[gdal-dev] Shapefile to Lambert Azimuthal Equal Area

[Ed McNierney]

Kent -

Frank's response is helpful here - I knew the 98xx codes were familiar but the light bulb didn't go on in my head.

If you have the PROJ library installed, you should have an EPSG file.  It is normally in /usr/local/share/proj.

The EPSG system is a set of numeric codes for standard geographic mathematical objects.  Those codes are used by PROJ to specify a specific projection, and act as both (a) a form of standardized shorthand for a specific projection and (b) a way to avoid typos, ambiguity, or confusion in describing a projection.

Not all EPSG code values refer to specific projections; some refer to families of projections, or projection types.  In your case, code 9820 refers to the entire class of all possible Lambert Azimuthal Equal Area (LAEA) projections.  There are an infinite number of LAEA projections; a specific projection is defined by the class/type of projection (LAEA in this example) and the particular parameters used to define the exact projection to be used.  Frank described, for example, one sort of LAEA projection you might want, centered on latitude=0, longitude=0.  An LAEA projection centered on latitude=0, longitude=90 is a different projection although it belongs to the same class.

If you are to use an EPSG code to describe a projection, you need to use one of the numeric codes found in your /usr/local/share/proj/epsg file.  Not all possible projections are described; only standard and commonly-used ones.

You don't have to use an EPSG code, and if you're not using a standard and well-known projection (not projection class) then there's no need to either look for or invent one.  You can instead describe the projection you want parametrically, using either the WKT or PROJ formats Frank described.  I usually like to use the PROJ format since it's a bit less verbose, but either will work.

	- Ed

Ed McNierney
Chief Mapmaker
Demand Media / TopoZone.com
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396

-----Original Message-----
From: Kent Eschenberg [mailto:eschenbe at psc.edu] 
Sent: Monday, January 07, 2008 1:57 PM
To: Ed McNierney
Cc: GDAL
Subject: Re: [gdal-dev] Shapefile to Lambert Azimuthal Equal Area

Hi Ed,

Thanks! Here's the sequence of pages that led to the 9820 code:

www.gdal.org
www.gdal.org/ogr
www.gdal.org/ogr/osr_tutorial.html
www.remotesensing.org/geotiff/proj_list
www.remotesensing.org/geotiff/proj_list/lambert_azimuthal_equal_area.html

That last page lists the EPSG code as 9820. I'm trying I'm trying to convert 
currently uses the WGS84 projection.

What file do you mean when you refer to my "EPSG file"? Currently I don't have 
such a file; that is what I am trying to create.

All advice appreciated!
Kent

Ed McNierney wrote:
 > Kent -
 >
 > The obvious question would be, "And do you think code 9820 actually exists 
in your EPSG file?".  There is no such projection code in my EPSG table from PROJ.
 >
 > What projection definition do you think EPSG:9820 describes?
 >
 >      - Ed
 >
 > Ed McNierney
 > Chief Mapmaker
 > Demand Media / TopoZone.com
 > 73 Princeton Street, Suite 305
 > North Chelmsford, MA  01863
 > ed at topozone.com
 > Phone: +1 (978) 251-4242
 > Fax: +1 (978) 251-1396
 >
 > -----Original Message-----
 > From: gdal-dev-bounces at lists.osgeo.org 
[mailto:gdal-dev-bounces at lists.osgeo.org] On Behalf Of Kent Eschenberg
 > Sent: Monday, January 07, 2008 11:29 AM
 > To: GDAL
 > Subject: [gdal-dev] Shapefile to Lambert Azimuthal Equal Area
 >
 > Any idea why I cannot convert a shapefile to the Lambert azimuthal equal area
 > projection? I tried
 >
 > ogr2ogr -t_srs EPSG:9820 -f "ESRI Shapefile" destination.shp source.shp
 >
 > and saw the message
 >
 > ERROR 6:
 > EPSG PCS/GCS code 9820 not found in EPSG support files.  Is this a valid
 > EPSG coordinate system?
 > Failed to process SRS definition: EPSG:9820
 >
 > I'm using GDAL 1.4.2 on Fedora core 6.
 >
 > TIA!
 > Kent
 > Pittsburgh Supercomputing Center