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发表时间 : 2008-5-19 11:17:23 | 浏览 : 1816    评论 : 0
Map projections can represent the earth as a flat surface, or can closely represent the
curvature of the earth. Ellipsoids are a set of control points that the map projection
method uses to define the 3-dimensional shape of the earth. Because the Earth is not a
perfectly round ball, different projection methods and ellipsoids have been developed
to compensate for the Earth’s irregularities at certain places. Terrain near the equator
needs a different projection than terrain near either pole.

The map projection and ellipsoid method you choose will depend on your terrain’s
location in the world, the size of the area being covered, and its shape. The goal of
choosing a map projection and ellipsoid setting is to introduce the least amount of
distortion into your generated terrain. Creator provides five map projections and four
ellipsoids to from which to choose.

? Flat Earth treats latitude and longitude coordinates as x and y coordinates. This results
in a rectangular database. Flat Earth projections are generally used for databases
which run more east-to-west and are not too close to either pole.
flat earth.jpg

? Universal Transverse Mercator (UTM) works well for databases that run more
north-to-south, or along polar routes. UTM divides the earth into six degree longitudinal
zones with a central meridian. This map projection is most accurate along the meridian, with distortion increasing as you move farther east or west of the meridian.
UTM.jpg
? Geocentric is a round earth projection where z radiates from the center of the Earth
through the North Pole. This projection is used for large area databases, or for simulating
the earth’s curvature.
Geocentric.jpg
? Lambert Conic Conformal uses two standard parallels as boundaries. Between the
boundaries, the latitude lines are perpendicular to the longitude lines. This projection
is most accurate for high latitude flights over areas located in the middle latitudes between 84 degrees North and 80 degrees South. Distortion increases closer
to the poles.
LCC.jpg
? Trapezoidal is an azimuthal projection that is most accurate at its central point, with
distortion increasing as the distance increases from this point. This projection
method is best for small (one degree by one degree or less) databases.
Trapezoidal.jpg
Ellipsoids are mathematical formulas that define the shape of the Earth, which bulges
in the middle and is flattened at the poles. Cartographic data uses several different
ellipsoids. For example, the ellipsoids used primarily for mapping data in the United
States are WGS 1972 and WGS 1984. Ellipsoids are used in conjunction with projections
to convert real world locations to places on a map. All maps specify the earth ellipsoid
best suited for the area.

Note: When you apply a geospecific texture to your terrain, you must make
sure it has the same projection and ellipsoid as your terrain or it may not
map correctly.

from:\MultiGen-Paradigm\docs\creator\Creator_Terrain_Guide.pdf

[ 本帖最后由 pillwkai26 于 2008-6-14 08:10 PM 编辑 ]

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