There are several reasons for obtaining such items, but two certainly stand out as being all-important. First, maps and aerial images provide geographers a "feel" for the area, even if they have never before been in the area under consideration. Second, they can be used as "base maps." Geographers by our very nature map things. That is, we record facts, draw boundaries, and make notes in a spatially graphic form. If a picture is worth a thousand words to most people, a map is worth ten thousand words to a geographer.
To select appropriate maps or aerial imagess to use as base maps, certain things must be considered.
Aerial images can be classified as being either oblique [sample], taken from an airplane at some angle, such as out of the side window, or vertical [sample], taken from a camera mounted on the bottom of the plane and pointed straight down. Vertical images have an advantage over oblique ones in that features on the landscape cannot be obscured by other features. They also benefit from not being as distorted. Indeed, in their center (the center point) distortion is nonexistent. Distortion does, however, increase toward the margins of the photos.
Vertical aerial images have been described as an actual picture of reality, because a large number and variety of control or orientation points are provided, for example landforms (e.g., hills, stream junctures), vegetation (e.g., tree lines), and cultural features (e.g., roads, buildings). They are particularly helpful if they are stereoscopic photographs.
Vertical aerial photographs are normally taken in parallel overlapping strips. Photos in each strip typically overlap each other 60%. The strips of photos normally overlap each other 30%. By virtue of the overlap, these photographs provide a three-dimensional perspective when viewed with a tool called a stereoscope. They are available for many parts of the United States from several government agencies and private commercial concerns.
A good source of information about obtaining aerial photographs is the United States Geological Survey, or USGS. This center maintains an informational data base of aerial photographic coverage of the U.S. and it's territories. It includes information from federal, state, and local government agencies.
Imagery from high altitude aircraft and space satellites can be obtained from the same source.
A wide selection of aerial imagess are sold by several private survey companies. These are usually taken at low altitude and at scales larger than 1:25,000. They are often quite recent but they suffer from being project specific. That is they were taken under contract for private individuals for very specific purposes. Names and addresses of leading photogrammetric concerns from all over the U.S. can be found by means of a Google search or in current issues of journals such as Photogrammetrical Engineering and Remote Sensing and Professional Surveyor.
One of the advantages of vertical aerial images is that they come in a variety of format sizes and scales. Their greatest disadvantage is that they are often not very recent.
Most "developed" and many of the "developing" countries publish topographical maps which can be purchased. For example, the Instituto Nacional de Estadistica Geografia e Informatica or INEGI , makes and markets such maps in Mexico [sample]. The principal source of these maps in the United States is again the U.S. Geological Survey,[sample] or USGS Copies, however, can be purchased in map stores everywhere, or found in various libraries.
Individual "quadrangle" map sheets do not have legends, per se, but a fold-out legend appropriate to all maps is available. There is nevertheless much information provided at the bottom of each map--name, map series, location in longitude and latitude, location in the state, date of edition, date of revision, compass declination, contour interval, and scale. Most quadrangle maps span 7 1/2 minutes of latitude at the scale of 1:24,000 or 2,000 feet to the inch. Some, however, especially in the more remote parts of the country span 15 minutes of latitude at the scale of 1:62,500.
Topographical maps can be very helpful in constructing base maps as landforms (assuming there is ample terrain or relief), cultural features, and locations are relatively easy to identify.
A special type of topo map/aerial image is the orthophotomap. These are usually color-enhanced aerial images on top of which topographic contour lines and other information (e.g., boundaries, names) have been added. In some respects, orthophotomaps are ideal for constructing base maps--they contain the best of both maps and photos, and they contain few of the limitations.
Created by William E. Doolittle. Revised 28 August 2017