off refers (ATO to) is the a verticaldifference distance between between the thealtitude UAV andin flight the andelevation the ground. of the take Another-off point important AMSL. distance The height is the AGL distance refers from to the the vertical projection distance centre between of the camera the UAV to groundin flightalong and the the ground. optical axis.Another When important the camera distance is pointed is the down distance in the from nadir the direction, projection the centre distance of the is equalcamera to to the ground UAV heightalong the AGL. optical axis. When the camera is pointed down in the nadir direction, the distance is equal to the UAV height AGL.

Figure 1. Unmanned aerial vehicle (UAV) heights, elevation and altitude. UAV's take-off point is Figure 1. Unmanned aerial vehicle (UAV) heights, elevation and altitude. UAV's take-off point is colored grey and the current UAV position is black. colored grey and the current UAV position is black. For oblique images, the slant distance from the projection centre of the camera to the ground point For oblique images, the slant distance from the projection centre of the camera to the ground in the direction of the optical axis is labeled as DCG (distance camera-ground). Figure2 shows the point in the direction of the optical axis is labeled as DCG (distance camera-ground). Figure 2 shows DCGs for vertical and oblique images. the DCGs for vertical and oblique images. 2.2. UAV Photogrammetry in Flat and Non-flat Areas 2.2. UAV Photogrammetry in Flat and Non-flat Areas If the terrain is flat or almost flat, the usual method of capturing the terrain with a UAV is to fly horizontallyIf the terrain at is a flat certain or almost height flat, ATO, the which usualrefers method to theof capturing vertical distance the terrain above with the a UAV launch is point.to fly Thehorizontally camera ofat thea certain UAV height is pointed ATO, in which the direction refers to of the the vertical nadir. distance The images above are the taken laun withch point. a certain The overlapcamera of to the allow UAV 3D is pointed reconstruction in the direction of the ground of the nadir. using The image images matching are taken algorithms. with a certain Numerous overlap applications,to allow 3D reconstruction such as Mission of the Planner, ground UgCS, using DJI image GS Pro,matching 3DSurvey algorithms. Pilot, Pix4Dcapture Numerous applications, and others osuchffer as automated Mission Planner, mission UgCS, flying. DJI A GS flight Pro plan, 3DSurvey is generated Pilot, Pix4Dcapture based on a user-defined and others offer area, automated the flight height,mission overlap flying. ofA theflight images plan andis generated camera type.based The on plana user is- uploadeddefined area, to the the UAV, flight which height, then overlap takes oofff the images and camera type. The plan is uploaded to the UAV, which then takes off automatically, flies to the waypoints, takes images and lands at the home point, provided there are no malfunctions. Remote Sens. 2020, 12, 1293 4 of 20 automatically, flies to the waypoints, takes images and lands at the home point, provided there are noRemote malfunctions. Sens. 2020, 12, x FOR PEER REVIEW 4 of 20 If the elevation of the area of interest changes significantly, flying at constant height ATO does not provideIf the optimalelevation image of the overlap area of for interest 3D reconstruction. changes significantly, However, flying we see atthe constant following height solutions ATO does to thenot problem: provide optimal image overlap for 3D reconstruction. However, we see the following solutions to the problem: A combination of several missions flown at different altitudes, • • A combination of several missions flown at different altitudes, Fly the UAV manually or, • • Fly the UAV manually or, Use a flight planning application that allows terrain following. • • Use a flight planning application that allows terrain following. TheThe first first option, option, a combination a combination of several of several missions, missions, is more is time-consuming more time-consuming than a single than mission, a single andmission, you must and beyou careful must tobe prevent careful gapsto prevent between gaps the between areas flown. the areas Manual flown. flying Manual is even flying more is time even consuming,more time itconsuming, is difficult toit maintainis difficult the to samemaintain height the AGL, same and height there AGL, is no and guarantee there is of no correct guarantee image of overlap.correct If image an adequate overlap. terrain If an model adequate is available, terrain an model autonomous is available, flight an mission autonomous following flight the terrain mission isfollowing the best option. the terrain is the best option. AA digital digital terrain terrain modelmodel (DTM) is is required required to to enable enable the the UAV UAV to follow to follow the non the- non-flatflat terrain terrain.. DTMs DTMsor DEMs or DEMs with with a certain a certain spatial spatial resolution resolution can can be be provided provided worldwide worldwide (mostly inin lowlow resolution), resolution), nationwidenationwide (mostly (mostly in in di ffdifferenterent resolutions) resolutions) and and locally locally from from previous previous surveys. surveys. The The biggest biggest problem problem is theis lackthe lack of software of software thatsupports that supports terrain terrain data for data mission for mission planning. planning. At the timeAt the of ourtime survey of our flights survey (endflights of 2018), (end we of found 2018), a we single found application a single for application DJI UAVs for with DJI DTM UAVs import with capabilities, DTM import namely capabilities, Drone Harmony.namely Drone According Harmony. to [7] from According 2019, UgCS to [ was7] from the only 2019 other, UgCS commercial was the application only other that commercial offered suchapplication possibilities that atoffer thated time. such possibilities at that time.

2.3.2.3 Basic. Basic aspects aspects of of UAV UAV Photogrammetry Photogrammetry in in Steep Steep Terrain Terrain AnAn important important aspect aspect of of UAV UAV photogrammetry photogrammetry in steepin steep terrain terrain is the is the direction direction of the of opticalthe optical axis axis of theof camera.the camera. The The image image scale scale in aerial in aerial surveying surveying is based is based on on the the ratio ratio between between the the distance distance from from the the imageimage element element to to the the projection projection centre centre and and the the distance distance from from the the projection projection centre centre to to the the object object on on the the terrain.terrain. When When the the optical optical axis axis of of the the camera camera was was directed directed vertically vertically to to the the ground, ground, as as is is common common in in flatflat areas, areas, the the image image scale scale of of steep steep terrain terrain changed changed significantly. significantly In. In these these cases, cases, the the oblique oblique images, images, whosewhose optical optical axes axes were were perpendicular perpendicular to to the the ground ground surface, surface, provided provided much much better better image image properties properties withwith respect respect to to the the image image scale scale and and the the subsequent subsequent image image GSD. GSD. Figure Figure2 displays 2 displays the the geometric geometric comparisoncomparison between between the the vertical vertical image image and and the the oblique oblique image image perpendicular perpendicular to to the the terrain. terrain.

(a) (b)

FigureFigure 2. 2Photographing. Photographing steep steep terrain terrain with with a (aa )(a vertical) vertical image image and and an an (b ()b oblique) oblique image. image.

An example of an extreme situation on a vertical image in steep terrain is shown in Figure 3. The height differences from the image projection centre to points A and B were 288.4 and 19.6 meters, respectively. The 3D distances to the same points were 362.0 and 53.7 meters.

