Geography Reference
In-Depth Information
Spatial Reference
Of course, every instance of every spatial data model discussed previously must have as a foundation a
spatial reference to the real world. As discussed, this is not simple. If it were, perhaps Microsoft, Google,
or IBM would be the principle vendor of GIS software. And if the world had been created as a big cube,
rather than a big sphere, life would be easier for GIS specialists.
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But we are stuck with the complexities
of spherical trigonometry, geodesy, and myriad coordinate systems and datums. You could say that the
spatial reference consists of three parts:
A coordinate system, with its associated datum, map projection and parameters, and in some cases an
elevation (denoted by z values), and, in some cases, distances along lines or paths (denoted by m values).
❏
A spatial extent (domain) which defines latitude and longitude (or x and y), boundaries.
❏
A scale, when display is involved, that relates units of linear measure on the map to those on the
ground.
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The method of coordinate system application is handled differently with different Esri products. One
way is a projection file, which might look like this:
PROJECTION
STATEPLANE
ZONE
3976
DATUM
NAD83
Zunits
NO
Units
FEET
Spheroid
GRS1980
Xshift
0.0000000000
Yshift
0.0000000000
Parameters
Of course, this is only part of the story. The lines of text of this file point to additional complexity, which
might look like this:
PROJCS[“NAD_1983_StatePlane_Kentucky_North_FIPS_1601_Feet”,
GEOGCS[“GCS_North_American_1983”,DATUM[“D_North_American_1983”,
SPHEROID[“GRS_1980”,6378137.0,298.257222101]],
PRIMEM[“Greenwich”,0.0],UNIT[“Degree”,0.0174532925199433]],
PROJECTION[“Lambert_Conformal_Conic”],
PARAMETER[“False_Easting”,1640416.666666667],
PARAMETER[“False_Northing”,0.0],
PARAMETER[“Central_Meridian”,-84.25],
PARAMETER[“Standard_Parallel_1”,37.96666666666667],
PARAMETER[“Standard_Parallel_2”,38.96666666666667],
PARAMETER[“Latitude_Of_Origin”,37.5],
UNIT[“Foot_US”,0.3048006096012192]]
And then there is the software that has to interpret all this.
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Except for those living on the edge. (Pun intended.)
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