Mapping Hacks

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Chapter 9, Mapping with Other People

#100 Map Imaginary Places

HACK

The GEOGCS[ entry is where we define our coordinate system, known as

A2484, with a prime meridian of New New Albemuth. For future spatial

analysis, the important part of this is the

SPHEROID definition inside the

WKT:

SPHEROID("made_up_name",<semi-major axis>,<inverse flattening>).

The

UNIT is 2π/360, the number of degrees per radian; degree-based coordi-

nate systems are (relatively) simple and familiar, so don’t change it. The

semi-major axis and inverse flattening tell PostGIS how to calculate distance

and area from points based on a latitude/longitude coordinate system. Ours

is an unprojected, geographical coordinate system. “Work with Different

Coordinate Systems”

[Hack #26] explains how different projections work in

more depth. Try

SELECT SRID,SRTEXT FROM SPATIAL_REF_SYS from your data-

base shell to see all the different projections normally available in PostGIS.

The last value,

PROJ4TEXT, tells proj4 how to perform coordinate transforma-

tions and isn’t really necessary for our purposes, so we just borrow the defi-

nition for WGS84 for the sake of completeness. For in-depth information

about the PostGIS spatial tables, check out the online documentation at

http://postgis.refractions.net/docs/ch04.html.

Now when you insert and retrieve points or other bits of geometry into Post-

GIS, use your new SRID and you can perform realistic ...