Modification of High Resolution Airborne Laser Scanning Dtms for Drainage Network Delineation

PFG 2014 / 1, 0041–0054 Report Stuttgart, February 2014

Modification of High Resolution Airborne Laser Scanning DTMs for Drainage Network Delineation

MICHAEL VETTER &GOTTFRIED MANDLBURGER, Vienna, Austria

Keywords:Hydrology,flowaccumulation,opensource,GRASS-GIS,anthropogeniceffects, street networks, DTM, ALS

Summary: In this paper a method is presented to Zusammenfassung: Anpassung eines hochauflö- adapt a 1 m ALS (Airborne Laser Scanning) DTM senden Airborne Laser Scanning DTMs zur Be- (DigitalTerrainModel)fordrainagenetworkde- rechnung von hydrologischen Modellen. In diesem lineation. In such DTMs small natural topographic Artikel wird eine Methode vorgestellt, mit der ein features as well as anthropogenic structures like hochauflösendes 1 m ALS (Airborne Laser Scan- roads and artificial embankments are contained, ning) DTM (Digital Terrain Model) für die Ablei- both influencing the drainage network delineation tung eines potentiellen Gerinnenetzes aufbereitet process in different ways. While natural topo- wird.In1mGeländemodellensindnatürlicheGe- graphic features lead water to drainage systems, ländemerkmale sowie Bereiche mit anthropogener anthropogenic structures can deflect water along- Reliefüberformung(StraßenundkünstlicheBö- side artificial surfaces, which are not flow-active schungen) abgebildet. Der natürliche Geländever- under regular hydrological conditions and, there- lauf und die anthropogenen Strukturen beeinflus- fore,resultinwrongdrainagesystems.Wepresent sendieAbleitungvonGerinnenetzwerkenbasie- aworkflowtosubtracttheinfluenceofroadsinthe rend auf Fließakkumulation unterschiedlich. So DTM, replacing the actual road profile by an aver- werdenGerinnenetzwerkeentlangdesnatürlichen agehillslopeoftheneighbouringterrain.This Geländeverlaufs in der Regel ein realistisches Ge- modified DTM is the basis for drainage network rinnenetzwerk ergeben. Entlang von künstlichen delineation using standard flow accumulation. The Strukturen können die Berechnungen in eine Rich- drainagenetworksarederivedforfourDTMvari- tung abgelenkt werden, in welche unter normalen ants(1mDTM,1mmodifiedDTM,5mDTMand hydrologischen Bedingungen kein Abfluss stattfin- 5 m modified DTM) and the results are compared det. Eine solche Ablenkung resultiert daher in ei- to a reference dataset. It is shown that the accuracy nem unrealistischen Gerinnenetzwerk. Mit dem of the derived drainage network can be increased vorgestellten Ansatz, bei dem der Einfluss der Stra- by 3–5% using the modified instead of the original ßen aus dem Geländemodell herausgerechnet und 1mDTM.Thegaininaccuracyamountsupto12% durch eine angenäherte natürliche Neigung entlang whenusingthemodified1mDTMcomparedto der Straßen ersetzt wird, ist eine deutliche Verbes- anyofthe5mDTM.Therefore,ourconclusionis serungderGerinneableitungmöglich.Fürvierver- that high resolution and modified 1 m DTMs with schiedene DTM-Varianten werden Gerinnenetz- anthropogenic structures such as roads strictly re- werke abgeleitet (1m DTM, modifiziertes 1m movedshouldbeusedfordrainagenetworkdeline- DTM, geglättetes 5 m DTM und modifiziertes 5 m ationinsteadofacoarsespatialresolutionoran DTM). Die Ergebnisse werden mit einem Referenz- original1mDTM. datensatzverglichen,umdieGenauigkeitderAb- leitungzudokumentieren.DieendgültigenResul- tate der Ableitung und der Qualitätskontrolle erge- beneineVerbesserungderGenauigkeitum3–5% desmodifiziertenimVergleichzumoriginalen1m DTMundeineVerbesserungvonbiszu12%des modifizierten 1 m DTM im Vergleich zum geglät- teten bzw. modifizierten 5 m DTM. Folglich ist un- sere Empfehlung, für die Ableitung von Gerin- nenetzwerken anstatt eines geglätteten 5 m DTM oder eines originalen 1 m DTM ein modifiziertes 1 m DTM zu verwenden, bei dem anthropogene Strukturen wie Straßen rigoros entfernt wurden.

