Syllabus & Marks Distribution
| Detail | Information |
|---|---|
| Chapter | 8 — Surface Model |
| Teaching Hours | 3 hours |
| Marks Allocation | 6 marks |
| Total Course Marks | 80 marks |
| Weight | 7.5% of total |
Syllabus Topics
- DEM (Digital Elevation Model)
- Slope
- Aspect
- Other raster functions
Importance & Frequently Asked Topics LOW
Marks Distribution by Each Exam
Asked 4 times across past exams. Total marks: 20. Average marks per exam: ~5.
| Exam | Marks |
|---|---|
| 2071 Bhadra | — |
| 2071 Magh | 6 |
| 2072 Ashwin | 5 |
| 2072 Magh | — |
| 2073 Bhadra | — |
| 2073 Magh | — |
| 2074 Bhadra | — |
| 2074 Magh | — |
| 2075 Bhadra | 6 |
| 2075 Magh | — |
| 2076 Bhadra | ~3 |
| 2078 Chaitra | — |
| 2079 Chaitra | — |
| 2080 Chaitra | — |
| 2081 Chaitra | — |
| 2082 Shrawan | — |
Frequently Asked Topics
| Topic | Times Asked | Frequency |
|---|---|---|
| DEM | 3 | |
| Slope | 1 | |
| Aspect | 1 |
Topic-wise Importance & Exam Tips
| Topic | Importance | Key Points / Exam Tips |
|---|---|---|
| DEM (Digital Elevation Model) | High | Define DEM, DTM vs DSM difference. Sources of DEM data. Applications in engineering and hydrology. |
| Slope calculation from DEM | High | Numerical problems frequently asked. Know the 3×3 neighborhood method, formula: slope = arctan(√(dz/dx)² + (dz/dy)²). Practice grid calculations. |
| Aspect calculation from DEM | High | Often combined with slope questions. Formula: aspect = arctan(dz/dy / dz/dx). Convert to compass direction (0°=N, 90°=E, 180°=S, 270°=W). |
| Hillshade and viewshed analysis | Medium | Hillshade for 3D terrain visualization (sun altitude + azimuth). Viewshed for line-of-sight analysis from observation points. |
| DEM creation methods | Medium | From contour interpolation, GPS points, photogrammetry, LiDAR, SRTM/ASTER satellite data. |
| Applications of surface modeling | Low | Road alignment, dam site selection, cut-fill volume estimation, flood modeling. |
1. DTM, Its Structures, and Applications (6 Marks)
PAST EXAM QUESTIONS ON THIS TOPIC:
- What is a DTM? List the various structure of DTM. Explain three applications of DTM [1+2+3] (2071 Magh, Q3b)
- What are the data sources of DEM? Describe about the applications of DEM [6] (2075 Bhadra, Q8)
A. What is a DTM?
A Digital Terrain Model (DTM) is a 3D digital representation of the bare-earth terrain surface. Unlike a Digital Surface Model (DSM) which includes buildings and tree canopies, a DTM mathematically filters out these above-ground features to represent only the natural ground elevation.
B. Structures of DTM:
DTMs can be structured in a GIS database in three primary ways:
- Raster Grid: A continuous matrix of equally sized square pixels, where each pixel stores a single elevation (Z) value.
- TIN (Triangulated Irregular Network): A vector-based model consisting of non-overlapping triangles formed by connecting scattered elevation points. It efficiently represents sharp ridges and valleys.
- Contour Lines (Isolines): Vector lines connecting points of equal elevation, traditionally used in topographic maps.
C. Three Applications of DTM:
- Hydrological Modeling: DTMs are the foundational data required to delineate watershed boundaries, calculate flow accumulation, and model runoff networks in software like the mesoscale Hydrologic Model (mHM).
- Transportation Engineering: Used extensively to extract topographic profiles for highway alignment and to calculate precise cut-and-fill earthwork volumes.
- Hazard Mapping: Used to identify extremely steep terrain profiles, which is critical for mapping landslide susceptibility or avalanche risks on high-altitude routes.
2. Creation of DEMs and Data Sources (5 Marks)
PAST EXAM QUESTIONS ON THIS TOPIC:
- Explain how DEM is created in Raster GIS? [5] (2072 Ashwin, Q2c)
- What are the data sources of DEM? Describe about the applications of DEM [6] (2075 Bhadra, Q8)
A. Data Sources of DEM:
- LiDAR (Light Detection and Ranging): Highly accurate laser pulses fired from aircraft or drones.
