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Open Source GIS 101, Part 8: Projections

Part 8: A Few Notes About Map Projections

There are plenty of books on Geodesy, but here we’ll focus on selecting your Coordinate Reference System (CRS) for your GIS project. Luckily, there is a well organized index of coordinate systems universally used by open source and proprietary GIS software. Using ArcCatalog, right-click one of our many feature classes and select Properties, select the XY Coordinate System tab, and examine the first two lines under Current coordinate system:

NAD_1983_StatePlane_California_V_FIPS_0405_Feet
WKID: 2229 Authority: EPSG

Most of us understand that the first line says the spatial data is expressed as coordinates in the North American Datum of 1983, California State Plane Coordinate System, Zone 5, U.S. Survey Feet. 

The second line is a different identifier for the same CRS. The WKID represents the Well Known IDentification number, and 2229 is the number designated by the European Petroleum Survey Group (EPSG – now part of the OGP), who originally compiled a widely used database of ellipsoids, datums, and coordinate systems. In a similar manner to the way the California Department of Motor Vehicles issues license plate numbers, the OGP Geodetic Subcommittee reviews new map projections, and, upon their approval, issues identification numbers for those projections. Whether you call it an ArcGIS WKID, a Quantum GIS CRS, or a Spatialite SRID, you will likely be using a number issued by the EPSG.

As a resource, some useful links are listed below:

EPSG Registry is worth bookmarking!

Spatialreference.org is another must-bookmark for its speed and simplicity.

California Public Resources Code, Section 8801-8819 is your legal reference. Definitions for each Zone in California are included here.

ArcGIS 10 Help is an excellent source of information for beginners and advanced users.

For those who would like to learn more about the challenges of integrating high accuracy data into GIS workflows, my colleague Victor Flores recommends this excellent webcast.

Does anyone remember how high school students like to amuse each other by spelling words with their calculators? Whatever the humor and mystery surrounding the use of 900913 (spells ‘google’) for the projection used by Google Maps, you won’t find that number in the EPSG Registry. However, Spatialreference.org does state that the EPSG equivalent is 3857.

Next, Part 9, An Open Source Alternative to the File Geodatabase (Sort of)…

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