When looking at imagery, the first function should be orientation of the imagery to a map or other reference guide. This will allow the interpreter to review patterns on the imagery which may indicate man-made or influenced objects. Such patterns can be very apparent like the streets of a city, or rows of planted crops. Others may be a simple fence line in an open field. Patterns are clues used to direct the interpreter to the next level of interpretation.
The surroundings of an object or site is another clue to identification. The surroundings may be a city, a city block, or a field. The important part is knowing what your surroundings should be when looking for a particular object or using the surrounding area as a clue to an unknown object. For example, a parking lot of cars could be a car dealership or a vehicle dismantle yard. If it is located in a commerical zone within the city, it's easy to determine that a dealership is present. However, if the lot is located in an industrial zone with other manufacturing businesses, then your process of interpretation should move to eliminate sales of cars.
The relative size of an object will help you with identifying an unknown object. The intrepreter should know the approximate scale of the photography to help determine different sizes. If the scale is unknown, you can measure a known object and compare it to an unknown object. Another helpful tip is the ratio of width v length. Cars generally have a 1:2 ratio, where trucks/vans have a 1:3. These and other types of relatives size ratios allow a qualified interpreter to correctly identify objects from high altitude photogrpahy.
Shapes are often easily recognized from imagery. Objects such as baseball fields, runways, or bridges can be identified with relative ease. Some shapes escape interpretation because the interpreter has limited experience with that object. An example that an inexperienced person often has trouble with are old drive-in movie theaters. A drive-in has the shape of a baseball field, but patterns that look like planted crops, the result can be an inaccurate conclusion based on the interpreters limited experience.
When viewing objects from vertical imagery; shapes, lines, and other key idetification features can be hidden by the sun and/or the reflectancy of the objects surface. However, a qualified interpreter can use the shadows provided by the sun to define the object's shape, identify changes in height, and possibly determine the way a structure is built; e.g. a bridge. This is not always the case, shadows can also be a foe, masking the object of interest.
Key Identification Features
Every object has "key I.D. features". The features that make an object not only unique but identifiable from a similar object. A good example are planes; the length and width are good features to help I.D. the plane, but the interpreter should be able to see further. The number of engines, placement of engines, wing location to fuselage and shape of wings all help I.D. the type of plane you are viewing.
Interpreters should develop specialties and personal I.D. keys not only to aid with the interpretation, but also to make the process the same each time. I.D. keys should outline the key features that will aid interpreter in identifying objects, buildings, vehicles, or sites from different views, scales, and altitudes.