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Introduction to Lidar-Based Aerial Surveys (Part 2)

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Introduction to Lidar-Based Aerial Surveys (Part 2) Surveying technical Introduction to lidar-based aerial surveys (Part 2) by Dr. Adriaan Combrink, CK Aerial Surveys In the first part of this series of articles the reader was introduced to lidar-based aerial surveys (LBAS) as a surveying technique, and the basic technologies and workflow of LBAS were outlined. In this article we will look at typical LBAS deliverables and applications. We will also consider the specifications of an aerial survey that a client needs to understand prior to commissioning surveys. he usefulness of three- would determine to what extent a dimensional lidar data is point’s elevation should differ from Legend Trooted in the ability to classify its neighbours’ to be considered individual survey points to specified as significant to be included in classes. These classes can then be the model key point DTM. In CAD Computer aided drawing used, separately or in combination, to the case of large flat areas, the maximum spacing will determine produce powerful applications. DTM Digital terrain model the maximum distance between Considering the desired outcome points in the resulting DTM. Geographical information and applications of an aerial survey GIS These parameters will depend on systems is absolutely critical in deciding on the application and scale of the the specifications of the deliverables. project. GSD Ground sampling distance After looking at typical deliverables, Contours: A contour is a line we will consider the specifications which connects points of equal LBAS Lidar-based aerial survey required for particular deliverables. elevation on a map. This is an older and still very efficient Light detection and The most common LBAS way of representing elevations, Lidar ranging deliverables especially in mapping, because it The first – and most common – deliverable of LBAS data is a digital terrain model (DTM). In order to produce this, we consider the points classified as “ground points” during classification of the point cloud. However, due to the high spatial sampling rate of modern lidar systems this could be an impractically large data set. In order to make it more usable to the average user in a CAD or GIS environment, one of the following strategies will be employed: Gridding: This is an older strategy which groups points together in a grid of blocks, say 5 x 5 m, and yields a single mean elevation for each block. Although this technique is very efficient as far as computing resources go, it has the major disadvantage of not being sensitive to significant elevation changes over short distances. Model key points: This is a thinning strategy which requires two input parameters from the user, namely sensitivity and Fig. 1: Half-metre contours draped over a model key point DTM for a railway route along a maximum spacing. The sensitivity street. 20 PositionIT – June 2011 SURVEYING technical such as power lines, vegetation or buildings. Power-line mapping (see Fig. 3): The catenaries of transmission and distribution power lines can be accurately surveyed and modelled using LBAS data. Apart from the obvious applications in line assessment and servitude encroachment detection, two of the very important applications are firstly to detect trees or other structures which pose a threat to the sustainability of power supply; and secondly to measure the amount of catenary sag, which is also dependent on ambient temperature and electrical current, in order to determine to what extent the maximum capability of the power line is being utilised. Hydrology (see Fig. 4): By combining rainfall data with LBAS-derived DTMs, hydrologists can determine 50- or 100-year flood-lines. These have application in infrastructure Fig. 2: An orthophoto tile corresponding to the area depicted in Fig. 1. development as well as risk management. Volume determination (see Fig. 5): is easy to interpret when studying LBAS deliverables. Fig. 2 shows an Auditing and resource reconciliation a particular area of interest. example of orthophotos produced by are parts of modern mining operations. Typically LBAS deliverables will this strategy. Due to its dense spatial sampling, have various levels of detail to use Although these LBAS deliverables at different scales. For instance, a lidar data sets are very useful and – terrain models and orthophotos minor contour layer in GIS or CAD cost-effective in three-dimensional – are not products in themselves, could be a quarter- or half-metre mapping of open-pits or stockpiles, for contour, and the basic and major they are the fundamental building which the volume can be determined contour layers could contain 1- blocks of most modern mapping relative to a base surface. and 5-m contours, respectively. products. For any three-dimensional Then, depending on the scale engineering projects, all features and application of the user, the are to be considered relative to the appropriate layers can be enabled. terrain model. For two-dimensional mapping