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An example of an extreme situation on a vertical image in steep terrain is shown in Figure3. The height differences from the image projection centre to points A and B were 288.4 and 19.6 meters, respectively. The 3D distances to the same points were 362.0 and 53.7 meters. Remote Sens. 2020, 12, x FOR PEER REVIEW 5 of 20

Figure 3. Extreme height differences in a steep terrain result in a varying GSD within the vertical Figure 3. Extreme height differences in a steep terrain result in a varying GSD within the vertical image image (~0.5 cm in point A and ~7.9 cm in point B). (~0.5 cm in point A and ~7.9 cm in point B). Since the image overlap is calculated according to the planned UAV height AGL, there is an Since the image overlap is calculated according to the planned UAV height AGL, there is an issue issue in vertical images because the distance to the upper part of the captured area in the image is in vertical images because the distance to the upper part of the captured area in the image is much much shorter than the height AGL. In Figure 3, point B was only 19.6 meters below and 53.7 meters shorter than the height AGL. In Figure3, point B was only 19.6 meters below and 53.7 meters away away from the UAV, whose projected height above the DTM was 80 meters. As a result, the overlap from the UAV, whose projected height above the DTM was 80 meters. As a result, the overlap was was reduced and could become critically low in very steep areas. The situation is depicted in Figure reduced and could become critically low in very steep areas. The situation is depicted in Figure4 for 4 for vertical images and horizontal flat terrain. Equation (1) provides the relationships between the vertical images and horizontal flat terrain. Equation (1) provides the relationships between the overlap overlap fraction (R), the image base (B) and the dimensions of the image sensor scaled to the ground fraction (R), the image base (B) and the dimensions of the image sensor scaled to the ground (W) [23]: (W) [23]: 퐵B 푅 R== 1 1− (1) (1) −푊W TheThe image image base basewas calculated was calculated from the from specified the specified height AGL height and AGL the projected and the overlap. projected With overlap. a constantWith a image constant base, image the overlap base, the was overlap reduced was to reduced 60% at the to 60% half at of the the half height of the AGL height and further AGL and reducedfurther to reduced20% for toa quarter 20% for of a the quarter height of theAGL. height AGL.

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Figure 3. Extreme height differences in a steep terrain result in a varying GSD within the vertical image (~0.5 cm in point A and ~7.9 cm in point B).

Since the image overlap is calculated according to the planned UAV height AGL, there is an issue in vertical images because the distance to the upper part of the captured area in the image is much shorter than the height AGL. In Figure 3, point B was only 19.6 meters below and 53.7 meters away from the UAV, whose projected height above the DTM was 80 meters. As a result, the overlap was reduced and could become critically low in very steep areas. The situation is depicted in Figure 4 for vertical images and horizontal flat terrain. Equation (1) provides the relationships between the overlap fraction (R), the image base (B) and the dimensions of the image sensor scaled to the ground (W) [23]: 퐵 푅 = 1 − (1) 푊 The image base was calculated from the specified height AGL and the projected overlap. With a Remoteconstant Sens. image2020, 12 , base, 1293 the overlap was reduced to 60% at the half of the height AGL and further6 of 20 reduced to 20% for a quarter of the height AGL.

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Figure 4. Decreased overlap in steep terrain. Figure 4. Decreased overlap in steep terrain. 2.4. Development of Custom DEMs 2.4. Development of Custom DEMs The creation of custom DEMs is applicable to any publicly available DTM. In our experience, we adviseThe againstcreation usingof custom the globalDEMs is SRTM applicable (shuttle to any radar publicly topography available mission)DTM. In our DEM. experience, Due to we its spatial advise against using the global SRTM (shuttle radar topography mission) DEM. Due to its spatial resolution of 3 arcseconds, the SRTM DEM is not accurate enough to plan a UAV mission in steep hilly resolution of 3 arcseconds, the SRTM DEM is not accurate enough to plan a UAV mission in steep terrain [9,24–26]. The national 5-meter DTM (NDTM) of The Surveying and Mapping Authority of hilly terrain [9,24–26]. The national 5-meter DTM (NDTM) of The Surveying and Mapping Authority Sloveniaof Slovenia was was selected selected for for the the study. study.

2.4.1.2.4. The1. The Plane Plane DEM DEM ToTo prevent prevent the the UAV UAV from from getting getting tootoo closeclose to an area area that that suddenly suddenly bec becameame very very steep steep—since—since the UAVthe height UAV height AGL AGL is the is vertical the vertical distance distance from from the the imported imported NDTM, NDTM, seesee Figure 55aa—the—the idea idea was was born to createborn to a create plane a surfaceplane surface over theover extreme the extreme parts parts of theof the area. area. Consequently, Consequently, thethe distance of of the the UAV fromUAV the from terrain the terrain was much was much more more favorable, favorable, as shown as shown in Figurein Figure5b. 5b.

(a) (b)

FigureFigure 5. 5.Oblique Oblique imagesimages acquired when when the the angle angle of inclination of inclination changes changes suddenly: suddenly: (a) use (ofa) the use of the nationalnational 5-meter 5-meter digital digital terrain model model (NDTM (NDTM)) for for mission mission planning planning and ( andb) use (b of) use the ofconstructed the constructed planeplane digital digital elevation elevation modelmodel ( (DEM)DEM) for for mission mission planning. planning.

Figure 6 displays the plane DEM set above the NDTM. Three reference points were selected for the calculation of the plane. Two reference points, marked 1 and 2 in Figure 6, were located in the lowest part of the NDTM, and point 3 was the one where the plane through points 1 and 2 touched the NDTM as it approached the NDTM from the vertical position. The reference XY coordinates of the plane DEM were the same as for the NDTM, only the elevations were recalculated. Therefore the resolution of the plane DEM was the same as for the NDTM.

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Figure6 displays the plane DEM set above the NDTM. Three reference points were selected for the calculation of the plane. Two reference points, marked 1 and 2 in Figure6, were located in the lowest part of the NDTM, and point 3 was the one where the plane through points 1 and 2 touched the NDTM as it approached the NDTM from the vertical position. The reference XY coordinates of the plane DEM were the same as for the NDTM, only the elevations were recalculated. Therefore the Remoteresolution Sens. 2020 of, 12 the, x FOR plane PEER DEM REVIEW was the same as for the NDTM. 7 of 20

