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Projection Information
for the Cartographic Boundary Files
To simplify the use and translation of these cartographic boundary files,
they have been posted here unprojected, where the geographic
coordinates are referenced by latitude and longitude (a.k.a. the
"Geographic" projection).
When viewed at a very large scale (small area mapped - e.g., census block
group to county subdivision), it is difficult to notice that the area is
unprojected. However, when viewed at a small scale (large area mapped -
e.g., viewing a county or larger geographic entity), as these boundary files
were intended to be used, it is very easy to notice that the unprojected
geographic area looks distorted. Therefore, when maps are created from
these boundary files, the Census Bureau strongly recommends that the
user re-project them into a projection suitable for the purpose of the map
and of the geographic area to be displayed.
Because it is impossible to produce a distortion free projection of the
Earth (a 3-D surface) onto a computer monitor or piece of paper (a 2-D
surface), a projection must be selected based on the purpose and
geographic area to be shown on the map. The relative importance of the
map's area, distance, shape, direction, and perspective all must be
considered and a "best fit" projection selected.
For example, for small-scale (large-area) maps of the U.S., the Census
Bureau has traditionally used the Albers Equal-Area Conic projection.
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