1 Map Projections
- Purpose: Transform 3D Earth onto 2D surface. All projections introduce distortion.
- Types by surface: Cylindrical (Mercator), Conical (Lambert), Azimuthal/Planar (Stereographic).
- Properties preserved: Conformal (shape), Equal-area (area), Equidistant (distance). No projection preserves all.
- UTM (Universal Transverse Mercator): 60 zones, each 6 degrees wide, central meridian scale 0.9996.
2 Nepal Datum and MUTM
- Nepal datum: Nagarkot as origin, Everest 1830 ellipsoid.
- MUTM (Modified UTM): Zone width reduced to 3 degrees (84–87°E and 87–90°E). Scale factor = 0.9999. More accurate for Nepal's narrow east-west extent.
3 GCS vs PCS
| Aspect | GCS | PCS |
|---|---|---|
| Units | Degrees (lat/long) | Metres/feet |
| Surface | 3D (sphere/ellipsoid) | 2D flat plane |
| Distortion | None (angular reference) | Some (depends on projection) |
| Use | Global referencing | Mapping, measurement |
4 RMS Error
Used in georeferencing to measure accuracy of transformation. Lower RMS = better fit. Acceptable threshold depends on scale (typically <1 pixel).
5 Accuracy vs Precision
- Accuracy: Closeness to true value. High accuracy = low systematic error.
- Precision: Repeatability/consistency. High precision = low random error.
- A measurement can be precise but inaccurate (consistently wrong), or accurate but imprecise (close on average but scattered).
6 Scale Types
- Verbal: "1 cm represents 1 km."
- Representative Fraction (RF): 1:50,000.
- Graphical (bar scale): Visual bar. Remains valid when map is resized.
- Large scale = small area, more detail (1:5,000). Small scale = large area, less detail (1:1,000,000).
Cylindrical (Mercator), Conical (Lambert), Azimuthal. Explain properties: conformal, equal-area, equidistant. Draw projection surfaces.
MUTM: 3-degree zones for Nepal (vs 6-degree UTM). Scale factor 0.9999 (vs 0.9996). More accurate for Nepal's narrow extent. Two zones: 84–87 E and 87–90 E.
GCS: angular (degrees), 3D reference. PCS: linear (metres), flat plane. GCS = location reference; PCS = measurement system. Use comparison table.
RMS = sqrt(sum of squared residuals / n). Show calculation step by step. Compare with acceptable threshold.
Draw target diagrams: (a) accurate + precise, (b) precise + inaccurate, (c) accurate + imprecise, (d) neither. Explain systematic vs random error.
Verbal, RF, graphical. Graphical best because it resizes with map. Large scale = large detail, small area.