Figure 6. The plane DEM (in red) of the case study above the NDTM (in green) with the reference points. Figure 6. The plane DEM (in red) of the case study above the NDTM (in green) with the reference points.This DEM ensures a certain minimum distance to the terrain. Furthermore, an appropriate overlap was ensured. The disadvantage is the greater distance to some areas, e.g., local depressions, whichThis leads DEM to ensures a lower a model certain resolution minimum of them. distance to the terrain. Furthermore, an appropriate overlap was ensured. The disadvantage is the greater distance to some areas, e.g. local depressions, which2.4.2. leads The to Fake a lower DEM model resolution of them. The plane DEM is a simple surface that is easy to calculate because only three reference points are 2.4.2. The Fake DEM needed. However, this simplicity has some shortcomings. The differences in elevations between the planeThe DEMplane and DEM the is DTM a simple can becomesurface large.that is Thereforeeasy to calculate another because custom only DEM three was developed.reference points are needed.The fakeHowever, DEM isthis specially simplicity designed has some for UAV shortcomings. surveying The with differences oblique images. in elevations The fake between DEM grid theis plane shifted DEM to the and NDTM the DTM grid can so thatbecome the DCGlarge. to Therefore the NDTM another is constant custom at givenDEM was values developed. of the projected heightThe fake above DEM the is fake specially DEM, UAVdesigned heading for UAV and gimbalsurveying angle, with all oblique of which images. are constant The fake during DEM a grid flying is shiftedmission. to Thethe NDTM recalculated grid so grid that points the DCG and theirto the elevations NDTM is form constant the fake at given DEM. values A graphical of the projected relationship heightbetween above the the fake fake DEM DEM and, UAV the headi NDTMng isand shown gimbal in Figureangle,7 all. of which are constant during a flying mission. The recalculated grid points and their elevations form the fake DEM. A graphical relationship2.4.3. The between tilted DEMs the fake DEM and the NDTM is shown in Figure 7. An unexpected UAV's behavior was observed during its practical deployment in a mountainous terrain. As the UAV height ATO increased, the height AGL decreased. A generalized situation is shown in Figure8a. The UAV starts the survey at the projected height AGL h_0. The height AGL at the top, denoted h_top, should be similar to h_0. However, in our case study, which is described in Section3, it was not, since h_top was in all cases about 20 meters smaller than h_0. To compensate for the error, the tilted DEMs were introduced. The idea is to tilt a DEM so that the actual flight height is more parallel to the basic DEM and h_top is similar to h_0. The situation is shown in Figure8b. The compensation value (CV) has been set at 30 meters, to be on the safe side. The tilted DEMs were recalculated DEMs so that the elevation increased linearly from 0 at the bottom to 30 meters at the top.

(a) (b)

Figure 7. Fake DEM and NDTM: (a) Graphical 3D comparison of fake DEM (blue) and NDTM (green) of the case study. The fake DEM is shifted so at 80 m above the fake DEM the distance camera-ground (DCG) at pitch angle—45 degrees is 60 m and (b) oblique image acquired using fake DEM in mission

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Figure 6. The plane DEM (in red) of the case study above the NDTM (in green) with the reference points.

This DEM ensures a certain minimum distance to the terrain. Furthermore, an appropriate overlap was ensured. The disadvantage is the greater distance to some areas, e.g. local depressions, which leads to a lower model resolution of them.

2.4.2. The Fake DEM The plane DEM is a simple surface that is easy to calculate because only three reference points are needed. However, this simplicity has some shortcomings. The differences in elevations between the plane DEM and the DTM can become large. Therefore another custom DEM was developed. RemoteThe Sens. fake2020, 12DEM, 1293 is specially designed for UAV surveying with oblique images. The fake DEM8 ofgrid 20 is shifted to the NDTM grid so that the DCG to the NDTM is constant at given values of the projected height above the fake DEM, UAV heading and gimbal angle, all of which are constant during a flying Themission. tilted values The recalculated were applied grid to the points NDTM, and the their plane elevations DEM and form the fake the DEM. fake DEM. Figure 8 Ac displays graphical anrelationship example of between a 3D comparison the fake DEM of NDTM and the and NDTM the tilted is shown NDTM. in Figure 7.

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planning. When approaching a steep feature, the fake DEM guides UAV to back off from the terrain to keep the DCG constant.

2.4.3. The tilted DEMs An unexpected UAV's behavior was observed during its practical deployment in a mountainous terrain. As the UAV height ATO increased, the height AGL decreased. A generalized situation is shown in Figure 8a. The UAV starts the survey at the projected height AGL h_0. The height AGL at the top, denoted h_top, should be similar to h_0. However, in our case study, which is described in Section 3, it was not, since h_top was in all cases about 20 meters smaller than h_0. To compensate for the error, the tilted DEMs were introduced. The idea is to tilt a DEM so that the actual flight height is more parallel to the basic DEM and h_top is similar to h_0. The situation is shown in Figure 8b.

The compensation value (CV) has been set at 30 meters, to be on the safe side. The tilted DEMs were (a) (b) recalculated DEMs so that the elevation increased linearly from 0 at the bottom to 30 meters at the top. TheFigureFigure tilted 7. 7.Fake Fake values DEMDEM wereand and NDTM: NDTM applied:( (aa )) to GraphicalGraphical the NDTM, 3D3D comparisoncomparison the plane ofof fakeDEMfake DEMDEM and (blue) (blue) the fake and and NDTM NDTM DEM. (green)(green)Figure 8c displaysofof the the an case caseexample study. study . Theof The a fake3D fake comparison DEM DEM is is shifted shifted of so NDTMso at at 80 80 m m and above above the the the tiltedfake fake NDTM.DEM DEMthe the distance distance camera-groundcamera-ground (DCG)(DCG) at at pitch pitch angle—45 angle—45 degrees degrees is is 60 60 m m and and ( b(b)) oblique oblique image image acquired acquired using using fake fake DEM DEM in in mission mission planning. When approaching a steep feature, the fake DEM guides UAV to back off from the terrain to

keep the DCG constant.

(a) (b)

Figure 8. Cont.

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(c)

FigureFigure 8. 8.DEMs DEMs and and tilted tilted DEMs:DEMs: (a) UAV flight flight heights heights when when the the basic basic DEM DEM is isused used for for mission mission planning;planning; (b ()b UAV) UAV flight flight heights heights whenwhen thethe tiltedtilted DEM is is used used for for mission mission planning planning and and (c) (ca) 3D a 3D comparisoncomparison of of NDTM NDTM (green) (green) and and tilted tilted NDTMNDTM (dark orange) of of the the case case study. study.

