Air Photo Interpretation
Objectives

To understand and practice the basic guidelines for interpreting an air photo.

Let's look at some basic guidelines to assist you in interpreting an air photograph.

FEATURE IDENTIFICATION

Shape

Cultural Features

- some features such as airport runways have a distinctive shape which make them easy to identify
- roads are usually regular in shape which tends to make them easy to identify
- railways are also of uniform width and have long, straight stretches and more gentle curves than roads
- bridges should be looked for wherever roads or railways cross a body of water; they are generally of uniform width, and will often cast a shadow which reveals their structure
- buildings have a variety of regular shapes that vary considerably in size depending upon the use of the structure

Natural Features

- generally have more irregular shapes
- quite easy to tell the difference, for example, between an orchard (rows of trees with regular spacing) and a forest area with irregular tree growth

Pattern

- formed by the repetition of a feature
eg farm fields, rows of trees in an orchard, oil storage tanks at a refinery

Shadow

- high objects such as towers, chimneys, bridges, or high buildings cast shadows which help you to identify the object
- shadows can help one to tell the time of day that the photo was taken

Tone

Cultural Features

- roads and tracks will usually have a light grey tone as they are fairly reflective of light
- railways: medium grey
- bridges: vary according to construction material

Natural Features

Water
- generally appears dark grey or black since it absorbs much of the light
- if the water body is fairly shallow with a light-coloured or weedy bottom, or if it carries a high silt load, it will reflect more light and appear lighter in colour
- rough water has many reflective surfaces so it may range from dark to almost white

Vegetation
- most will vary from shades of grey to black
- since deciduous, broad-leafed trees reflect more light than coniferous trees, they appear in lighter shades

Soil
- bare ground usually has a light colour but the wetter it is, the darker the tone

Crops
- cultivated fields and fallow fields are almost white in tone
- well-developed crops will be dark grey but tones vary according to the season, type of crop, and stage of growth

General Guide
Spring: sharp, distinct field patterns due to differences in tillage and crop development
Summer: dark tones for maturing crops and heavily leafed trees
Fall: distinct field patterns because of the various stages of crop development and harvesting
Winter: when there is little or no snow cover, tones are generally drab and dull

  INTERPRETATION

Location

- refer to a topographic map to locate the area shown in the air photo. Often compass directions are not given on an air photo; therefore if you are required to find a particular direction on an air photo, you will need to utilize an accompanying topographic map.

Scale

- see next lesson

Physical Features

Broad Patterns
- look at the whole photograph to get a general picture of the area

Drainage
- describe the distribution of lakes, rivers, and streams, and general drainage conditions

Natural Vegetation
- determine the approximate percentage of the area which is covered by natural vegetation

Soils
- if field patterns are obvious, a determination of the crops grown could indicate particular
soil types

Climate
- use information about drainage, vegetation, and soils to makes inferences about the climate
(see “Topographic Map Interpretation” notes)

Cultural Features

Settlements
- how many settlements are there?
- where are they located?
- how are they distributed (i.e. clustered, scattered, regularly or irregularly spaced)


  Transportation and Communication

Roads
- what variety of roads are found (i.e. dual highways, single highways, paved, gravel, etc.)?
- are the routes direct or twisting?

Airport
- where is it located in relation to any visible settlements?
- how long are the runways?

Other
- are there hydroelectric transmission lines, telegraph or telephone lines?
- are any communication towers visible?
- how do these features relate to physical and cultural patterns?

Economic Activities

- note any evidence of fishing, forestry, mining or quarrying, manufacturing, recreation or tourism, or agriculture and its distribution
- establish any relationships between economic activity and the physical and cultural features of the area



Assignment Work

1) Download, print out, and complete the Air Photo Exercise.

In order to complete the exercise, you will need to view the following air photos and topographic maps:

New Glasgow a.p.New Glasgow t.m.

St. John's a.p. St. John's t.m.

Beloeil a.p. Beloeil t.m.

2) Check over your answers using the Key.

 

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