Topographic Mapping Questions and Answers

Answer: C

Correct answer: a digital terrain model

The terrain model is the bare earth, with buildings, trees and other surface features removed. The surface model is what the sensor first sees, tops of trees and roofs included. The distinction is critical in use: a flood model or an earthworks calculation needs the terrain model, while a line-of-sight or solar study needs the surface model.

Answer: D

With no vegetation or buildings to intervene, the first return and the ground are the same surface. The two diverge exactly where something stands above the ground, which is why forests and towns show the largest differences.

Answer: A

Timing the pulse gives the range, and combining that with the position and attitude of the aircraft gives a coordinate for each return. Because some pulses reach the ground through gaps in a canopy, it can map terrain beneath forest.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the surface model includes vegetation and buildings, and a LiDAR pulse spreading over a footprint can return from a canopy and again from the ground beneath, which is exactly how the bare earth is extracted. Statement 3 is impossible - contours derived from a model inherit its errors and add interpolation of their own, so they can never be better than the data they came from.

Answer: C

Topology alone cannot say whether one feature passes over the other or through it, so the crossing carries its own symbol. Reading a crossing without checking that symbol is how a route is planned through an unfordable river.

Answer: D

The spot height is printed as a number and needs nothing further. Gradient, area and intervisibility all require measurement and working before an answer can be given, and the last of them needs a profile drawn along the line of sight.

Answer: A

The rise is 120 metres over 1,200 metres of ground, which reduces to 1 in 10. Converting the distance to metres before dividing keeps the two quantities in the same unit, and forgetting to do so gives a gradient a hundredfold too gentle.

Answer: B

Correct answer: 5 per cent, about 3 degrees

\[ \frac{1}{20} = 0.05 = 5\%, \qquad \arctan(0.05) = 2.86^{\circ}. \] Per cent and degrees are easily confused because they are close for gentle slopes - a 5 per cent grade is under 3 degrees - but they diverge sharply as the slope steepens, since 100 per cent is 45 degrees and no percentage corresponds to vertical.

Answer: C

A gradient of 100 per cent means the ground rises as far as it advances, which is 45 degrees. A vertical face has an infinite percentage gradient, since it advances no distance at all, which is why cliffs are shown by symbol rather than by contour.

Answer: D

The term describes the ground itself, not how it has been drawn. Gently rolling country with little height range is subdued relief, however boldly the contours happen to be printed. The word describes the landscape rather than the cartography.