3.3. Site Site Description, Description Field, Field Work Work and and DataData ProcessingProcessing TheThe developed developed DEMs DEMs (plane, (plane, fake, fake, tilted tilted plane, plane, tilted tilted fake and faketilted and NDTM)tilted NDTM) and terrain-following and terrain- aerialfollowing surveys aerial were surveys tested were on the tested Belca on active the Belca rockfall active in rockfall the NW in part the ofNW Slovenia. part of Slovenia. It lies only It lies a few 100only m abovea few 10 the0 m village above ofthe Belca, village while of Belca, a sawmill while a is sawmill located is justlocated below just the below unstable the unstable slopes. slopes. In 1953 a largeIn 1953 rockfall a large event rockfall with event considerable with considerable damage damagewas reported was reported in this area. in this A area. forest A roadforest crossing road crossing the area is shown in Figure 9a. The next major rockfall event in February 2018 triggered the the area is shown in Figure9a. The next major rockfall event in February 2018 triggered the debris debris flow that accumulated on the road that has been closed since then. Another massive rockfall flow that accumulated on the road that has been closed since then. Another massive rockfall event event occurred in October 2018, eroding the left bank of the Belca torrent in addition to the material occurred in October 2018, eroding the left bank of the Belca torrent in addition to the material on the on the rockfall area. The debris has accumulated in and around the Belca river bed. It has largely rockfall area. The debris has accumulated in and around the Belca river bed. It has largely damaged damaged the smaller hydroelectric power station and the local sawmill. Thereafter, some cracks in thethe smaller rock mass hydroelectric near the top power of the stationslide caused and concern, the local so sawmill. that a controlled Thereafter, removal some of cracks the threatening in the rock massblock near with the explosives top of the was slide initiated. caused The concern, rockfall so extends that a controlled over an area removal of about of the100 threateningm × 300 m and block with explosives was initiated. The rockfall extends over an area of about 100 m 300 m and spreads spreads between elevations 710 m AMSL at the toe and 1020 m AMSL at the crown.× between elevations 710 m AMSL at the toe and 1020 m AMSL at the crown. Remote Sens. 2020, 12, x FOR PEER REVIEW 10 of 20

(a) (b)

Figure 9. Belca rockfall: (a) on the national orthophoto and (b) on a photograph taken at the bottom. Figure 9. Belca rockfall: (a) on the national orthophoto and (b) on a photograph taken at the bottom. In the period from November to December 2018, several UAV surveys were carried out in which different DEMs and types of images were tested. Table 1 shows an overview of the surveys.

Table 1. Surveys of the Belca rockfall with information on DEMs and the type of images used in each mission.

Date DEM/imagery DEM/imagery DEM/imagery 8 Nov 2018 NDTM/oblique / / 4 Dec 2018 plane/oblique plane/oblique fake/oblique 11 Dec 2018 tilted_NDTM/vertical tilted_plane/oblique tilted_fake/oblique The UAV height AGL dictates the GSD of the images. The DJI Phantom 4 Pro used in research achieved a GSD of 1 cm at a height of 36.5 m, 2 cm at a height of 73 m, etc. The NDTM was geolocated in the plane coordinate system in XYZ format. For use with the Drone Harmony app, it must be transformed into WGS-84 and then exported in Esri ASCII format. The NDTM was initially used for vertical and oblique image acquisition. For the latter, the gimbal pitch was set to -45 degrees, as the average slope angle is close to 45 degrees. The planned height AGL for all flights was set at 80 meters. The front overlap was set to 80% and a side overlap at 65%. As already mentioned in Section 2.3, the image overlap is reduced in steep terrain. To keep the actual overlap at the desired rate, the projected overlap can be set higher, but this would mean more images and longer flight times. In order to facilitate the comparison of the two sets of images (vertical and oblique), it was decided to keep the same projected heights and overlaps. Since the topography of the area is diverse and the slope angle changes rapidly, the NDTM was considered unsuitable for oblique imagery. Two user-defined DEMs were created to support the acquisition of the oblique images. The plane DEM was generated in the national coordinate system and transformed into WGS84 and Esri ASCII format, same as for the NDTM. The steepest inclination of the plane DEM was 43.9 degrees with an azimuth of 332 degrees. Since the surface of the plane DEM differs from the NDTM, the GSD on the NDTM varies when the plane DEM was used in mission planning. For the fake DEM of the Belca case, the elevations of the DTM were recalculated in such a way that from any point 80 meters above the DEM, the DCG at the pitch angle -45 degrees and heading 330 degrees was 60 meters. In practical use, the fake DEM was uploaded to the mission control app, the flight height AGL was set to 80 meters, the gimbal pitch angle to -45 degrees and the heading to 330 degrees. The DCG to the NDTM terrain was 60 meters. The value of 60 meters was a rounded value of 80 m × cos (45°) = 56.6 m, where 45° was the pitch angle.

Remote Sens. 2020, 12, 1293 10 of 20

In the period from November to December 2018, several UAV surveys were carried out in which different DEMs and types of images were tested. Table1 shows an overview of the surveys.

Table 1. Surveys of the Belca rockfall with information on DEMs and the type of images used in each mission.

Date DEM/Imagery DEM/Imagery DEM/Imagery 8 Nov 2018 NDTM/oblique // 4 Dec 2018 plane/oblique plane/oblique fake/oblique 11 Dec 2018 tilted_NDTM/vertical tilted_plane/oblique tilted_fake/oblique

The UAV height AGL dictates the GSD of the images. The DJI Phantom 4 Pro used in research achieved a GSD of 1 cm at a height of 36.5 m, 2 cm at a height of 73 m, etc. The NDTM was geolocated in the plane coordinate system in XYZ format. For use with the Drone Harmony app, it must be transformed into WGS-84 and then exported in Esri ASCII format. The NDTM was initially used for vertical and oblique image acquisition. For the latter, the gimbal pitch was set to -45 degrees, as the average slope angle is close to 45 degrees. The planned height AGL for all flights was set at 80 meters. The front overlap was set to 80% and a side overlap at 65%. As already mentioned in Section 2.3, the image overlap is reduced in steep terrain. To keep the actual overlap at the desired rate, the projected overlap can be set higher, but this would mean more images and longer flight times. In order to facilitate the comparison of the two sets of images (vertical and oblique), it was decided to keep the same projected heights and overlaps. Since the topography of the area is diverse and the slope angle changes rapidly, the NDTM was considered unsuitable for oblique imagery. Two user-defined DEMs were created to support the acquisition of the oblique images. The plane DEM was generated in the national coordinate system and transformed into WGS84 and Esri ASCII format, same as for the NDTM. The steepest inclination of the plane DEM was 43.9 degrees with an azimuth of 332 degrees. Since the surface of the plane DEM differs from the NDTM, the GSD on the NDTM varies when the plane DEM was used in mission planning. For the fake DEM of the Belca case, the elevations of the DTM were recalculated in such a way that from any point 80 meters above the DEM, the DCG at the pitch angle -45 degrees and heading 330 degrees was 60 meters. In practical use, the fake DEM was uploaded to the mission control app, the flight height AGL was set to 80 meters, the gimbal pitch angle to -45 degrees and the heading to 330 degrees. The DCG to the NDTM terrain was 60 meters. The value of 60 meters was a rounded value of 80 m cos (45 ) = 56.6 m, where 45 was the pitch angle. × ◦ ◦ The georeferencing of the UAV images was performed using ground control points (GCPs), which were signaled with targets (a black circle 27 cm in diameter on a white background). Ideally, the GCPs should be evenly distributed over a surveyed area [27–29] to achieve optimal accuracy of results. The terrain of the Belca rockfall is not only steep, but it is also a challenge to reach some areas in order to set the GCPs optimally. Figure 10 shows the geometric distribution of the GCPs. In the middle of the area, in this case in the central part of the rockfall, there should be some GCPs. Firstly, it is very dangerous to cross the rocky area and secondly, it would be difficult to fix a GCP target on the unstable ground. Since some of the targets were set under adverse GNSS conditions, we used two different GNSS receivers and two modes, RTK and fast-static, to achieve high quality results. The acquired accuracy of the GCP positions was about 5 cm. The GCP coordinates were determined in the national reference coordinate system D96/TM (EPSG: 3794). Remote Sens. 2020, 12, x FOR PEER REVIEW 11 of 20