© 2014 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany www.schweizerbart.de DOI: 10.1127/1432-8364/2014/0208 1432-8364/14/0208 $ 3.50 42 Photogrammetrie • Fernerkundung • Geoinformation 1/2014

1Introduction The differences between drainage network delineation based on high spatial reso- Forthelasttwodecades,AirborneLaserScan- lutionandcoarseDTMsareshowninLI & ning (ALS) has become the prime acquisition WONG (2010).Thequalityofadrainagenet- technique for collecting topographic data in work based on high spatial resolution data, highspatialresolution(>1point/m2)witha e.g. a 1 m DTM grid, is mainly influenced by height accuracy of less than 15 cm (WEHR & man-made objects such as streets and dams, LOHR 1999, BALTSAVIAS 1999),whichisusedin wheretheflowdirectionisdeflectedalong many different fields of application, e.g. HÖF- the gradient of the street. Remaining brid- LE & RUTZINGER (2011), MANDLBURGER et al. gesormissingobjectslikeculvertsorpipes (2009), and SOFIA et al. (2011). While country- actingasflowbarriers,whicharenotrepre- wideALSdataacquisitionisalreadyfinished sentedintheDTMandyieldunrealisticflow in some European countries, e.g. The Nether- paths (VIANELLO etal.2009).ADTMfreeof lands, Denmark, and Switzerland, some other the above mentioned flow barriers, which ad- countries will complete ALS data acquisition ditionally guarantees a monotone height pro- inthenearfuture,e.g.AustriaandFinland. gressing along the streams, is referred to as a Almost all European countries maintain riv- “hydrologicallyenforcedDTM”.SuchaDTM ernetworkdataderivedfromlowresolution isaprerequisiteforobtainingcorrectdrainage Digital Terrain Models (DTMs) or topograph- network results. icmaps.ThisisalsotrueforAustria,butthe planimetric accuracy and the completeness of the Austrian river network are poor, especial- 2 Objectives ly for small catchments. The European Water FrameworkDirective(WFD,EU2000)ob- We present a study to map the potential river ligates the member countries to provide de- drainagenetworksystemusingahighresolu- tailed, up-to-date river network data in high tionALSDTMinordertoincreasethelev- planimetricaccuracywithadditionalattri- el of detail, correctness and completeness of butes per river reach, e.g. length, bed slope, thefinaldrainagesystem.Duetothefactthat width, and stream ordering. Currently, activi- streets produce massive errors for drainage tiesareundertakentostandardizethedataex- network delineation (VIANELLO et al. 2009) it change on a transnational level. Guidelines for isouraimtoshowthepositiveimpactofma- basic datasets have already been implemented nipulating artificial structures in a high spatial (INSPIRE, EU 2007). resolution ALS DTM on the delineation qual- The standard drainage delineation meth- ity.Thegoalofthiscontributionisthederiva- ods implemented in proper GIS-Software are tion of potential river drainage systems. Here- basedeitheronsingle-neighbourflowalgo- by, the term “potential river” describes the wa- rithms (D8, O’CALLAGHAN & MARK 1984) or tercourse following the terrain gradient under multiple-neighbour flow algorithms like mul- regular hydrological conditions. At locations tipleflowdirection(MFD,QUINN et al. 1991). where streets block the potential flow path, Both flow algorithms are used to compute pipes and culverts are usually built to guar- drainage networks which indicate the poten- antee a continuous watercourse along ditches. tial watercourses. A comprehensive overview Thosepipesandculvertsaredimensionedto about flow algorithms and their differences cope with regular run-off volumes. As regular are given by GRUBER & PECKHAM (2009) and hydrological conditions are presumed in this WILSON et al. (2008). Various implementations article, no deflections alongside the streets are areavailableinstandardGISsoftwareprovid- expected. As subsurface man-made structures ingspecializedtoolsforindividualenviron- under roads, e.g. pipes, are undetectable in ments. Besides flow algorithms, other meth- ALSdata,theroadsactaswallsorbarriersif ods exist to derive drainage networks and fea- theyarenotremovedfromtheALSDTMprior tures related to hydrology (PASSALACQUA et al. tothedrainagedelineation.As1mDTMsare 2010, 2012). availableinmanycountries,thegoalistoex- ploitthefullpotentialofthosedataassource MichaelVetter&GottfriedMandlburger,ModificationofHighResolutionALSDTMs 43 fordrainagenetworkdelineation,whichisnot covers an area of 93 km2 and is divided into thecaseuptonow(MANDLBURGER et al. 2011). an upper (Bolgenach) and a lower (Weißache) Therefore, we present a method to modify a sub-catchment (Fig. 1). The terrain elevations DTMbysection-wiserecalculatingthenear- rangefrom450mattheconfluenceofthe naturalslopebeforetheroadwasconstructed WeißachandtheBregenzerAchto1645mat andbyreplacingtheactualroadprofilebased the Feuerstätterkopf. The dominant geological ontheaveragehillslope.Thisyieldsamodi- formation is Molasse with a dense drainage fied 1 m DTM without streets. The results are system (OBERHAUSER & RATAJ 1998). comparedtoareferencedatasetprovidedby ForthetestsiteALSdataareavailablefrom the local mapping authority. We assume that differentepochs.ForthisstudytheDTMde- themodificationoftheDTMleadstoabet- rivedfromthepointcloud,usingthehierarchic ter drainage network mapping accuracy when robust filtering approach described by KRAUS applying automatic delineation based on flow & PFEIFER (1998),oftheNovember2003cam- accumulation. paign, was chosen. The average point density is 1.6 points/m2 (last echoes). The data were collected with Optech’s ALTM 2050 scanner 3 TestSiteandData in discrete echo recording with a maximum of four reflections per laser shot. The grid width Thechosentestsiteisasub-catchmentofthe oftheDTMis1m.Inaddition,aresampled Bregenzer Ach (Vorarlberg, Austria) drain- versionoftheDTMwithaspatialresolution ing the Bregenzerwald. The Bregenzer Ach of5mwasgenerated. is an alpine river with an approximate length Besidesthetopographicdata,astreetand of80km,atotalcatchmentareaof830km2, adrainagenetworklayerwereprovidedby an average annual discharge of 46 m3/s and the local mapping authority (Landesamt für amaximumdischargeof1350m3/s, which Vermessung und Geoinformation, LVG). The was measured in August 2005, as a 100 year street layer contained several files, each of flood eventA ( MT DER VORARLBERGER LANDES- them representing different street/road types REGIERUNG 2005). The selected sub-catchment such as high order streets, municipal roads,

