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# 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.16 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.

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", ...``

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