projects, the features Fig. 1 shows an example of half-metre identified in the orthophotos can be contours plotted over a model key point line mapped manually and, together DTM. with the contours, form the basis of As discussed in the previous article topographic maps. in this series (see PositionIT Jan/Feb 2011), it is very common to operate a LBAS applications to add value medium-format camera in conjunction Since topographic mapping with the lidar system. Individual is usually not the sole aim of pixels from the resulting images are lidar-based surveys, we should projected onto the DTM in order to consider some of the value-add geo-reference and ortho-rectify the deliverables from LBAS. The images. This projection requires applications listed here are by no knowledge of the camera’s lens means a complete list, but rather distortion, which is calibrated in an attempt to demonstrate some a laboratory, as well as the exact of the creative ways in which position and orientation of the camera lidar data sets have been applied. at the moment the image is sampled. These applications often require After mosaicing and colour-balancing information in addition to the all the individual images, they will most common LBAS deliverables be cut into rectangular orthophoto mentioned in the previous section, Fig. 3a and b: Horizontal perspectives of a tiles. After DTMs and contours, such as weather data, climate data, lidar point cloud along and perpendicular to orthophotos are the most common and lidar point classification to classes electricity transmission lines. 22 PositionIT – June 2011 SURVEYING technical Agriculture and forestry: Apart from combining multi-spectral sensors with LBAS data, lidar applications in precision farming and forestry include biomass determination, inventory recording and measurement of vegetation heights or tree crown sizes. Three-dimensional mapping (see Fig. 6): Most engineering projects require accurate three-dimensional data to form the basis of design work, such as three-dimensional break-lines draped onto lidar data at the edge of a slope. Risk assessors and insurers often use three-dimensional building information for the purpose of property valuation, with information such as building height, surface area and volume being readily available Fig. 4: Lidar-derived DTMs can be used, in conjunction with climate data, to determine from digitised buildings. accurate flood-line positions. As is evident from the examples in this section, LBAS routinely finds application in industries such as mining, environmental management, agriculture, forestry, civil engineering and service utilities such as roads, electricity and water supply. Aerial survey specifications The specifications for an aerial survey will depend on the client’s application requirements as well as what can be offered by service providers in the industry. In this section the most important specifications will be mentioned as well as some current Fig. 5: High density point clouds surveyed in mining environments can be used for volume industry standards, which are mostly audits and resource reconciliation. technology-dependent. Point cloud density: This parameter refers to the number of points to be surveyed per square metre in the area of interest. The point density depends on various factors, including the flying speed and elevation, the side-to-side scan rate of the laser scanner and the pulse rate of the laser system. As a general rule, a higher point density is considered to be better. The reason for this is that large point spacing might miss significant features or elevation changes which are of interest for Fig. 6: Lidar data can be used to model complex building structures, as shown in this three- specific applications. For normal dimensional wire-mesh, for use in civil engineering projects. topographic mapping, the current standard is approximately five or more points per square metre, Orthophoto pixel size: A digital the resolution of pixels reduce as the while one could get up to 60 points image is composed of a large square of the pixel size increases, per square metre by focussing the number of square picture elements, e.g. a 7-cm pixel (49 cm²) has laser pulses in a narrow strip for called pixels. Pixel size, or ground double the resolution of a 10-cm power-line surveys to ensure one sampling distance (GSD), refers pixel (100 cm²), and like-wise a point per linear metre surveyed along to the side length of the individual 10 cm pixel has double the each conductor and earth wire. squares. It is important to note that resolution of a 15 cm pixel. The PositionIT – June 2011 23 SURVEYING technical current industry standard for horizontal and vertical coordinates. Because these specifications are very aerial photography from fixed-wing Usually the 1-sigma standard dependent on the current frontiers aircraft is 15 to 20 cm pixel size, deviation of the imagery position error of technology, it is expected that while helicopter-based surveys should be better than 1½ to 2 times these will improve as rapidly as the typically yield 7 to 10 cm resolution.
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