Geography Reference
In-Depth Information
the vertical datum are related to this zero elevation. Local datums, in fact,
are used for areas up to the size of continents—for example, the North Amer-
ican Datum of 1927, which made a location on Meades Ranch in Kansas the
starting point of the triangulation that measured the earth's undulations and
put them into relationship with the Clarke 1866 ellipsoid. Geocentric
datums—for example, the World Geodetic System Datum of 1984—take the
entire earth into consideration and lack an origin point; they don't have a
defined datum point, but are calculated from a network of geodetic observa-
tions. The difference between local datums can be several hundred meters—
for instance, between NAD 1927 and NAD 1983 in some areas of the United
States. Conversions of measurements between the two systems can become
quite complex. Fortunately, programs are widely available to transform
between popular datums—for example, between NAD 1927 and NAD 1983—
for most areas. A few important datums in North America and globally are
listed in Table 4.2.
Types of Projection and Their Characteristics
Theoretically, the number of possible projections is unlimited; practically,
the number is limited only by the creativity of mathematicians and geode-
sists and the needs of organizations to coordinate their creation, mainte-
nance, and use of geographic information. To start, you should familiarize
yourself with the three basic developable surfaces, also called “projection
families,” used to create map projections.
Developable surfaces
, which are an
actual or imaginary drawing of the projection, were used to help cartogra-
phers visualize the projection process. They are no longer used to project
maps, but they are helpful in understanding projections.
Developable surfaces can be drawn, but many projections are created
without them. Projections created with developable surfaces can be demon-
strated using a light hung in the middle of a transparent globe or by shining
a flashlight through a portion of a globe onto the developable surface. For
example, a two-liter plastic bottle, cut off at both ends and marked with a
constant interval of vertical lines, with a light bulb hung in the middle to pro-
ject the lines on a wall, will show how a cylindrical projection projects lati-
TABLE 4.2. Selected Datums
Horizontal
Datum Name
Ellipsoid
Local/geocentric
Where used
NAD 1927
Clarke 1866
Local
North America
NAD 1983
GRS 1980
Geocentric
North America
WGS 1984
GRS 1980 with additional
measurements
Geocentric
World
New Zealand Geodetic
Datum (NZGD) 2000
GRS 1980
Geocentric
New Zealand
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