10+Times Asked
~40Total Marks
6Topics
MediumPriority
1 DEM / DTM / DSM
- DEM (Digital Elevation Model): Grid of elevation values representing bare earth.
- DTM (Digital Terrain Model): DEM plus additional terrain features (breaklines, ridges, rivers).
- DSM (Digital Surface Model): Includes everything on the surface — buildings, trees, terrain.
2 DEM Data Sources
- Topographic surveys and contour digitisation.
- GPS ground surveys and total station.
- LiDAR (Light Detection and Ranging).
- SRTM (90 m resolution), ASTER GDEM (30 m).
- Photogrammetry from stereo aerial photographs or satellite imagery.
- InSAR (Interferometric Synthetic Aperture Radar).
3 DEM Structures
- Grid (raster): Regular matrix. Simple, efficient for computation.
- TIN: Irregular triangles. Variable resolution — dense in complex terrain.
- Contour lines: Lines of equal elevation. Traditional representation.
4 Creating DEM in Raster GIS
- Collect source data (contour lines, spot heights, GPS, LiDAR).
- Input data into GIS (digitise or import).
- Apply interpolation (IDW, Kriging, or Spline).
- Convert to raster grid (choose cell size).
- Validate against known control points.
5 Slope and Aspect from DEM
slope = arctan(√((∂z/∂x)² + (∂z/∂y)²))
Slope: Rate of elevation change from 3×3 window. Output in degrees (0–90) or percent.
Aspect: Compass direction of steepest descent. 0–360 (N=0, E=90, S=180, W=270). Flat = −1.
6 Applications of DEM
- Watershed delineation, flow direction, stream network.
- Slope/aspect analysis for engineering and agriculture.
- Visibility analysis: viewshed, line of sight.
- Volume calculation: cut-fill for construction.
- 3D visualisation and contour generation.
Past Exam Questions
+What is DTM? List structures and applications. (2071 Magh, 6 marks)
Define DTM. Three structures: Grid, TIN, Contour. Applications: watershed, slope for construction, viewshed for towers.
+How is DEM created in Raster GIS? (2072 Ashwin, 5 marks)
Five steps: data collection, data input, interpolation, rasterisation, validation. Name specific interpolation methods.
+Data sources of DEM and its applications. (2075 Bhadra)
Sources: surveys, contours, GPS, LiDAR, SRTM, ASTER, photogrammetry, InSAR. Applications: hydrology, engineering, military, telecom.
+Explain slope and aspect functions with illustration. (2076, 5 marks)
Draw 3×3 grid with sample elevations. Show partial derivatives. Compute slope and aspect. Include aspect compass diagram.