The georeferencing of the UAV images was performed using ground control points (GCPs), which were signaled with targets (a black circle 27 cm in diameter on a white background). Ideally, the GCPs should be evenly distributed over a surveyed area [27–29] to achieve optimal accuracy of results. The terrain of the Belca rockfall is not only steep, but it is also a challenge to reach some areas in order to set the GCPs optimally. Figure 10 shows the geometric distribution of the GCPs. In the middle of the area, in this case in the central part of the rockfall, there should be some GCPs. Firstly, it is very dangerous to cross the rocky area and secondly, it would be difficult to fix a GCP target on the unstable ground. Since some of the targets were set under adverse GNSS conditions, we used two different GNSS receivers and two modes, RTK and fast-static, to achieve high quality results. The acquiredRemote Sens. accuracy2020, 12, 1293 of the GCP positions was about 5 cm. The GCP coordinates were determined11 of in 20 the national reference coordinate system D96/TM (EPSG: 3794).

FigureFigure 10. 10. DistributionDistribution of of GCPs GCPs (shown (shown on on the the dense dense point point cloud). cloud).

TheThe images images were were processed processed with with Agisoft Agisoft Metashape Metashape software software [30]. The [ images30]. The' exterior images' orientations, exterior orientations,calculated during calculated the bundle during block the bundle adjustment block (BBA)adjustment were (BBA) used to w analyzeere used theto analy distancesze the of distances the UAV offrom the the UAV ground. from Together the ground. with Together the BBA, withdense the point BBA, clouds dense and point orthophotos clouds andwere orthophotos created to visually were createdevaluate to the visually effects evaluate of various the flight effects missions of various on theflight final missions products. on the final products.

4.4. Results Results ThisThis section section presents presents the the ma mainin height height-related-related results results of of the the Belca Belca case case study, study, which which used used custom custom DEMsDEMs that that would would ensure ensure a a constant constant distance distance between between the the UAV UAV and and the the terrain terrain in in a a vertical vertical or or oblique oblique direction.direction. The The practical practical aspects aspects of of vertical vertical and and oblique oblique images images were were discussed discussed in in Secti Sectionon 55.2.2..

4.1.4.1. UAV UAV heights heights over over NDTM NDTM and and custom custom DEMs DEMs DJI'sDJI's technical technical support support would would not not reveal reveal to to us us the the principles principles behind behind the the estimation estimation of of aircraft aircraft height.height. Based Based on on information information collected collected in in the the forum forum on the on DJIthe website DJI website and [31 and], DJI [31's],non-RTK DJI's non aircraft-RTK operate with relative heights, using the on-board barometric altimeter and height 0 being the point at which the UAV takes off from the ground.

When using the terrain file in Drone Harmony Pro application, it is necessary to select the starting point, as the height at this point is set to 0. According to [31] the mission planning procedure creates a grid over the user selected polygon. Each grid point height was calculated as a weighted average over the adjacent DTM grid points. In Figures 11–15, the UAV height ATO refers to the barometric height of the UAV at the time an image is taken. The height AGL was calculated as the height ATO, reduced by the interpolated elevation value of a DEM at the point vertically below the projection centre (PC) of an image. The barometric height ATO was taken from the flight log. The coordinates of PCs for all images were Remote Sens. 2020, 12, x FOR PEER REVIEW 12 of 20

aircraft operate with relative heights, using the on-board barometric altimeter and height 0 being the point at which the UAV takes off from the ground. When using the terrain file in Drone Harmony Pro application, it is necessary to select the starting point, as the height at this point is set to 0. According to [31] the mission planning procedure creates a grid over the user selected polygon. Each grid point height was calculated as a weighted average over the adjacent DTM grid points. In Figures 11-15, the UAV height ATO refers to the barometric height of the UAV at the time an

Remoteimage Sens. is taken.2020, 12 , The 1293 height AGL was calculated as the height ATO, reduced by the interpolated12 of 20 elevation value of a DEM at the point vertically below the projection centre (PC) of an image. The barometric height ATO was taken from the flight log. The coordinates of PCs for all images were estimatedestimated inin thethe bundlebundle adjustmentadjustment duringduring thethe SfM.SfM. TheThe DCGsDCGs toto DEMsDEMs werewere analyzedanalyzed eithereither inin thethe verticalvertical directiondirection oror inin thethe slantslant directiondirection ofof thethe opticaloptical axis. axis. FigureFigure 1111 showsshows thethe UAVUAV heightsheights aboveabove thethe NDTMNDTM usingusing NDTMNDTM oror customcustom DEMDEM inin missionmission planning.planning. When When using using NDTM, NDTM, some some of the of spikesthe spikes could could be caused be caused by NDTM by NDTM smoothing smoothing in the mission in the planningmission planning application. application. Since the Since surface the of surface the plane of the DEM plane is smooth DEM is by smooth itself, suddenby itself, changes sudden in changes height arein height smaller are than smaller when than using when the NDTMusing the for NDTM mission for planning. mission planning.

FigureFigure 11.11. UAVUAV flightflight heightsheights aboveabove thethe NDTMNDTM usingusing NDTMNDTM inin missionmission planningplanning (blue)(blue) andand usingusing planeplane DEMDEM inin missionmission planningplanning (red).(red). The flight heights of the same plane DEM based flight over the plane DEM are shown in Figure 12a. The flight heights of the same plane DEM based flight over the plane DEM are shown in Figure The corresponding image block is shown in Figure 12b. The first spike (around image number 140) was 12a. The corresponding image block is shown in Figure 12b. The first spike (around image number a consequence of the PC heights of the images named 297-300, which should be almost equal, but the 140) was a consequence of the PC heights of the images named 297-300, which should be almost UAV descended 12 meters in between. The last spike was located between the images named 310 and equal, but the UAV descended 12 meters in between. The last spike was located between the images Remote311. Sens. The 2020 di, 12fference, x FOR PEER in height REVIEW should be more than 15 meters, but the actual lift was only 4.513 meters. of 20 named 310 and 311. The difference in height should be more than 15 meters, but the actual lift was only 4.5 meters.

