Landslides in relation to terrain parameters - A Remote Sensing and GIS approach
Data
The data used to generate drainage, lineament, road and landslide maps of Darjeeling Himalaya is as follows :
All the four thematic layers using the above data are generated in GIS environment on 1:25,000 scale. The softwares used for this are ARC View GIS and ERDAS Imagine. The toposheets are initially registered with geographic lat-long coordinates. The PAN image is then co-registered with the geo-referenced toposheets by taking input ground control points from PAN image and reference ground control points (GCP) from registered toposheets. Then LISS-III image is co-registered with the geo-referenced PAN image. The registered images are then re-projected from geographic lat-long to polyconic projection so that the image co-ordinates are in meters. The merging of gray scale and multi-spectral images can enhance the satellite data interpretation because of both high spatial and spectral resolutions. Hence, a merged image from LISS III and PAN images is created through digital image processing. Feature extraction for all the four themes involves interpretation of individual LISS III, PAN and their merged product. The technique followed for map generation is described below  Fig.2: Drainage density map Initially the PAN image is looked into and landslide slopes are very well interpreted. In this image the high contrast in the gray level due to thickly vegetated land and landslide affected barren slope is very well judged. However, the landslide areas with insignificant gray level contrast but having other spectral attributes are difficult to identify on PAN image. Hence in the next step, the merged PAN and LISS-III image is used. It is observed that landslide slopes are very nicely emerged as different identity even where it is not completely barren. All the identified landslides are mapped in the vector layer. The landslide vector layer is then exported to ARC View and landslide map is prepared after cleaning and building polygon topology. The theme table showing the area and perimeter of each landslide is automatically generated. In total 254 landslides are mapped. Few of these landslides are cross checked in the field also. The landslide map of the area is shown in the Fig. 1 Drainages from toposheets are digitised onscreen and then modified using satellite images. The vector file is then exported as shape file and the drainage map is generated in ARC View. Lineaments are the linear morpho-tectonic features of the terrain which include faults, fractures, ridges, major discontinuities etc. To extract these lineaments, satellite images are studied. It is observed that interpretation of merged data is more judicious than of individual image because of high spatial and spectral resolutions. The lineaments interpreted are digitised in vector mode and converted to shape file to generate the lineament map in ARC View. Considering road as one of the important anthropogenic factors inducing instability, a road map is generated showing only metalled roads of the study area. It is observed that the roads are very well identified on the merged image. All the roads are mapped initially from the toposheets and modified using the merged satellite data in vector form. After converting it to shape file, the road map is generated in ARC View. |