Fig. 1: Test site (Bregenzerwald, Vorarlberg, Austria); shading superimposed with elevation cod- ing, roads and existing river network. Left: map of Vorarlberg with the upper (red) and lower (green) sub-catchments of the Bregenzer Ach. Map sources: Austria: Wikimedia.ogr as svg; Vorarlberg: http://vogis.cnv.at/ (WMS); catchments, streets and official drainage network: Landesamt für Vermessung und Geoinformation (LVG, Amt der Vorarlberger Landesregierung). 44 Photogrammetrie • Fernerkundung • Geoinformation 1/2014 forestroadsorhikingtrails.Thelayerswere have been built and no major mass movements produced by different departments within orothernaturalhazardshaveoccurredsince the authority of Vorarlberg on the basis of or- thetimetheALSdatawereacquiredin2003. thophotos and shaded relief maps of the 1 m Onlyinonepartahugelandslideiscurrently DTM. Before the actual processing, a consist- activebutinthisareanostreetsarelocated. ency check was performed by visual inspection and the different layers were merged into a single dataset. It could be verified that the 4Methods deviation of the street locations compared to theshadedreliefmapofthe1mALSDTMis Themethodreliesontwodatasets.Thefirst less than 5 m. datasetistheALSDTM,whichisusedasba- The reference drainage network dataset sic input for the processing workflow present- wasprovidedbythelocalhydrologyandgeo- edlater,andfordrainagenetworkdelineation. informationdepartment(AbteilungWasser- Theseconddatasetisavector(polyline)layer wirtschaft,LandesamtfürVermessungund representing the axes of the existing streets. Geoinformation). It was mapped in a semi- Whereverpossible,streetdatafromthelocal automatic way by deriving the main drain- mapping authorities should be used, because age network using standard flow accumu- thesedatasetsareusuallywellmaintainedin lation methods based on the 1 m ALS DTM Europe concerning both, accuracy and up- from 2003. The resulting vector dataset was to-dateness. If no such official data source is interactively edited using orthophotos and 1 m available, street layers can also be extract- shaded relief maps to improve the correctness ed from OpenStreetMap (OSM) or obtained and completeness of the dataset. This refer- from commercial navigation and routing sys- encedatasetalsoincludesallculverts,piped tem providers with an expected lower accura- sectionsunderbridgesandfictitiousriveraxes cy.Ifnocentrelinesofthestreetsareavail- through lakes. The provided reference drain- able, the street edges can be derived by using a age network is constantly improved and en- multi-scale segmentation approach (BAATZ et tirely supervised by hydrology experts and is al. 2003), raster-based classification using el- the best available dataset covering the entire evation, slope, aspect and curvature (first and area of one single federal state in Austria. The second order derivatives) as shown in WOOD latest timestamp of the river network refer- (1996), BRÜGELMANN (2000) or RUTZINGER et ence data is October 10, 2012. Those data are al.(2011)orbreaklinemodellingbasedonthe used for evaluating the presented methods. Al- ALS point cloud, e.g. BRIESE (2004). The re- though there is a time difference between the sultingstreetedgescanbeusedtoobtainthe ALSDTMandthereferencedataset,thistime centre lines. gap is of minor importance, because no streets

Fig. 2: Workflow and processing chain for DTM modification. MichaelVetter&GottfriedMandlburger,ModificationofHighResolutionALSDTMs 45