- Satellite Imagery (Stereo-photogrammetry): Global datasets like SRTM (Shuttle Radar Topography Mission) or ASTER.
- Ground Surveys: Spot elevations collected using DGPS or Total Stations during field research.
- Digitized Topo Maps: Scanning and digitizing historical paper contour maps.
B. How is a DEM created in Raster GIS?
A raster DEM is rarely captured perfectly as a grid in the field. It is created through Spatial Interpolation.
- First, scattered elevation data (like a GPS track from a Mardi Himal or Tsho Rolpa trek) is imported into the GIS as a point shapefile.
- An interpolation algorithm (such as Inverse Distance Weighted, Spline, or Kriging) is applied to these discrete points.
- The software calculates the estimated elevation for all the "empty" spaces between the surveyed points.
- The output is a continuous raster grid where every single pixel has an assigned elevation value, effectively creating a smooth digital surface.
3. Slope and Aspect Functions (5 Marks)
PAST EXAM QUESTIONS ON THIS TOPIC:
- Explain about slope function with illustration. [Part of 5] (2076 Bhadra, Q7b) 2. (Also Covered under 7.2 in questions like 2071 Bhadra, Q6c, 2073 Bhadra, Q7d, 2078 Chaitra, Q7a, and 2079 Chaitra, Q7a.)
In the exam, when asked to explain these with illustrations, define the concept and draw a simple grid showing a central pixel surrounded by its 8 neighbors.
Slope Components (Lectures p.30)
A slope is formally described by a plane tangent to a point on the surface. It has two components:
- Gradient: The maximum rate of change of elevation of the plane (steepness). Expressed in degrees (0°–90°) or percent rise.
- Aspect: The compass direction of the maximum gradient (direction the slope faces), with respect to an arbitrary zero (usually North).
A. Slope Function:
- Concept: Slope calculates the maximum rate of change in elevation between a specific cell and its eight immediate neighbors. It identifies the steepness of the terrain.
- Output: The output raster displays values either in Degrees (0° for flat, 90° for vertical) or Percent Rise (elevation difference divided by horizontal distance, multiplied by 100).
B. Aspect Function:
- Concept: Aspect calculates the downslope direction of the maximum rate of change in value from each cell to its neighbors. In simple terms, it identifies which compass direction the slope is facing.
- Output: The output raster displays values in degrees from 0° to 360°, measured clockwise from North. (e.g., 90° is an East-facing slope, 180° is a South-facing slope). Flat areas are usually assigned a value of -1.
Interactive Slope & Aspect Calculator
Enter elevation values into the 3×3 grid below and set the cell size. The calculator will compute the slope (in degrees and percent) and aspect (compass direction) using Horn’s method — the same algorithm used by ArcGIS and QGIS.
⛰️ Enter 3×3 Neighbourhood Elevations
Curvature Functions (Lectures pp.30–31)
Curvature describes the rate of change of slope, providing information about the shape of the terrain surface.
Profile Curvature (rate of change of gradient along the slope direction):
- Convex: Gradient increases downslope (e.g., top of a ridge — water flows faster)
- Concave: Gradient decreases downslope (e.g., valley bottom — water slows and accumulates)
- Planar: Constant gradient (uniform slope)
Plan Curvature (rate of change of aspect perpendicular to slope):
- Convex: Aspect lines diverge downslope (e.g., nose of a ridge — water spreads)
- Concave: Aspect lines converge downslope (e.g., valley — water concentrates)
- Planar: Constant aspect (parallel flow)
Profile curvature affects flow acceleration/deceleration; plan curvature affects flow convergence/divergence. Together they determine erosion/deposition patterns.
Chapter Summary
This chapter covers surface modeling in GIS, focusing on Digital Elevation Models (DEMs) and derived products. A DEM is a raster representation of terrain elevation, created from contour maps, GPS surveys, photogrammetry, LiDAR, or satellite imagery. Two main types exist: Digital Terrain Model (DTM, bare earth) and Digital Surface Model (DSM, includes buildings/trees). Slope analysis calculates the steepness of terrain at each cell (in degrees or percent), critical for construction feasibility, erosion modeling, and agricultural suitability. Aspect determines the compass direction a slope faces (N, NE, E, etc.), important for solar radiation analysis, snow melt modeling, and vegetation studies. Other raster surface functions include hillshade (3D visualization), viewshed analysis (visibility from a point), contour generation from DEM, and curvature analysis. These tools are fundamental for engineering design, environmental impact assessment, hydrological modeling, and terrain visualization.