Geography Reference
In-Depth Information
Units, Projection, and Scale
In Kentucky, eight different coordinate systems for spatial information are in use by state agencies alone.
No doubt, other states share this problem. How should distances on a map be presented—metric or
otherwise? What shows up on the product should take into account the nature of the audience. If you
can, avoid cluttering products with multiple coordinate systems. If the user doesn't mind the maps
looking weird, you might make the map using latitude and longitude coordinates. Usually though, users
like maps whose scales are consistent in all directions.
Thoughts on Resolution and Scale
Resolution is, basically, the smallest length in ground units at which the identity or characteristics
of something can be resolved by looking at the product. For example, what is the diameter of the
smallest object that can be seen in an aerial photograph? Clearly, the answer depends on several
factors besides the diameter of the object. It involves the reflectance of the object compared with the
surrounding ground, the quality of the vision of the person looking at the photo, and the scale of
the photo. An 8
10 photo of Arizona might allow only an object the size of Phoenix to be visible.
Through enlargement of the same photo, however, it might be possible to increase the resolution
so that an object 100 feet across could be identified. If further enlargement—no matter how
extensive—does not allow identification of objects with diameters smaller than 100 feet, then the
resolution of the photo is said to be 100 feet. Thus, it is obvious that resolution and scale are closely
linked—the larger the scale, the greater the resolution—up to a certain limit. Beyond that limit, no
further information can be obtained from the product simply by enlarging the scale. In previous
exercises you have had the experience, looking at raster images, of watching the information
disappear as you zoomed in.
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If a specific ground area is to be covered, designers must then weigh the relative factors of size, scale, and
resolution to determine the most appropriate method of producing an information product. Too often,
these determinations are made without sufficient thought, or are influenced by habit and conference
room table size. The issue of appropriate scale must be considered early in the process because some GIS
products utilize overlays and base maps of other materials to make them meaningful. Obviously, a nearly
exact physical match must occur. Further, the size, scale, and resolution of both base maps and overlaid
products must be appropriate. It is worth mentioning here that reproductions of paper map output, from
any source, are not always the same size as the original—and hence do not have the same scale as the
original. Unfortunately, statements regarding scale printed on the map, such as 1 inch = 1000 feet, are
reproduced along with everything else, creating a built-in lie.
Making Sure There Is a Base Map
Your GIS will include, probably, a multitude of layers. The existence of a base map—a cartographically or
photographically produced product of, usually, great accuracy, high precision, and great detail to which
all other products of the GIS can be referenced—is essential. To attempt to build GIS products without
first developing (or otherwise obtaining) a geographic base map is folly. But be aware: To use primarily
GPS to create GIS data sets intended to serve the base map function is a monstrous undertaking.
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