OurworkflowisshowninFig.2.First,the erage filter. As a consequence, four different centre lines of the streets (1) are buffered to DTMs are used in our experiments (original define the extent for DTM modification (2). 1mDTM,modified1mDTM,5mDTM,and Then,perpendicularlines(crosssections) modified5mDTM). withafixeddistancefromeachotheralongthe For drainage network delineation, a stand- street axis and a predefined length are created ard flow accumulation method based on Mul- (3).Foreachcrosssectiontheheightsofthe tipleFlowDirections(MFD),asimplemented startandendpointsareinterpolatedfromthe in GRASS-GIS (r.stream.extract, JASIEWICZ & DTMandtheheightofthecentrepointiscal- METZ 2011, GRASS DEVELOPMENT TEAM 2012), culated by linear interpolation from the start is used to compute the drainage network. andendpoints,resultinginthreeX,Y,Zcoor- Fortheerrorassessmentalinebasedap- dinate triples (4) for each cross section. After proachasdescribedinRUTZINGER et al. (2012) that, all extracted start, centre, and end points is chosen. For each dataset, i.e. reference and ofasinglestreetlinearetriangulated(5)and the derived drainage network, we interpo- fromtheresultingTINaregulargridofthe late points along the lines at a regular inter- buffer area (6) is derived, representing the re- val,andwereplacethelinesbythese(dense) constructed model of the terrain with near- point sets. The evaluation procedure is based naturalslope.Finally,thebufferfromstep(2) on an analysis of the reference and the derived isusedtocliptheDTM(7)and,byamapalge- drainage point sets. Points from both datasets braoperation,theclippedDTMandtherecon- areacceptedascorresponding(TruePositives, structedsurfacealongthestreetarefusedto TP)iftheyarewithinadefinedsearchradius themodifiedDTM(8),containingnostreets. (Fig. 3). False Positives (FP) denote points of the derived dataset for which no correspond- ingpointinthereferenceexists.Pointsofthe 5 Experiments reference with no corresponding point in the deriveddatasetarecountedasFalseNegatives Themethoddescribedintheprevioussec- (FN). Correctness (1), completeness (2) and tionwasappliedtotheoriginal1mDTMde- quality (3) are calculated for selected search scribedabove,yieldingamodifiedDTMof radii(1m,5m,10m,and15m),takinginto 1mresolution.Tosimulatetheinfluenceof accountthenumbersofTP,FPandFN,re- acoarserresolutiononthefinaldrainagenet- spectively. work product, both the original and the modi- TP fied1mDTMweredown-sampledtoaspatial Correctness = (1) resolution of 5 m by applying a moving av- TP+ FP TP Completeness = (2) TP + FN

TP Quality = (3) TP + FP + FN

6 Results

In this section we present the results of data processing (DTM modification) as well as the comparison of the different drainage networks obtainedbyusingthefourDTMvariantsde- scribedinsection5.Thecomparisoniscar- ried out both, by visual inspection of the de- Fig. 3: Classification of the nodes of the derived and reference network into True Positives riveddrainagenetworksandbyastatistical (TP), False Positives (FP) and False Negatives analysis of the accuracy with respect to the (FN); schematic diagram. reference data. 46 Photogrammetrie • Fernerkundung • Geoinformation 1/2014

6.1 DTM Modification fect of the DTM modification. In the plots the original DTM is shown in blue and the modi- TheparametersrequiredfortheDTMmodifi- fiedprofileinred.Themodifiedsurfacerepre- cationarea)thebufferingdistance,b)thecross sents the natural slope before road construc- sectiondistancealongthestreetaxis,andc) tion. The interpolation is carried out for all thelengthofthecrosssections.Thebuffering streetsandroadsoftheprovidedstreetlayer, distance a) is strongly related to the cross sec- whichresultsinamodifiedDTMapproximat- tion length c), because the length c) has to be ing the original slope. This modified DTM is larger than the buffering distance to guaran- thebasisforthefurthercalculationsofthe tee that no void data areas are produced dur- drainage network. The modified 5 m DTM ing data fusion, e.g. Fig. 2, step 8. The cross is calculated by resampling the modified 1 m sectiondistanceb)controlsthegranularityof DTM. theinterpolatedslopesandthereforeshouldbe kept small. The parameters used in our experi- mentsarea):7mbufferingdistance,resulting 6.2 Drainage Derivation in a buffer width of 14 m, which corresponds tothewidthofastreethavingfourlanes;b): Asapreconditionforthesubsequentderiva- 3mprofiledistancealongtheroadaxes;c): tionofthedrainagenetworktheMultipleFlow 16mlengthoftheprofiles.Duringthework- Direction was calculated for all four DTMs. flow(Fig.2,step4)theX,Y,Zcoordinatesof The parameters for drainage network delin- thestart,centreandendpointsofthecross eationareI)minimumcatchmentarea(2.5ha) sections are triangulated. and II) minimum segment length (> 100 m). Fig. 4 shows the colour coded height differ- These parameter values are applied to all encesbetweentheoriginalandthemodified DTMs to produce comparable results. A dis- 1 m DTM for a short street section. Plots of cussion about different drainage delineation threecrosssections(A,B,C)illustratetheef- parametersisnotgiveninthisarticle,because

