Photographs and satellite images as evidence

Overview

Difference between a vertical and an oblique aerial photograph, satellite image and ground truthing.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is the difference between a vertical and an oblique aerial photograph, and what is each good for?

   Model answer

   A vertical photograph is taken with the camera pointing straight down, so it shows the ground in plan and features keep their true relative positions, which makes it close to a map and good for measuring areas, comparing the extent of land uses and detecting change between dates. An oblique photograph is taken at an angle, so it shows the sides of buildings and the shape of relief and gives a more natural, recognisable view; it is better for showing landform shape and the character of a place, but the scale changes across the image so distances cannot be measured reliably from it.

2. 2

   What can a ground-level photograph contribute to an investigation, and what are its limits?

   Model answer

   A ground-level photograph records the detail of a site as it actually appeared - the condition of buildings, the size of bedload, the state of a sea wall - and provides evidence that can be re-examined and annotated after the fieldwork is over. Its limits are that it captures only what was in front of the lens at one instant, gives no scale unless an object of known size is included, carries no grid reference of its own, and can be framed, deliberately or not, so as to support a particular impression.

3. 3

   How should a photograph be annotated so that it counts as geographical evidence?

   Model answer

   Give it a title saying what it shows, where it was taken from, in which direction the camera was facing, and the date and time. Add lines pointing to features with annotations that explain rather than merely name - not just sea wall, but sea wall showing undercutting at its base, indicating continued wave attack. Include an object of known size for scale, or state the scale, and where possible quote a grid reference for the camera position so that the view can be reoccupied later.

4. 4

   Why should an object of known size be included when photographing a landform or a sediment sample?

   Model answer

   Because a photograph carries no inherent scale: a boulder and a pebble photographed close up look identical. Placing a ruler, a coin, a ranging pole or a person in the frame gives the reader a reference of known dimensions, so the size of the feature can be judged and, in a series of photographs, sites can be compared with one another. Without it the photograph shows shape and texture but says nothing about magnitude.

5. 5

   How are photographs of the same place from different dates used in geography, and what must you be careful about?

   Model answer

   They are used to measure change over time - cliff retreat, the growth of a settlement, the retreat of a glacier snout, the loss of woodland - by comparing the position of features against fixed reference points visible in both images. The cautions are that the two photographs must be taken from as near the same position and angle as possible, or apparent change may be an artefact of the viewpoint; that season, time of day, tide state and weather should be comparable; and that the dates must be known, since without them no rate of change can be calculated.

6. 6

   What is a satellite image, and how does it differ from an aerial photograph?

   Model answer

   A satellite image is produced by sensors on an orbiting satellite recording radiation reflected or emitted from the Earth's surface, and it is built from digital data rather than being a conventional photograph. Compared with an aerial photograph it covers a much larger area at one time, is repeated regularly over the same ground so that change can be tracked systematically, and can record wavelengths the eye cannot see. Aerial photographs generally offer finer detail over a small area and can be flown on demand.

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