(a) (b)

FigureFigure 12. ( 12.a) UAV(a) UAV flight flight height height above above the theplane plane DEM DEM and and (b) (hborizontal) horizontal image image-block.-block. Labels Labels are are imageimage filenames. filenames.

TheThe height height of the of theUAV UAV above above a certain a certain DEM DEM should should remain remain constant constant throughout throughout the mission. the mission. Ideally,Ideally, this this height height should should be be 80 80 meters, meters, as it as was it planned was planned for all flights. for all Dueflights. to unknown Due to inconsistencies unknown inconsistenciesof the initial of height the initial ATO theheight starting ATO height the starting deviated height from deviate 80 meters.d from However, 80 meters. the followingHowever, heightsthe followingAGL should heights be AGL close should to the initial be close height. to the The initial cases height. presented The di casesffer from presented the expected differ ones frombecause the expected ones because the height AGL decreased steadily with increasing amount of the height ATO. All other flights of the first two days show the same trend. Even though the DCGs are only relevant for the actual terrain represented by the NDTM, the example in Figure 12 is presented to further strengthen the hypothesis that the barometric height AGL of the UAV differed from the actual height AGL. To investigate the issue, the PCs' heights ATO of one flight were compared with the barometric heights ATO of the UAV of the same flight. The adjusted PC positions can be considered as the true positions. The comparison is depicted in Figure 13. The deviation increased to about 20 meters at the largest heights ATO, which was consistent with the cases shown above.

Figure 13. Differences of the barometric heights and the actual heights above take-off (ATO). The values in the figure are calculated as the barometric heights subtracted by the PC's heights ATO.

The flights were carried out under different weather conditions. Most of the flights were from bottom to top, some also from top to bottom. The height difference at the top was always in the range of 15-25 meters, so that it can be considered a systematic error.

4.2. Tilted DEMs

Remote Sens. 2020, 12, x FOR PEER REVIEW 13 of 20

(a) (b)

Figure 12. (a) UAV flight height above the plane DEM and (b) horizontal image-block. Labels are image filenames.

The height of the UAV above a certain DEM should remain constant throughout the mission. RemoteIdeally, Sens. this2020 , height12, 1293 should be 80 meters, as it was planned for all flights. Due to unknown13 of 20 inconsistencies of the initial height ATO the starting height deviated from 80 meters. However, the following heights AGL should be close to the initial height. The cases presented differ from the the height AGL decreased steadily with increasing amount of the height ATO. All other flights of the expected ones because the height AGL decreased steadily with increasing amount of the height ATO. first two days show the same trend. All other flights of the first two days show the same trend. Even though the DCGs are only relevant for the actual terrain represented by the NDTM, Even though the DCGs are only relevant for the actual terrain represented by the NDTM, the the example in Figure 12 is presented to further strengthen the hypothesis that the barometric height example in Figure 12 is presented to further strengthen the hypothesis that the barometric height AGL of the UAV differed from the actual height AGL. AGL of the UAV differed from the actual height AGL. To investigate the issue, the PCs' heights ATO of one flight were compared with the barometric To investigate the issue, the PCs' heights ATO of one flight were compared with the barometric heights ATO of the UAV of the same flight. The adjusted PC positions can be considered as the true heights ATO of the UAV of the same flight. The adjusted PC positions can be considered as the true positions. The comparison is depicted in Figure 13. The deviation increased to about 20 meters at the positions. The comparison is depicted in Figure 13. The deviation increased to about 20 meters at the largest heights ATO, which was consistent with the cases shown above. largest heights ATO, which was consistent with the cases shown above.

FigureFigure 13.13.Di Differencesfferences of of the the barometric barometric heights heights and and the actualthe actual heights heights above above take-o takeff (ATO).-off ( TheATO values). The invalues the figure in the are figure calculated are calculated as the barometric as the barometric heights heights subtracted subtracted by the PC's by the heights PC's ATO.heights ATO.

TheThe flightsflights werewere carriedcarried outout underunder didifferentfferent weatherweather conditions.conditions. MostMost ofof thethe flightsflights werewere fromfrom bottombottom toto top,top, somesome alsoalso fromfrom toptop toto bottom.bottom. The height didifferencefference atat thethe toptop waswas alwaysalways inin thethe rangerange ofof 15-2515-25 meters,meters, soso thatthat itit cancan bebe consideredconsidered aa systematicsystematic error.error.

4.2.4.2. TiltedTilted DEMsDEMs To solve the issue, the tilted DEMs were developed, see Section 2.4. As an example, the heights above the fake DEM and the tilted fake DEM are shown in Figure 14. The actual height above the DEM decreased again as the height ATO increased. However, the heights above the original fake DEM were much more constant. For comparison, the barometric heights of the UAV are shown. Remote Sens. 2020, 12, x FOR PEER REVIEW 14 of 20 Remote Sens. 2020, 12, x FOR PEER REVIEW 14 of 20 To solve the issue, the tilted DEMs were developed, see Section 2.4. As an example, the heights To solve the issue, the tilted DEMs were developed, see Section 2.4. As an example, the heights above the fake DEM and the tilted fake DEM are shown in Figure 14. The actual height above the above the fake DEM and the tilted fake DEM are shown in Figure 14. The actual height above the RemoteDEM Sens.decrease2020, 12d ,again 1293 as the height ATO increased. However, the heights above the original14 fake of 20 DEM decreased again as the height ATO increased. However, the heights above the original fake DEM were much more constant. For comparison, the barometric heights of the UAV are shown. DEM were much more constant. For comparison, the barometric heights of the UAV are shown.

Figure 14. Heights above the fake DEM (blue), heights above the tilted fake DEM (red) and barometric FigureFigure 14.14. Heights above the fake DEM (blue), heights above the tilted fake DEM (red) and barometric heights (orange) above the fake DEM. heightsheights (orange)(orange) aboveabove thethe fakefake DEM.DEM. If the tilted DEMs were used for missionmission planning, the actual heights AGL were much more If the tilted DEMs were used for mission planning, the actual heights AGL were much more constant, as can be clearly seen in Figure 14 (see(see blueblue line).line). constant, as can be clearly seen in Figure 14 (see blue line). Moreover, whenwhen using using oblique oblique images, images, the the DCGs DCGs along along the the optical optical axis axis were were more more important important than Moreover, when using oblique images, the DCGs along the optical axis were more important thanthe actual the actual flight flight heights heights AGL. AGL. The DCGs The DCGs of all threeof all missionsthree missions on the on third the day third to theday actualto the terrainactual than the actual flight heights AGL. The DCGs of all three missions on the third day to the actual terrainrepresented represented by the NDTMby the NDTM are shown are inshown Figure in 15Figure. 15. terrain represented by the NDTM are shown in Figure 15.

