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I need to integrate against the USDA's cropland data layer. They've got a demo and some API docs.

The important query I need to run is this, given an (x, y) of say (1551459.363, 1909201.537):

 http://nassgeodata.gmu.edu:8080/axis2/services/CDLService/GetCDLValue?year=2010&x=1551459.363&y=1909201.537

But coordinates are only useful if you know what coordinate system they belong to. The documentation is rather sparse, but I found this information:

The CDL is processed using the Albers Equal-Area Conic Projection with a spheroid of GRS 1980 and datum of NAD83. The downloadable zip files from the SARS website and CropScape are offered in the native Albers projection.

In order to conform to Geospatial Data Gateway technical specifications, any CDL data downloaded through the Geospatial Data Gateway is reprojected from Albers to the dominant Universal Transverse Mercator (UTM) zone with a spheroid and datum of WGS84. The one exception to the UTM projection is for Wisconsin. Wisconsin is projected using the Wisconsin Transverse Mercator (WTM) projection. This WTM projection is based on the 1991 adjustment to NAD83, and is called WTM83/91.

So I headed over to PostGIS to look for a match (hopefully this wasn't too narrow):

SELECT * FROM spatial_ref_sys WHERE srtext ILIKE '%DATUM%83%SPHEROID%80%' AND srtext ILIKE '%albers%';

I found 17 matches, but the projections are all regional like "Texas Centric Albers Equal Area".

How do I boil this information down to the correct SRID?

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The metadata are provided here. – whuber♦ May 2 at 2:13
Could you elaborate? I'm new to GIS, I don't understand the process I need to follow to take the information I have and make the desired queries. – Brad Koch May 2 at 2:34
The biggest problem is I don't recognize the coordinates I'm supposed to supply, so I don't know what I'm supposed to generate. They look kind of like UTM coords, but there's no zone attached. – Brad Koch May 2 at 2:51
I would like to suggest you first search our site on keywords related to "projection" and "coordinate system." Then, when you understand more, come back here to refine your question. In so doing, please specify what GIS software you are using. (Although this question is closed, it is still visible to people and you will be able to reopen it.) – whuber♦ May 2 at 13:23
Sorry about that, my question was a bit unclear and too "do my work for me". I explained what I did more thoroughly and cleaned up the scope. And I understand projections (a surface representing the earth) and datums (the rules that define the mapping of coordinates to a location on the projection). Is this edited question clear enough, or am I still neglecting something? – Brad Koch May 2 at 15:13
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1 Answer

up vote 2 down vote accepted

The metadata are available on the web site. Clicking on any of the choices gives an extensive metadata listing in a standardized (extremely verbose) format. For instance, from the metadata for Alabama 2012 I find (in the "spatial reference section" near the end) the following:

FOR CROPSCAPE USERS: Albers Conical Equal Area is the native projection used in the production of
the Cropland Data Layer. The projection parameters for the Albers projection are as follows:

Map_Projection_Name: Albers Conical Equal Area
Albers_Conical_Equal_Area:
Standard_Parallel: 29.500000
Standard_Parallel: 45.500000
Longitude_of_Central_Meridian: -96.000000
Latitude_of_Projection_Origin: 23.000000
False_Easting: 0.000000
False_Northing: 0.000000
...
Planar_Distance_Units: meters
Geodetic_Model:
Horizontal_Datum_Name: North American Datum of 1983
Ellipsoid_Name: Geodetic Reference System 80
Semi-major_Axis: 6378137.000000
Denominator_of_Flattening_Ratio: 298.257223563

The standard parallels of 29.5 and 45.5 degrees indicate this projection is adapted for the 48 conterminous states, so we can expect all the metadata to consistently provide the same parameters. A search of the spatialreference.org site shows it matches projection 6630, where it is described as "Albers Conical Equal Area as used by mrlc.gov (NLCD)". M. Kennedy points out that this appears to match EPSG 5070.

The trick to reading metadata is to wade past all the boilerplate garbage to get to what really matters. Searching a text-based metadata document for key works like "spatial reference" or "projection," etc., is often a good tactic.

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Thanks for your patience and help! Didn't realize PostGIS's spatial_ref_sys table is somewhat incomplete; 6630 isn't in mine. But that's a separate problem. – Brad Koch May 2 at 16:10
1  
@whuber Bill, you referred to spatialreference.org which <> EPSG. SR's IDs aren't EPSG's or Esri's. The EPSG entry is probably 5070, NAD83 / Conus Albers. Link to a report: epsg-registry.org/… – mkennedy May 2 at 19:02
@mkennedy Thanks for the clarification, Melita. I had somehow navigated past the EPSG database to the spatialreference site. – whuber♦ May 2 at 19:06

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