map_projections.hqx  1.7 Mb

The purpose of this HyperCard guide is to describe a few of the common map
projections that can be used to present thematic data.  This description of
map projections enables a compiler of thematic data to select a map
projection that will portray the data effectively, and it allows the map
reader to identify the map projection so that there is a mutual
understanding as to how to obtain information from the thematic map.   In
regional mapping, the choice of a map projection is usually based on
convention or availability rather than on how well the map projection
portrays a particular set of data.  The best projection for portraying
thematic data about people, countries, agriculture, geology, and so on (all
areal phenomena) is a map projection that preserves areal relationships.. 

Many map projections are derived on a developable surface (see glossary,
page 4)  producing a basic grid system of longitude and latitude known as
the earth's graticule.  From the graticule, a map projection can be defined.
 In this manual, four main types of map projections are described. 
Projections onto (1) Planes (Azimuthal), (2) Cones, (3) Cylinders, and (4) a
miscellaneous category.  Each of these main types has some variations, and
this guide will show some of these variations.  Eighteen map projections
commonly used to present thematic data are briefly described and
illustrated.  Also included are several examples of the map projections used
by the U. S. Geological Survey for thematic maps.  This guide consists of
descriptive material only and does not contain a program for creating map
projections.

Requirements for the diskette version of this report are: Apple Computer,
Inc., HyperCard 2.0 software (not supplied) and Apple Macintosh Plus,
Classic, SE, or II.  The date of this Open-File Report is 10/04/1991.
OF91-533-A, paper copy, 92p.  OF91-533 -B, 3.5-in. HD Macintosh diskette.

 To order this report, contact:  U. S. Geological Survey Books and Open-File
Reports Sales, P.O. 25425 Denver, CO. 80225, or call (303) 236-7476.