Figure 15. Slant DCGs to the NDTM from the tilted fake DEM (green), tilted plane DEM (blue) and FigureFigure 15.15. SlantSlant DCGsDCGs toto thethe NDTMNDTM fromfrom thethe tiltedtilted fakefake DEMDEM (green),(green), tiltedtilted planeplane DEMDEM (blue)(blue) andand tilted NDTM (red). The slant distances from the tilted NDTM are simulated with the values of pitch tiltedtilted NDTMNDTM (red).(red). The slant distances from the tiltedtilted NDTMNDTM areare simulatedsimulated withwith thethe valuesvalues ofof pitchpitch angle – 45 degrees at azimuth 330 degrees. angleangle –– 4545 degreesdegrees atat azimuthazimuth 330330 degrees.degrees. The DCGs from the tilted fake DEM mission are the most constant, ranging from 47 to 72 meters. TheThe DCGsDCGs fromfrom thethe tiltedtilted fake DEM mission are the most constant, ranging from 47 to 72 meters. As a reminder, the fake DEM was designed so that the DCGs are at least theoretically 60 meters long. AsAs aa reminder,reminder, thethe fakefake DEMDEM waswas designeddesigned soso thatthat thethe DCGsDCGs areare atat least theoretically 60 meters long. 5. Discussion 5.5. Discussion 55.1..1. UAV over over-Terrain-Terrain FlyingFlying in SteepSteep TerrainTerrain 5.1. UAV over-Terrain Flying in Steep Terrain To execute a terrain-following mission survey, a special software application and a DEM are required. When flying in steep terrain, the flight planning software must allow the user to import Remote Sens. 2020, 12, x FOR PEER REVIEW 15 of 20 Remote Sens. 2020, 12, 1293 15 of 20 To execute a terrain-following mission survey, a special software application and a DEM are required. When flying in steep terrain, the flight planning software must allow the user to import an an accurate terrain description [7]. We used Drone Harmony application as one of the few that were accurate terrain description [7]. We used Drone Harmony application as one of the few that were able able to import a DEM for a DJI-made UAV at the time of our surveys. to import a DEM for a DJI-made UAV at the time of our surveys. Figure 16 shows the comparison between the SRTM DEM and the NDTM. Since the SRTM DEM Figure 16 shows the comparison between the SRTM DEM and the NDTM. Since the SRTM DEM generalizes the terrain due to the low resolution, it is clear that the SRTM DEM cannot be used for generalizes the terrain due to the low resolution, it is clear that the SRTM DEM cannot be used for UAV missions in steep hilly terrain. It could even become dangerous if the aircraft follows the SRTM UAV missions in steep hilly terrain. It could even become dangerous if the aircraft follows the SRTM terrain, because it could crash to the ground. The SRTM DEM was more than 75 m below the ground terrain, because it could crash to the ground. The SRTM DEM was more than 75 m below the ground at some spots of our test area, see Figure 16b. The authors of [9] also claim that the global DTMs may at some spots of our test area, see Figure 16b. The authors of [9] also claim that the global DTMs may not be able to provide sufficient accuracy in the estimation of height AGL. not be able to provide sufficient accuracy in the estimation of height AGL.

(a) (b)

FigureFigure 16 16.. ComparisonComparison of ofSRTM SRTM DEM DEM and and NDTM NDTM:: (a ()a Spatial) Spatial comparison comparison of of SRTM SRTM DEM DEM in in green green tonestones and and NDTM NDTM in in brown brown tones tones and and (b ()b h)ypsometric hypsometric display display of of height height differences differences,, which which range range from from - - 7575 to to 100 100 meters.

5.5.2.2. Vertical Vertical and and O Obliqueblique Images Images in in Steep Steep Terrain Terrain TheThe use use of of oblique oblique images images is particularly appropriateappropriate in in hilly hilly terrain terrain with with rugged rugged topography topography and andoverhangs, overhangs as, as shown shown in in Figure Figure 17 1.7 The. The images images in in Figure Figure 17 17a–ca, 1 show7b and dense 17c show point dense clouds point from clouds single fromsurveys, single while survey Figures, while 17d Figure displays 17d a displays merged pointa merged cloud point from cloud the vertical from the images vertical and images the oblique and theimages oblique taken images on the taken plane on DEM. the plane If only DEM. the vertical If only imagesthe vertical are used, images many are topographicused, many detailstopographic may be detailslost, as may shown be lost, in Figure as shown 17a. in With Figure the 1 use7a. ofWith the the oblique use of images the oblique and the images combination and the ofcombination vertical and ofoblique vertical images, and oblique see Figure images, 17 d,see the Figure result 1 may7d, the still result not bemay perfect, still not but be there perfect, is much but there less void is much space lessand void the terrainspace and modeling the terrain is more modeling accurate. is more Rossi accurate. et al. [32] Rossi confirmed et al. that[32] theconfirmed use of aerial that the nadir use view of aeriis notal nadir very suitable view is for not surveying very suitable subvertical for surveying walls. On subvertical the other hand, walls. the On contribution the other hand, of oblique the contributionimages increases of oblique the consistency images increases of the reconstructed the consistency surfaces. of the Tadiareconstructed et al. [33 ]surfaces. improved Tadia the vertical et al. [33]accuracy improved of the the photogrammetric vertical accuracy model of the when photogrammetric they included model oblique when images they within included their oblique nadiral imagesdataset. within Similarly, their thenadiral authors dataset of [.34 Similarly,] state that the the authors use of of a tilted[34] state camera that improvesthe use of the a tilted robustness camera of improvesthe geometrical the robustness model. Theof the use geometrical of oblique imagesmodel. isThe crucial use of to improveoblique images the density is crucial of the to point improve cloud, theespecially density of in the connection point cloud, to peculiar especially features in connection of the surveyed to peculiar object features [35]. of the surveyed object [35].

Remote Sens. 2020, 12, 1293 16 of 20

Remote Sens. 2020, 12, x FOR PEER REVIEW 16 of 20

(a) (b)

(c) (d)

FigureFigure 17.17. DenseDense pointpoint cloudsclouds ofof aa ruggedrugged terrainterrain andand overhangs,overhangs, generatedgenerated from:from: ((aa)) verticalvertical imagesimages aboveabove NDTM;NDTM; ((bb)) obliqueoblique imagesimages aboveabove thethe fakefake DEM;DEM; ((cc)) obliqueoblique imagesimages aboveabove thethe planeplane DEMDEM andand ((dd)) mergedmerged densedense pointpoint cloudcloud from from vertical vertical and and oblique oblique images images above above the the plane plane DEM. DEM.

