Comparison between Lidar and Photogrammetry

LiDAR is functional for group of elevation data in case of dense forests, where photogrammetry fails to expose the accurate terrain information, due to dense canopy cover. Not limited by the environmental conditions restricting aerial photography, airborne LiDAR is emerging as an attractive alternative to the traditional technology for large-scale geospatial data capture. Because it is an active illumination sensor a laser system can collect data at night and can be operated in any weather and at low sun angles that prohibits aerial photography. Rural and remote areas can be surveyed easily and quickly because each XYZ point is individually geo-referenced, aerial triangulation or orthorectification of data is not required (Flood and Gutelius, 1997).

Photogrammetric methods for DTM generation are very time consuming and labor intensive. In photogrammetric method of DTM generation using stereoplotters, firstly photogrammetric model needs to be formed into stereoplotter using interior, relative and absolute orientations. Stereo-compiler manually digitizes geomorphic feathers such as, drainage, road edges, sides and bottom of ditches in one layer. These lines are called as “hard break lines”. The undulations in the topography are mapped by so called, “soft breaklines” (shown in yellow colour). Then spot heights are added up at regular interval (cyan colour) manually by keeping floating mark on the ground (in model). Later on the DTM is generated from these breaklines and spot heights by using sophisticated softwares currently available in market. On the contrary, on an average, 90-100 sq. km. area can be measured in one hour using LiDAR system. Typical post-processing time for LiDAR are, two to three hours for every hour of recorded flight data with additional processing time required for more sophisticated analysis for target classification (Lohani, 2000). Studies showed that LiDAR requires only 25 to 33% of the budget needed for photogrammetric compilation (Petzold et al., 1999)