Basics of Airphoto Interpretation

What follows are some general guidelines for interpreting a black and white infrared photo, such as those obtained from the DNR. Most of the information was taken from Lillesand and Kiefer's Remote Sensing and Image Interpretation published by John Wiley & Sons, Inc. (1994).

Keep in mind that airphoto interpretation is a skill that can only be developed with practice, and is not like interpreting an everyday photo that you might place in a photo album. Airphoto interpretation differs from conventional photo interpretation in the following ways:

• Airphotos are taken from an overhead perspective, which we normally don't see.
• Airphotos are taken using wavelengths outside of the visible portion of the spectrum.
• The earth's surface is shown at unfamiliar scales and resolutions. (Lillesand and Kiefer 1994, p. 150).

All three of these facts can make interpreting airphotos difficult for the beginner. There are many aspects to consider when interpreting an airphoto, including:

Shape

The general outline of the object can help you determine what it is. Some objects have a very distinct shape, while others may have a non-distinct shape. The capital building in Madison, for example, has a unique shape that differs from the surrounding buildings.

Size

The size of the object needs to be considered in relation to the scale of the airphoto. For example, perhaps you know that you are looking at a body of water, but is it a small pond, or a large lake?

Pattern

How the are objects arranged on the airphoto? Certain objects have a distinct pattern, such a pine plantation where the trees are in straight rows, rather than randomly scattered.

Tone (or Hue)

The brightness of the object can assist you in identifying it. For example, in black and white infrared photos, conifers appear darker than deciduous trees.

Texture

The visual smoothness or coarseness of the image creates its texture. The smoothness or coarseness is caused by features that are usually too small to discern as individuals (e.g., tree leaves).

Shadows

Shadows can both help and hinder the interpretation process. A shadow allows you to see the profile of an object, potentially making the object more easy to identify. However, objects within shadows may be difficult to identify because they reflect little light.

Site

Consider what types of trees would grow in the geographic location of the photo, or what types of trees would grow in the soil type.

Association

Consider placement of cultural features. Would a farm be located in the middle of an urban area? Would a skyscraper be located in the country? Have you identified an area as a carnival? If so, then perhaps the object that you cannot identify is a Ferris wheel.

General Tips

To help with airphoto interpretation, you might be able to obtain an interpretation key. Keys might have pictures with text labels so that you can identify an object by comparing it to the pictures. Selective keys are also available that allow you to identify an object using the process of elimination. One example of such a key is a dichotomous key that gives you 2 choices at each level, continually narrowing down your options. Because the appearance of objects can change according to geographic location and season, be sure that you are using a key appropriate for your photo.