1. Photo Interpretation

2. Photogrammetry is the art and science of determining the position and shape of objects from photographs. –object interpretation (What is it? Type? Quality? Quantity) and object measurement (Where is it? Form? Size?). The result of photo-interpretation is the classification of objects by various features

3. Principles of Aerial Photo Interpretation Color infrared photo. When interpreting aerial photographs, we're concerned with identifying features on the earth's surface and judging their significance.

4. We usually distinguish photo interpretation from photogrammetry, where we're concerned with taking measurements from photos......and using them to make maps, for example.

5. Photogrammetry equipment. Photogrammetry usually requires trained operators using expensive equipment. But most photo interpretation is done with nothing more complex than a stereoscope which allows you to see a three-dimensional image: as if you were flying over the terrain. A few of the uses of photo interpretation include inventorying forests or agricultural crops...

6. ...or routes for highways, railroads or powerlines.....estimating populations...

7. But no matter what the purpose of interpretation, there are seven basic principles used. They are: shape, shadow, pattern, association, texture, tone or color, and size. After viewing this slidetape you should be able to list these principles, and use them to identify objects on aerial photos.

8. The shape of an object, often gives enough information to identify the object. This is a baseball diamond next to a track and field area. The shape of this object indicates it's a building. Interpretation of its shape might lead us to conclude that it's an apartment or dormitory building.

9. Forest and river: Association can help identify trees, too. Pattern is another principle that helps us identify trees. The spoke-like branching pattern of western hemlock is quite different from the branching pattern of pacific silver fir. The man-made patterns of fields, orchards, and roads contrast with nature's patterns of river and forest.

10. Color, or tone on black and white photos, is another principle of photo interpretation...can easily separate tree species…pine tress are darker ones. Color photos can give much more information than black and white.

11. Bridge (Shadow): Shadow is another principle of photo interpretation. An object's shadow often gives us as much information as the object itself. Mountains: Shadows help define topography on single aerial photos. Buildings and street: They indicate relative height, too. The building on the left casts a much longer shadow and therefore is much higher than the one on the right.

12. Both absolute and relative size are used in photo interpretation. Absolute size means that we know the actual size of an object either by knowing the scale of the photograph or by visiting the object on the ground. Relative size means that an object is larger or smaller than surrounding objects in the photograph. This cone shaped object is 40-60 feet in diameter. As indicated by its surroundings, it's an old wigwam burner. House and apartments: Relative size helps us distinguish apartments from houses even though we don't know the exact dimensions of either.

13. Vehicles on street: These two vehicles are about the same shape. But when compared to the relative size of parked cars, we might conclude that the larger one is a truck, and the smaller a van. When interpreting photos we use many principles together, and we often do it unconsciously.

14. Most photo interpretation skills are learned by studying lots of photos, visiting the areas on the ground, and by acquiring background information in forestry, soils, and geology. Viewing photos in stereo helps immensely! Unfortunately most people can't do it without a stereoscope. You'll have to practice in the lab.