Fig. 4: Difference between original and modified 1 m DTM with street centre line (black), cross sections (blue), interpolation points (red) and triangulation (green); sections through original and modified DTM (DTM blue solid line, modified DTM red dashed line). MichaelVetter&GottfriedMandlburger,ModificationofHighResolutionALSDTMs 47 this is discussed in numerous papers, e.g. WIL- original1mandthe5mDTMsthanforboth SON et al. (2008). Our focus is on presenting modified models. By resampling the DTM, theadvantagesofusingamodified1mDTM small but sometimes relevant terrain features for deriving drainage systems, not on optimiz- are removed. If an important terrain feature ing parameters for drainage delineation. isremoved,thedirectionofthedrainagecan Thedrainagenetworksbasedonthefour bechangeddramaticallyandcanresultina different DTM variants are shown in Fig. 5. wrong final network. This phenomenon can It is clearly visible that the original 1 m DTM be seen in Figs. 5b and 6a alike. The level of resultsinmanydrainagesegmentsalong- detail of the derived drainage network also sidethestreets(Fig.5a).Adrainagesegment decreasesifa5mDTMisused.Ontheone is a part of the stream network between two hand,thelengthsofthesourcesegmentsde- nodes (nodes are intersection points, conflu- crease and on the other hand, the spatial reso- encepoints,sourceorendpoints).Theseprob- lution and, therefore, the stream mapping ac- lems are reduced but not completely removed curacy also decreases. byusingaresampled5mDTM(Fig.5b).Us- ing the proposed DTM modification, the artificial flow patterns along the streets are more 6.3 Accuracy of the Derived Drainage effectivelyremoved,andatthesametimea Network high level of detail is preserved (Fig. 5c). The useofamodified5mDTM(Fig.5d)showsno Inthissectionwepresenttheaccuracyofthe deflectionatallalongtheroads. deriveddrainagenetworkwithrespecttothe Fig. 5 also reveals that the deflection of de- reference data. For the quantitative assessment lineated drainage segments is higher for the allpipedandfictitiousriversegments(7.5%of

Fig. 5: DrainagenetworkbasedonoriginalandmodifiedDTMs(1m,5m).a)1mDTM;b)resam- pled 5 m DTM; c) modified 1 m DTM; d) modified 5 m DTM. 48 Photogrammetrie • Fernerkundung • Geoinformation 1/2014 theoriginalreferencedata)havebeenexclud- terns in the derived data caused by rasterisa- ed from the reference network. As can be seen tion.However,mostofthedifferencesaredue inFig.6a,thetestsitecontainspipedsections, toahighernumberofdrainagesegmentsin mainlyalongsideroads,and,consequential- thederiveddatasetsandtheirextendedlength ly, the reference dataset does show some flow compared to the reference. pathsfollowingthecourseoftheroad.Inthese For the quantitative error assessment, a specificcasesthepipedsectionsaremeasures point spacing of 1 m is used for all drainage to prevent mass movements. Usually, piped networks and the reference data. The percent- sectionsarebuiltasshortestpossibleconduit ages of completeness, correctness and the across the road to allow water flow under the qualityareshowninTab.2. road.Ourapproachisdesignedtodealwith that normal kind of underpass conductor but notwithpipedsectionsalongtheroad.There- fore, this particular part of the reference data 7 Discussion was neglected for the quantitative evaluation. A visual comparison of the derived drain- The presented method for DTM modifica- agesystemofbothmodifiedDTMs(1mand tion, using existing street data to replace and 5 m) and the reference is presented in Fig. 6a. remodelnear-naturalslopesalongtheroads, Thecorrespondenceofthederiveddrainage usesaglobalconstantstreetbufferwidth.A network based on the modified 1 m DTM is global width was chosen as the GIS metada- satisfactory. A few locations at the stream taoftheroadsdoesnotentirelycontainroad head are generally missing in the reference classificationinformation.Itisassumedthat network, where the stream head is close to bufferingtakingintoaccountdifferentstreet the catchment boundary. This results from the widthscouldincreasethequalityoftheDTM used catchment area threshold for drainage modification process. As shown in Tab. 2, the network delineation. It can be stated that the completeness of the derived river network is additionalflowlengthinupstreamdirection much better than the correctness for all consid- provided by the automatic delineation process eredDTMvariantswhentakingintoaccount is useful information although it is not cov- allderivedstreamsegments,i.e.,alldrainage ered in the reference. As can be seen in Figs. sections between two nodes, section 6.2. The 6aand6b,thedrainagenetworkderivedfrom completeness is almost the same for both 1 m the modified 1 m DTM delivers more streams models, but there is a difference of up to 17% than are contained in the reference network. between the 1 m and the 5 m DTMs. On the Thefactthattheseextrastreamsarecontained other hand, the correctness and the quality in the derived dataset means that they are well are much better for the 5 m models. The lower pronounced in the DTM. But this does not correctnessofboth1mDTMsbasicallystems necessarily mean, that they are permanently fromthehighernumberofautomaticallyde- drained, for which reason the expert respon- rivedstreamsandtheirextendedlength,both, sible for the reference dataset may have dis- with respect to the 5 m DTMs and the refer- regardedthemasrelevantrivers.Atleast, ence. The added value of these extra streams in case of major precipitation events, these was already discussed in section 6. streams are likely to contribute to the surface Tab.2clearlyshowsamoderateandunsat- runoff. isfactoryquality.However,thisglobalcom- The cumulative lengths of the reference parison using the entire automatically derived dataandthederiveddrainagenetworksare network is unjustified as rivers of minor im- analysed in Tab. 1. The derived network based portance for the countrywide river network on the original and the modified 1 m DTM were deliberately ignored by the human op- is more than twice as long as the reference erator providing the reference network. To (Fig.6)andthedrainagebasedonboth5m improvethecomparability,threesubsetsof models are approximately 1.4 times longer the derived network were compiled and ana- than the reference. A small part of the differ- lyzed independently with respect to complete- encesinlengthsstemsfromthezig-zagpat- ness, correctness and quality. The subsets are MichaelVetter&GottfriedMandlburger,ModificationofHighResolutionALSDTMs 49

