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

Black, white and grey tints can be seen on aerial pictures. The tint depends on the amount of light that is reflected by the photographed object. Light colours appear as light grey and dark colours as shades of darker grey.

The tint of water varies between white and black. Clear water appears darker than muddy water, because muddy water reflects more sunlight. Humid soil surfaces also appear darker than sandy soil.

Vegetation can be divided in two types according to their appearance on aerial photos:

• Natural vegetation like forests and grasslands is easy to identify due to their diverse pattern. Trees usually have dark tints.
• Planted trees and agricultural crops are also easily identified by the straight lines in which they are planted. The tint of a cultivated land depends on the height of the plants. For example, maize-fields with tall plants will appear darker than fields with small plants, because the tall plants absorb more sunlight than the small ones. Cultivated land usually has a patchwork quilt appearance of darker and lighter tints.

Gradients facing the sun reflect more light and therefore appear lighter than those with a southern gradient.

• Texture of objects

It refers to the general impression that objects make and can be described as SMOOTH, FINE, COURSE, ROUGH or DOTTED. Photos of cultivated land and water show a fine, smooth appearance. Shrubs and forest vegetation has a dotted appearance, whereas uninhabited mountainous areas have a rough appearance.

• Shadows

Aerial photos are usually taken between 10:00 and 14:00 when shadows are at their shortest. Shadows are of great help in identifying objects. Shadows that fall outwards indicate a higher area such as a hillside, mountain, or high building, whereas shadows falling inward indicate lower lying areas such as riverbeds and excavations.

• Patterns

When analysing aerial photos, much information can be obtained from patterns. Certain patterns are characteristic of specific objects and phenomena. Bushveld vegetation has a diffused pattern, but citrus orchards or vineyards have linear patterns.

The arrangement of buildings and streets show a typical urban settlement pattern. The older, central business centre has a grid pattern, and the newer residential areas appear to have rather concentric patterns.

We therefore have two main pattern types that can be identified on an aerial photo:

• the physical landscape, and
• the cultural landscape.
• Tips for reading, analysing and interpreting vertical aerial photos and orthophoto maps

• Mark the general patterns that clearly appear on the photo. It is usually done by placing trace paper on the photo and then the profiles of the patterns are traced. The different characteristics are named.
• Carefully study the shape, size, shadow, tint, and texture of the related characteristics to identify objects.
• Ask certain kinds of questions that can assist in identifying objects:

Relief characteristics

• Are there mountains, hills and valleys?
• How steep are the slopes?
• Are there any rivers?
• How wide are the river valleys or plains?
• In which direction does the river flow?
• Is the river perennial or non-perennial?

Vegetation

• Which types of vegetation exist?
• Which areas are covered with natural vegetation?

Transport systems

• Which types of transport occur?
• Which types of roads criss-cross the photo?
• Are the railway lines single or double lines?

Farming

• Do they farm with livestock or is it crop farming? Or both?
• What is the size of the cultivated land?
• What kind of crops are grown?

Settlements

• What types of buildings can be seen in the urban areas?
• What is the layout of farmsteads and outbuildings?
• Are there any industries? If so, which types?

Activity 1:

To identify and interpret information on a topographic and orthophoto map of the known environment

[lo 1.4]

• Your teacher will hand you a 1:50 000 topographic map and an orthophoto map of your area. Try to apply all the knowledge that you gained from the previous pages by answering your teacher’s questions.

Activity 2:

To identify a problem on an orthophoto map and to compare it with the real situation

[lo 1.4, 1.5]

• First work individually and then in your groups. Try to identify a problem that appears on the orthophoto map of your environment. If no problem exists, try to identify a potential problem, such as a flood, landslide, fires, erosion, the need for reservoirs, poor road or street planning, etc. Suggest a solution to the problem.

Activity 3:

To investigate an identified problem during a fieldwork excursion and to write a report on it

[lo 1.6]

• If possible your teacher will organise a fieldwork excursion to visit the site of such an identified problem. Collect data by making use of maps, drawings, photos, etc. After the excursion each group must write a report on their findings and also suggest a solution for the problem. The report must be written in the space below.

FIELD OBSERVATION

Position of site :

P roblem:

Aids used in investigation :

Findings :

Suggested solution :

N ames of group members:

Assessment