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LiDAR in Mapping
4. Applications
Coastal zones, beaches and wetlands are the areas with limited contrast and texture. It is very difficult if not impossible to achieve satisfactory results in such area with traditional photogrammetry. In above case LiDAR gives good results (Flood and Gutelius, 1997). Some application areas where LiDAR outperforms photogrammetry are (Green et al., 1997):
In the digital era today, GIS is increasing in complexity, sophistication and detail. But despite the increasing sophistication of these tools, at the heart of every GIS is the “base layer” that contains and displays the rudimentary spatial information on which all other GIS layers are built. If elevation data is available then a DTM can be created as the base layer. However collecting accurate elevation data to describe the terrain can be difficult and is most often costly and time consuming by traditional methods (Green et al., 1996). The LiDAR DEM tends to be much denser that those available through current sources. This potentially leads to more accurate orthorectification of aerial photographs. User enjoys the benefit of high spatial resolution from the imagery and a very accurate, dense DEM from the LiDAR system. The imagery can be used to add breaklines to the LiDAR data to reveal the terrain more accurately for contour interval of less than 2 foots. LH Systems ALS40 Airborne Laser Scanner claims to offer much the same swath width as an aerial film camera with a six-inch (15 cm) lens, which is ideal for simultaneous mapping such as corridor mapping. Simultaneous recording of LiDAR data and digital imagery requires joint operation of the two sensors on board of the aircraft. In corridor surveys with narrow widths, simultaneous mapping is very useful for estimating amount of cut and fill along the alternative alignments. This helps in selecting best economical route from available alternatives. But for projects with larger areas, aerial photography and LiDAR missions needs to be flown separately because of different field of views between aerial cameras and LiDAR scanners. The combination provides more realistic representation of the area than LiDAR data alone. If the project needs 1 foot or smaller contour interval and planimetric details, then it’s good to begin with traditional aerial photogrammetry and analytical triangulation. The LiDAR data can be imported into stereoplotter for each stereomodel so as to enable stereo-operator to easily edit LiDAR data, draw breaklines and generate contours. The combination could be ideal in obscured areas in heavy vegetation where LiDAR data can be useful to fill in the obscured areas not visible in aerial photography. Pure LiDAR products are normally used when schedules outstrip the need for a traditional large-scale map product. LiDAR data filtered to a bare-earth model, or in its raw form, can be grided to create DEM files for viewing as shaded relief or color-coded elevation data. These forms of data can be useful in an emergency situation such as a natural disaster. The following figure shows the area mapped by photogrammetry and LiDAR separately. The area was mapped by flying separately once for photogrammetry and then for LiDAR. The photogrammetrically compiled base maps were then modified for creating tax maps of the area as shown in the figure. These maps are then attached to the database for creating query based maps, the one shown in figure 3. Figure 3 shows photogrammetrically compiled digital map, resulting map with spatial query, contours generated by procedure described earlier and point data imported into GIS software, respectively. Gram++, a package developed by CSRE, IIT, Bombay was used for GIS purpose.  Figure 3 Combined LiDAR-Photogrammetry data product Indian and international scenario The technology is well tested and accepted by carrying out pilot projects in various countries around the world. LiDAR is commercially operational in various countries like USA, Canada and in Europe. So far no LiDAR project has been carried out in India. There is a proposed project that will be carried out by Survey Of India (SOI) in near future, on the pilot basis. In India, there are lots of ongoing and proposed projects related to roadways, railways, oil and gas pipelines, electric transmission lines, communication network, ports and harbors, for which speedy collection of accurate topographic data is an important factor, which reduces the cost of the entire project dramatically. Delays in project work due to the disadvantages of conventional data collection approaches may also be minimized (Lohani, 2000). India is prone to natural disasters of varied forms, resulting in heavy losses of life and wealth. LiDAR data have potential to be effective in many disaster management programs, including most frequently occurring floods, as in case of state of Orissa in India. The LiDAR technology can be very useful for such application in India. Conclusions The main advantages of LiDAR are accuracy of measurements, high automation and fast delivery times. Due to its typical characteristics, both in data collection and data type, LiDAR has opened up several new applications that are not economically feasible with the conventional techniques. In India the technology is yet to make its way and has still a long way to go. Looking at the potentials of this technology, it is obvious that LiDAR will play a major role in geospatial community in near future. References
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