ThereThere areare alsoalso somesome issuesissues withwith thethe obliqueoblique images.images. IfIf anyany objectsobjects withwith significantsignificant heightheight aboveabove thethe groundground areare present (e.g. (e.g.,, vegetation vegetation),), they they could could cause cause a disturbance a disturbance in inimaging imaging because because they they are arecaptured captured from from the theside, side, not not from from above above as in as the in the vertical vertical imagery. imagery. As Asa result, a result, it is it isvery very difficult difficult to togenerate generate an an orthophoto orthophoto from from the the oblique oblique images images if if such such is is required. Vertical Vertical images are are more more appropriateappropriate toto produceproduce orthophotosorthophotos[ [36].36]. AsAs anan example,example, FigureFigure 1818 showsshows twotwo insertsinserts ofof orthophotosorthophotos ofof aa forestforest areaarea nextnext toto thethe rockfall.rockfall. ItIt isis veryvery obviousobvious thatthat thethe softwaresoftware hashas problemsproblems toto createcreate aa clearclear orthophotoorthophoto fromfrom thethe obliqueoblique images.images. AmrullahAmrullah etet al.al. [[3377]] suggestedsuggested thatthat onlyonly verticalvertical imagesimages shouldshould bebe usedused toto produceproduce orthophotos.orthophotos.

Remote Sens. 2020, 12, 1293 17 of 20 Remote Sens. 2020, 12, x FOR PEER REVIEW 17 of 20

(a) (b)

Figure 18. InsertsInserts of oforthophoto orthophoto with with 5-cm 5-cm resolution resolution:: (a) from (a) vertical from vertical images images and (b) and from (b oblique) from obliqueimages. images.

The main problem with vertical images in steep terrain is the large difference difference in scale in a single image, see Figures2 2–-44,, whichwhich probablyprobably causescauses longerlonger processingprocessing times.times. NotNot onlyonly thethe scalescale isis anan issue,issue, a big factor is thethe appearanceappearance of a detaildetail inin manymany images.images. For example, a detail in the lower part of the rockfall appears in more thanthan 100100 verticalvertical images.images. The number of oblique images with the same detail was about 10. In figures, figures, a dense cloud generation from 218 vertical images took 10 hours and 54 minutes minutes and and 78.3 78.3 million million points points were were calculated. calculated. A dense A dense cloud cloud calculation calculation from 203 from oblique 203 oblique images tookimages 1 hour took and 1 hour 24 minutes,and 24 minutes, in which in 129.8 which million 129.8 pointsmillion were points calculated. were calculated.

5.3.5.3. Height D Decreaseecrease An unexpected issue occurred on all our flights,flights, namely the decreasedecrease of the actual height of the UAV aboveabove any any DEM DEM as as the the height height ATO ATO increased. increased. At aAt total a total height height of about of about 300 meters 300 meters ATO, theATO, actual the heightactual height AGL decreased AGL decreased by about by 20-25 about meters 20-25 atmeters the top. at the We weretop. We able were to compensate able to compensate for this by for tilting this theby tilting DEMs the upwards, DEMs upwards, see Section see 2.4.3 Section. These 2.4.3. values These appliedvalues appl to theied Belca to the site. Belca This site. issue This needsissue needs to be furtherto be further investigated investigat at othered at other sites atsites diff aterent different elevations. elevations.

6. Conclusions 6. Conclusions The execution of a UAV mission in steep terrain is much more complex than in flat terrain. In the The execution of a UAV mission in steep terrain is much more complex than in flat terrain. In latter, the UAV flies at a constant height ATO and the images are captured vertically. In non-flat terrain, the latter, the UAV flies at a constant height ATO and the images are captured vertically. In non-flat the UAV must follow the terrain at a certain height AGL. Mission flying in steep terrain is much more terrain, the UAV must follow the terrain at a certain height AGL. Mission flying in steep terrain is comfortable than manual flying, mainly because of the changing elevation of the terrain, but still much more comfortable than manual flying, mainly because of the changing elevation of the terrain, special care is needed during preparation and even more during the flights. In addition to the terrain but still special care is needed during preparation and even more during the flights. In addition to itself, high trees or other tall objects need to be taken into account when setting the flight height. the terrain itself, high trees or other tall objects need to be taken into account when setting the flight To be able to follow the terrain, a DEM with a suitable resolution and topographical accuracy height. is required. Using low resolution global DEMs, such as SRTM, is not only inadequate, but can also To be able to follow the terrain, a DEM with a suitable resolution and topographical accuracy is be dangerous. Based on our research, a 5-meter NDTM contained sufficient topographic detail to required. Using low resolution global DEMs, such as SRTM, is not only inadequate, but can also be adequately follow the terrain from a UAV height of 80 meters. A special application is required for the dangerous. Based on our research, a 5-meter NDTM contained sufficient topographic detail to use of a DEM for terrain-following flying. adequately follow the terrain from a UAV height of 80 meters. A special application is required for the useA UAVof a DEM survey for of terrain a steep-following terrain should flying not. contain only vertical images, because there are large scale changes in the images and not all details are captured in the rugged terrain. The processing time A UAV survey of a steep terrain should not contain only vertical images, because there are large ofscale the changes vertical in images the images is much and longer not all than details the are processing captured of in the the oblique rugged images. terrain. TheThe obliqueprocessing imagery time alsoof the has vertical an issue images generating is much an longer orthophoto than the image. processing of the oblique images. The oblique imagery also hasTo overcomean issue generating the problems an orthophoto mentioned image. above, the optimal solution is to use a combination of verticalTo andovercome oblique the images. problems It is truementioned that the above, combination the optimal of both solution types of is images to use requires a combination even more of vertical and oblique images. It is true that the combination of both types of images requires even more

Remote Sens. 2020, 12, 1293 18 of 20 processing time, but the result is a point cloud with much more detail, and a proper orthophoto can be produced. Regular DTMs as a basis for the terrain following flying are only suitable for the vertical images. Custom DEMs can be generated to create a surface for the oblique images. The plane DEM is easy to construct, it assures the minimum distance to the surface and thus a sufficient overlap. However, the fake DEM is preferred, as the DCGs are preserved and thus the GSD is kept constant. If due to limited time or UAV batteries only one flight mission can be performed and the orthophoto is not required, we recommend the use of the fake DEM and oblique images. For the optimal complete solution, select NDTM for the vertical images and the fake DEM for the oblique images. If there is a height issue as we encountered it, use a tilted NDTM for the vertical images and a tilted fake DEM for the oblique images. The plane DEM is a reasonable alternative to the fake DEM.

Author Contributions: Conceptualization, K.K.T., D.G. and D.P.; Data curation, K.K.T. and D.P.; Formal analysis, K.K.T. and D.G.; Funding acquisition, D.P.; Investigation, D.G. and D.P.; Methodology, K.K.T.; Project administration, D.P.; Resources, D.P.; Supervision, D.P.; Validation, D.G. and D.P.; Visualization, K.K.T.; Writing—original draft, K.K.T.; Writing—review and editing, K.K.T., D.G. and D.P. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge the financial support from the Slovenian Research Agency (research core funding No. P2-0227 Geoinformation infrastructure and sustainable spatial development of Slovenia and No. P2-0406 Earth observation and geoinformatics). Conflicts of Interest: The authors declare no conflict of interest.

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