Tab. 1: Cumulative length of networks entire reference data, revised reference data (fictitious and pipedsectionsremoved)andderivednetworksbasedonoriginalandmodifiedDTM(1m&5m).

Data Reference Reference 1mDTM 1mDTM 5mDTM 5mDTM subset modified modified Length (m) 246,163 227,688 518,250 495,106 353,501 346,980 Percentage 100.0 92.5 210.5 201.1 143.6 141.0

Fig. 6: Results of drainage network delineation. a) From modified 5 m DTM (green), modified 1m DTM(darkblue)andthereferencenetwork(red);b)Drainagenetworkoftheentiretestsitede- rivedfrommodified1mDTMandreferencedata.Pipedreferencesectionsaremarked.

Tab. 2: Error assessment (completeness, correctness and quality); complete drainage network compared with revised reference data.

1 m radius 5 m radius 10 m radius 15 m radius All comp. corr. quality comp. corr. quality comp. corr. quality comp. corr. quality segments DTM1m 0.50 0.21 0.17 0.85 0.35 0.33 0.90 0.37 0.35 0.92 0.38 0.36 DTM1m 0.50 0.22 0.18 0.86 0.37 0.35 0.91 0.39 0.38 0.92 0.40 0.39 modified DTM5m 0.33 0.20 0.14 0.78 0.47 0.42 0.83 0.50 0.46 0.85 0.51 0.47 DTM5m 0.33 0.20 0.14 0.79 0.48 0.43 0.84 0.51 0.47 0.86 0.53 0.48 modified 50 Photogrammetrie • Fernerkundung • Geoinformation 1/2014 introduced step by step with the final goal to reference data segments were compared to restrict the nominal-actual comparison to the thespatiallycorrelatingpartsofthederived vindicated parts. segments,i.e.thederivedsegmentswerecut First, all extra segments of the derived data- to the same lengths as the reference. In this set located entirely outside a 20 m perimeter way,allpartsofthederivedsegments(i)cor- around the reference network were excluded responding to piped sections of the reference (Selection I, blue lines in Fig. 7a). Addition- or (ii) extending the reference near the source ally, all major rivers featuring a width larg- wereremoved(bluelinesinFig.7cspatially er than 20 m, i.e. the Weißache and the Bol- correspondingtotheredlinesinFig.7d). genache, were removed (selection II, cf. blue Tab.3showsthatthecompletenessishigh- linesinFig.7b).Finally,basedonSelection er for the derived drainage network using the IIthedetailedareashowninFig.6awasan- modified 1 m DTM than for the drainage de- alysed separately (Selection III). Hereby, the rivedbasedontheoriginal1morboth5m

Fig. 7: Subsets of the river network derived from modified 1 m DTM (a-c, blue lines corresponding toSelectionI-III)andreferencedata(d,redlines).

Tab. 3: Error assessment (completeness, correctness and quality) for Selection I, II and III (Fig. 7).

1 m radius 5 m radius 10 m radius 15 m radius Selection I comp. corr. quality comp. corr. quality comp. corr. quality comp. corr. quality DTM1m 0.49 0.36 0.26 0.83 0.61 0.54 0.88 0.65 0.60 0.90 0.66 0.62 DTM1m 0.49 0.38 0.27 0.85 0.65 0.58 0.90 0.68 0.63 0.91 0.70 0.65 modified DTM5m 0.32 0.27 0.17 0.77 0.65 0.54 0.82 0.69 0.60 0.84 0.71 0.62 DTM5m 0.33 0.27 0.17 0.78 0.65 0.55 0.84 0.69 0.61 0.86 0.71 0.64 modified 1 m radius 5 m radius 10 m radius 15 m radius Selection II comp. corr. quality comp. corr. quality comp. corr. quality comp. corr. quality DTM1m 0.54 0.40 0.30 0.85 0.63 0.57 0.80 0.65 0.59 0.89 0.66 0.61 DTM1m 0.55 0.42 0.31 0.87 0.67 0.61 0.89 0.69 0.64 0.90 0.70 0.65 modified DTM5m 0.34 0.29 0.19 0.77 0.66 0.55 0.80 0.68 0.58 0.81 0.69 0.60 DTM5m 0.35 0.29 0.19 0.79 0.65 0.56 0.82 0.68 0.59 0.84 0.69 0.61 modified 1 m radius 5 m radius 10 m radius 15 m radius Selection III comp. corr. quality comp. corr. quality comp. corr. quality comp. corr. quality DTM1m 0.63 0.55 0.41 0.97 0.84 0.83 0.99 0.86 0.85 0.99 0.86 0.86 modified MichaelVetter&GottfriedMandlburger,ModificationofHighResolutionALSDTMs 51

