Topographic Mapping Questions and Answers

Answer: A

Drawn to true scale, most real terrain flattens into a barely visible ripple, so some exaggeration is necessary for the profile to be read at all. What matters is that the factor is stated, so slopes are not taken as steeper than they are.

Answer: B

Correct answer: drawing a profile along the line joining them

Intervisibility depends on whether any intervening ground rises above the line of sight, and only a profile answers that. The heights along the joining line are read from the contours, plotted against distance, and the straight line between the two end points drawn on the section - if the ground breaks that line anywhere, the view is blocked. Counting contours or comparing spot heights cannot detect an intervening ridge.

Answer: C

One carefully measured baseline plus a network of observed angles fixes a whole framework of stations, because a triangle is determined by a side and two angles. Measuring every distance directly would have been impossible over rough country.

Answer: D

Angles can only be observed between stations that are mutually visible, and high ground gives the sightlines the network needs over long distances. Accessibility is in fact usually worse rather than better, and the ground no more stable than anywhere else.

Answer: A

A level and a graduated staff give the height difference between successive points, and a chain of such differences carries a height from a benchmark to wherever it is needed. Angles and distances are measured by other instruments entirely.

Answer: B

Correct answer: overlapping photographs of the same area

Photogrammetry works from parallax: the same ground point appears in a different relative position on two photographs taken from different stations, and that displacement gives its height. A single vertical photograph carries no height information at all, which is why the flight lines are planned with about sixty per cent forward overlap.

Answer: C

Height requires two overlapping views, because it is derived from the displacement of a point between them. One photograph records position on a plane but says nothing about how far the ground stood above or below that plane.

Answer: D

Photograph scale is the focal length divided by the flying height, and 0.15 metres over 3,000 metres gives one twenty-thousandth. Both figures must be converted to the same unit before the division, which is where the millimetre focal length catches people out.

Answer: A

Removing the displacement caused by terrain and camera tilt gives an image with a single consistent scale, so distances can be measured on it as on a map. An uncorrected photograph has a scale that varies across the frame.

Answer: B

Both are true and the reason gives the cause: a hilltop is nearer the camera than the valley beside it, so it appears at a larger scale and is displaced outward from the centre of the frame. Correcting both effects is exactly what produces an orthophoto.