Base Maps and Aerial Images

One of the first things a geographer, or for that matter any field worker, should try to get his or her hands on well before going into the field is a good map and/or aerial image (Google Earth perhaps) of the study area. In some cases, for example when doing foreign field work in remote locales, such items may not exist, much less be available. In other cases, however, some very good maps and aerial imagess not only exist but are readily available and at little or no cost.

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.

The nature and detail of the data that are to be collected.
The scale or minimal areal unit at which the data are to be mapped.
The types of mapping tecniques to be employed: single feature or multiple feature, total mapping coverage or transect or sample-area mapping, etc.
The system(s) of data classification to be used.
The types and number of control or reference points that a given type of base map will provide.

There are several types of materials that can be used as base maps, but three or commonly used.

Vertical areal images (either aerial photographs or Google Earth images).
Topographic (quadrangle) maps.
Cadastral (plat) maps.

Each of these "standardized" items has its advantages and limitations for the construction of base maps.

Vertical Aerial Photographs

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.

Topographic Maps

Unlike aerial images, "topo" maps are not a complete image of a place at a particular time. Only some, usually major, features are show. However, some nonvisible information such as land survey points and lines, and political boundaries have been added.

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.

Problems of Scale

The principal weakness with vertical aerial images, topographic maps, and orthophotomaps is their scale. They typically cover too large an area to portray "microlevel" typically gathered during field studies.

Cadastral Maps

This type of map contains very little information other than property boundaries. Cadastral maps can cover large portions of the earth, for example surveys of the American West [Cadastral Surveys], or small parcels such as tracts of land in urban areas. Sometimes these maps are plat specific [sample], and sometimes they include aggregates of lots [sample]. The scale of these maps is usually quite good, ranging from 1:600 to 1:10,000. Unfortunately, they do not normally contain any control or reference points that are identiable in the field. For this purpose alone, they do not normally help in making base maps in rural areas. They are nearly perfect, in contrast, for making base maps in urban areas.

Existing Maps of Large Areas and the Need for Maps of Small Areas

Base maps need not be accepted in the condition and format in which they are found. They can be improved. This is typically done by enlarging or "blowing up" vertical areal photographs [sample], or topographic maps [sample], and then tracing relevant information onto a blank sheet of paper [sample]. In the course of doing this, irrelevant and therefore confusing information can be elimated resulting in a less-cluttered or "cleaner" map. The resulting map as good, and as an improvement, as it may be over the base, is still a "rough draft" [sample] that needs to be improved upon further by field work [sample]. In some cases, base maps and their second generation drafts are so rough that their only value is in getting the field worker close to the area to mapped and studied in great detail.