DTMs.Thecorrectnessishigherforthemodi- sents a commensurable basis of the reference fied1mDTMcomparedtotheotherDTMs andthederiveddataset.Again,forthe5m withinthe1msearchradius(Tab.2).Ifthe search radius the correctness of the modified main river is included (Tab. 2 and Tab. 3, se- 1mDTMrisesto84%. lectionI)intheevaluation,thecorrectness From Tab. 3 it can be seen that the correct- ofthestreamsbasedonthecoarseresolution ness gradually increases from Selections I to DTMs is higher. In Tab. 3, both, correctness III.Furthermore,anincreaseofthecorrect- and completeness are higher for the 1 m modi- ness is noticeable from 1 m search radius to fied DTM streams. The correctness (in Tab. 2) 5m,butlargersearchradiidonotleadtoa is much lower than the completeness, because furtherimprovement.Fromthatobserva- piped sections and fictive river axes through tion we conclude that the planimetric accu- lakesandlargerivershavedeliberatelybeen racyofthederivednetworkisinthemetre removed from the reference dataset as men- range. Concerning the different DTM vari- tioned in section 6. Due to our DTM modifi- ants,itcanbestatedthatwhereasTab.2,i.e. cationthesepartsarecontainedinthederived all derived segments, exhibits higher cor- dataset, causing a high level of FP and, con- rectness for the coarse resolution DTMs, the sequently, a low correctness. Furthermore, modified1mDTMstrictlyoutperformsthe the automatic delineation algorithm produc- otherDTMsinTab.3.Thisisthereasonwhy es more streams (Tab. 1) than are contained theevaluationforSelectionIIIwasonlycar- in the reference, which further contributes to riedoutforthisDTMvariant.Theaccuracy the low correctness. This does not necessarily of the derived drainage network as measured mean that the automatically derived network by the completeness, the correctness and the is incorrect, but a separation into parts where quality of the results, is on average 3 to 5% thederivednetworkreallydeviatesfromthe higher when using the modified DTM rather reference and extends the reference is neces- than the original 1 m DTM. We consider a sary. 3to5%increaseinqualityasarelevantim- Forthatreasonthequalityassessmentwas provement. It is crucial to mention that not all carried out for Selection I, II and III. Selection man-made structures necessarily have high Iconsidersthefactthattheautomaticprocess impact on the natural run-off. In other words, delivers more streams and that rivers in sub- inmostcasestherun-offisdominatedbythe catchments are contained, where they have overall natural relief. Thus, the improvement been disregarded in the reference. For the 5 m potential in terms of completeness, correct- search radius, the increase in correctness is ness and quality is within narrow bounds, but more than 25% for the original and modified nonethelesstheremaining5%improvement 1mDTMs(Tab.2:0.35/0.37vs.Tab.3/Selec- may locally make a big difference. Compar- tion I: 0.61/0.65). Selection II deals with riv- ingtheresultsachievedforthemodified1m erswiderthan20m,wheretheemployedflow DTMwiththoseachievedusingtheDTMat accumulation based delineation procedure is a lower resolution; the average is between 1% not the method of first choice. For streams of and12%betterusingthemodifiedDTM.The that size, the planimetric accuracy of the river main differences between the results based on axisbasedonflowaccumulationislowasthe DTMsof1mand5mresolutionoccurinthe derivedlineismeandering.Forderivingac- completeness and only minor differences oc- curate centre lines of such rivers more suitable curinthecorrectness.Byusingcoarsereso- approaches exist, e.g. HÖFLE et al. (2009) and lution input data, the possibility of deflect- VETTER et al. (2011). The additional increase is ing the derived run-off is much more evident. 2%forboth1mDTMs.SelectionIIIaddition- Some terrain features are removed due to the ally takes into account that (i) underground resampling process. Therefore, the flow accu- pipedsectionsarebuiltmoreorlessstraight mulation follows a wrong slope, e.g. drain to a and not following the surficial discharge and neighbouring valley, Fig. 6a. As this increases (ii) that the derived source segments generally thenecessarymanualeditingeffort,thepro- extend their counterpart in the reference net- posed method facilitates the drainage network work near the river head. This subset repre- delineation process. 52 Photogrammetrie • Fernerkundung • Geoinformation 1/2014

