[Gdal-dev] What are georeferenced coordinates?

[Frank Warmerdam]

Kent Eschenberg wrote:
> What are georeferenced coordinates? I can't find a definition in any of 
> the GDAL documents. I would guess they are longitude and latitude.
> 
> I'm trying to use the "-projwin" option for gdal_translate with valid 
> longitude and latitude. But this always fails saying they are outside 
> the bounds of the input image. "-srcwin" works as expected.
> 
> "gdal_translate --version" yields "GDAL 1.4.2.0, released 2007/06/27". 
> I'm using a Redhat Fedora Core 5 system.

Kent,

I use the term "georeferenced coordinates" to refer to coordinates in some
sort of cartographic reference system.  Mainly this means a projected
coordinate system like "UTM 11 WGS84" in which case the locations are measured
in some linear units like meters or feet from some origin, or a geographic
coordinate system like "WGS84" in which case the coordinates are angular
units (usually degrees) relative to the intersection of the prime meridian
(usually greenwich) and the equator.  This is the latitude and longitude
we see in lots of places.

So the arguments to -projwin need to be in the same coordinate system as
the image is georeferenced in.  If the image is projected (I think you
mention in an earlier message that it is in lambert implying a projected
coordinate system) then the values need to be linear values (meters or
feet) in that projection.

Regarding that email:

 > What are the options for gdalwarp that will produce an image on a simple
 > longitude/latitude coordinate system?
 >
 > The input file, lambert.tif, is a GeoTiff file from the National Atlas
 > using the Lambert_Azimuthal_Equal_Area projection. It covers the 48 states.
 >
 > I'd like the output X pixel coordinate to represent equally-spaced
 > values of longitude and Y to represent latitude.
 >
 > I tried this:
 >
 >    gdalwarp -t_srs "+proj=ortho +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0"
 >       lambert.tif ortho.tif

I think you want to use:

gdalwarp -t_srs WGS84 lambert.tif wgs84.tif

The ortho projection is not at all the same thing as unprojected.

Best regards,
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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