Trying to figure out epsg for a GeoTIFF

[Ed McNierney]

Dennis -

The EPSG coding system is just a list of various projections.  There's
no way to "figure out" what an EPSG code should be (with a few limited
exceptions for related projections).  You just need to get the list and
look it up!

EPSG provides a set of tools, but they're hard to use.  There are
various other EPSG lists kicking around the Web.  However, ultimately
you will be depending on the list of EPSG projection codes that you have
installed (via the PROJ library) on your own server, so you might as
well look there.  In the nad subdirectory of your PROJ installation (I
don't know whether you're running on Windows or UNIX) you'll find a file
simply named "epsg".  This is a text file; open it with your favorite
editor.

The EPSG projection definitions in that file are labeled with common
names, so if you know you've got "Massachusetts State Plane" coordinates
you can search for that term (there are several variations).  If you
only have the projection parameters (which you do) then you need to
search for those.

I searched the standard PROJ 4.4.9 distribution and could not find
"-81.45" or "38.66" anywhere in that file.  That makes me suspect that
this is a projection that may be "non-standard".  There's nothing
"wrong" with that - you can define a projection any way you like.  But
if you want to use it in some contexts (like WMS) you need an EPSG
definition for it.  The EPSG code number simply provides a reliable
shorthand for the definition.  It's more compact, and it is easier to
figure out whether two projections are the same or not (comparing all
the parameters leads to confusion when the precision of parameters is
different - are 81.67 and 81.66667 the same number or not?).

In your MAP file you can also simply spell out the projection parameters
(i.e. the PARAMETER values below) rather than using the EPSG file.  But
I'd like to ask what you need to do with this source data?  If you need
to have an EPSG code to refer to it, that leads down one path - if you
just need to display it with other layers in MapServer, that's a
different path.  And if it's the ONLY layer you want to display in
MapServer you can simply ignore the whole problem!

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Dennis Megarry
Sent: Tuesday, February 21, 2006 7:48 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: [UMN_MAPSERVER-USERS] Trying to figure out epsg for a GeoTIFF

I'm going to sound like as complete newbie be assking this (which I am
really).

I have a GeoTIFF raster image and I'm not sure how to setup the
projection in my map file.  Can someone take a look at the below info
from gdalinfo and tell me what I need to do?  If at all possible, some
details so I can learn a bit?

Driver: GTiff/GeoTIFF
Size is 11873, 8123
Coordinate System is:
PROJCS["unnamed",
    GEOGCS["NAD83",
        DATUM["North_American_Datum_1983",
            SPHEROID["GRS 1980",6378137,298.2572221010042,
                AUTHORITY["EPSG","7019"]],
            AUTHORITY["EPSG","6269"]],
        PRIMEM["Greenwich",0],
        UNIT["degree (supplier to define
representation)",0.01745329251994328],
        AUTHORITY["EPSG","4269"]],
    PROJECTION["Lambert_Conformal_Conic_2SP"],
    PARAMETER["standard_parallel_1",38.6666666666667],
    PARAMETER["standard_parallel_2",33.33333333333337],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",-81.45000000000009],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]]]
Origin = (-439988.052575,241474.587546)
Pixel Size = (63.88386800,-63.88386800)
Metadata:
  AREA_OR_POINT=Area
Corner Coordinates:
Upper Left  ( -439988.053,  241474.588) ( 84d48'17.02"W,  1d46'42.74"N)
Lower Left  ( -439988.053, -277454.072) ( 84d40'25.94"W,  2d 7'20.21"S)
Upper Right (  318505.112,  241474.588) ( 79d 1'15.86"W,  1d48'22.30"N)
Lower Right (  318505.112, -277454.072) ( 79d 6'57.06"W,  2d 5'48.28"S)
Center      (  -60741.470,  -17989.742) ( 81d54'14.71"W,  0d 8'11.59"S)
Band 1 Block=11873x1 Type=Byte, ColorInterp=Palette