10. Topographic Surveying
Topographic Surveying
This domain tests terrain representation, spot heights, contours, breaklines and accuracy standards. Expect questions on interpolation, feature coding and the suitability of a survey for design.
Core rules
- Contours connect points of equal elevation; they never cross except at overhangs, which a standard contour map cannot represent.
- Contour interval is chosen from map scale, relief and purpose; closer contours imply steeper ground.
- Spot heights and breaklines improve terrain models; a DEM without breaklines can smooth ridges and channels.
- Feature codes and attributes make the survey usable in GIS and design.
- Accuracy should be stated in plan and height, with a confidence level and check method.
Worked example
Two spot heights are 100.0 m and 102.0 m, 20 m apart. A point 5 m from the 100.0 m spot lies at elevation 100.0 + (5/20)(102.0 - 100.0) = 100.0 + 0.25 x 2.0 = 100.5 m. This linear interpolation is valid only between points on a uniform slope; a breakline would require separate interpolation.
Traps
Interpolating across a breakline; treating contours as boundaries; using too coarse a contour interval for flat terrain; omitting drainage lines; failing to state the height datum.