8Conclusion therefore, increases, both, the reproducibility and the efficiency of the river network deriva- Forthederivationofarivernetworkbasedon tion process. standard flow accumulation techniques a -hy drologically enforced DTM without barriers hinderingtherun-offisrequired.Theavail- Acknowledgements able high resolution DTMs do not fulfil this prerequisite as they contain objects like brid- The authors want to thank the “Landesamt ges and street dams acting as flow barriers. fürVermessungundGeoinformation”ofVor- In this contribution we therefore presented an arlberg, Austria, namely to PETER DREXEL approach to automatically modify the DTM for providing the DTM as well as the refer- withinabufferzonearoundstreets.Dams, ence dataset of the drainage network. Thanks cuttings and bridges are removed and the alsogotoourcolleaguesBERNHARD HÖFLE of near-natural slope before anthropogenic relief theUniversityofHeidelberg,Germany,and transformation is approximated. MARTIN RUTZINGER from the University of Thedrainagenetworksderivedfromthe Innsbruckforsupportingtheaccuracymeas- original and modified 1 m DTM as well as urecalculation.Thisstudywasco-fundedby for downsampled 5 m models were compared the Austrian Federal Ministry of Agriculture, with reference data manually digitized by hy- Forestry,EnvironmentandWaterManage- drologyexperts.Itcouldbeshownthatthe ment (Project: BlMLFUW-UW.3.2.5/0040- overall accuracy gain is 3–5% when using VII/2010). We would also like to acknowl- themodifiedinsteadoftheoriginal1mDTM edge the financial support from the Austrian and12%comparedtothedown-sampled5m Science Funds (FWF) as part of the Vienna DTM. Whereas the down-sampling of the Doctoral Programme on Water Resource Sys- high resolution DTM speeds up the delinea- tems (DK-plus W1219-N22), the Austrian Sci- tion process, this potentially results in incor- ence Promotion Agency (FFG-COMET) pro- rect deflections of the run-off. The availabil- ject“AlpineAirborneHydromapping–from ityoftopographicdetailscontainedinthe1m research to practice” (AAHM-R2P), the Aus- DTM are especially important near the catch- trianClimateandEnergyFund–ACRPPro- ment boundaries. gram – Climate induced system status chang- The presented DTM modification strategy es at slopes and their impact on shallow land- is designed for regular discharge condition, i.e. slide susceptibility (C3S-ISLS) and the Insti- mean-flow,wheretheupstreamwatersfully tute of Geography, University of Innsbruck. drain-offunderthestreetsviapipedsections. Inourapproach,theimpactofthesepipedsec- tionsissimulatedbytheappliedDTMmodifi- References cation. Beyond that, additional hydrologically AMT DER VORARLBERGER LANDESREGIERUNG,2005: relevant constructional measures exist, e.g., to Das Starkregen- und Hochwasserereignis des stabilizeslopespronetolandslidesviapiped August 2005 in Vorarlberg. – Amt der Vorarl- sections alongside the slopes and/or streets. berger Landesregierung, www.vorarlberg.at/ AsthesefeaturesareinvisibleintheDTM, pdf/naturereignisdokumentatio.pdf (10.11.2013). they are generally not available for the flow BAATZ,M.,2003:eCogniction – user guide 3, accumulation based river network derivation. Definiens Imaging GmbH. Thisresultsindeviationsofthederivednet- BALTSAVIAS, E.P., 1999: Airborne laser scanning: work from the reference and in a decreased basicrelationsandformulas.–ISPRSJournalof accuracy in these areas. As a consequence, Photogrammetry&RemoteSensing54 (2–3): 199–214. manualeditingofthefinalstreamsbyanex- BRIESE,C.,2004:Three-dimensional Modelling of pert becomes necessary whenever the data ba- Break lines from Airborne Laser Scanner Data. sisdoesnotfulfiltherequirementsofahydro- –IAPRS35 (B3): 1097–1102. logically enforced DTM. However, it can be BRÜGELMANN, R., 2000: Automatic breakline detec- concluded that the proposed method contrib- tionfromairbornelaserrangedata.–IAPRS33 utes to reduce the manual editing effort and, (B3): 109–116. MichaelVetter&GottfriedMandlburger,ModificationofHighResolutionALSDTMs 53

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Addresses of the Authors: DI Dr. GOTTFRIED MANDLBURGER,ResearchGroups Photogrammetry&RemoteSensing,Department Mag. Dr. MICHAEL VETTER,1.)ResearchGroups of Geodesy and Geoinformation, Vienna Universi- Photogrammetry&RemoteSensing,Department ty of Technology, Austria, Tel. 0043-1-58801- of Geodesy and Geoinformation, Vienna Universi- 12235, e-mail: [email protected]. ty of Technology (TUW), Austria; 2.) CWRS – ac.at Centre for Water Resource Systems, TUW; 3.) In- stituteofGeography,UniversityofInnsbruck,Aus- Manuskript eingereicht: Februar 2013 tria, e-mail: [email protected] Angenommen: November 2013