4.1 Reading, analysing and interpreting vertical aerial photos and  (Page 12/2)

Social sciences: geography

Grade 7

Map work

Module 11

Reading, analysing and interpreting vertical aerial photos and orthophoto maps

• Before it can be expected of you to interpret vertical aerial photos and orthophoto’s any further, you will need more information.

A vertical aerial photo of a specific area is taken vertically from above from an aeroplane.

Advantages

• Most objects are visible.
• The layout of roads and railway tracks are clearly visible.
• High trees, mountains and buildings can easily be distinguished by their shadows.

Disadvantages

• The photos are as seen from above and the view is therefore odd to man. It is an unusual visual angle and one needs a lot of practice, knowledge and experience to read it.
• It is in black and white and can be confusing.

• Vertical aerial photos are used as the basis when topographic maps are drawn. Information from the aerial photos is transferred by cartographers (people trained to draw accurate maps) on paper by using the international symbol key and colours, which you are familiar with.
• Today orthophoto maps are very popular in map studies. The advantages of the 1:50 000 topographic map and vertical aerial photo are combined to draw an orthophoto map. An orthophoto map is therefore a vertical aerial photo on a lower level of 1:10 000, with contour lines added.
• Contour lines of 5 metres intervals are used in order to identify detail. Even names of places, railway lines and roads are shown on the photo. The objects are therefore easily identifiable. Maps and photos therefore have there own characteristics, advantages and disadvantages. A clear presentation of a landscape is actually only possible when using a topographic map and photo together.
• A topographic map is the presentation of the surface of an area on scale, using a symbol key. This provides a simplified picture on which only certain characteristics are shown. A topographic map is therefore not always accurate and can even be outdated.
• A vertical aerial photo and orthophoto map are very useful tools when used together with a topographic map. Only then accurate map and photo readings can be made.

1. How do you study or interpret a vertical aerial photo or orthophoto maps?

• Size of objects

If the scale of the photo is calculated, then the length of objects can be determined and the distance and square area can be calculated. This method is used to determine the area of an urban settlement with a grid pattern.

• Shapes of objects

• Man made phenomena are generally evenly distributed. Here you can consider the shape of buildings, ploughed lands, roads, railway lines and dam walls. Natural phenomena and objects are evenly distributed, like mountains, rivers, forests, and grasslands.
• Roads are broad grey lines on a photo. National roads are usually straight with no sharp curves. Secondary roads appear narrower and have many curves. Railway lines appear as small dark grey lines with gradual curves. Roads often have perpendicular connections with other roads, whereas railway lines gradually connect up with other lines.
• Rivers are wide and dark grey lines with curves and bends where they meander through different areas, and other side streams connect up with the main stream. The V-shape of a side stream connection will ALWAYS point down stream and can be used to determine the direction of flow.
• The tint and shading of objects