[Geotiff] Images vs DEMs in GeoTiff

[Frank Warmerdam]

Cody / Gail,

Cody is right.  I really only read your question as far as "contains pixels or
posts" before I answered.  The GTRasterTypeGeoKey cannot be reliably used
to differentiate image data from DEM data.

Note there are many types of raster data beyond "visual image", and DEM.

Best regards,

Nagle, Gail A (US SSA) wrote:
> Hi Cody,
> 
> Thanks for adding this info. If the misinterpretation of the value of the GTRasterTypeGeoKey has become commonplace, I do think a better solution is available in the 1.5 Draft spec of GeoTIFF. We have a copy of a document numbered Specification version 1.9.3 dated 14 June 2001 from Niles Ritter. It contains the following proposed key in section 4.4.1:
> 
> DataInterpretationGeoKey
> Key ID = 1103
> Type = SHORT (code)
> Values:
> DataMultispectral=1
> DataElevation=2
> DataVector=3
> DataThematic=4
> 
> I don't know if the 1.5 Draft is still being considered or not. But it does seem that the power of GeoTIFF is such that it is being used in ways that were not originally planned for. So having a definitive key describing the content of the file would be very useful.
> 
> In the meantime, when we are trying to determine what kind of data we have, an image or a DEM, we are going to look for the value RasterPixelIsPoint and assume it is a DEM and try to load it as such. Likewise, if the value is RasterPixelIsArea, we will try to load it as an image. If the user is not happy with our interpretation of the data, we will provide a way for the user to specify how to load the data: DEM or Image.
> 
> Thanks for taking the time to weigh in on this. Things do get complicated!
> 
> -Gail
> 
> 
> -----Original Message-----
> From: Cody Benkelman [mailto:benkelman at yahoo.com] 
> Sent: Wednesday, June 25, 2008 6:58 PM
> To: Nagle, Gail A (US SSA)
> Subject: FW: [Geotiff] Images vs DEMs in GeoTiff
> 
> Gail
> 
> I never like to disagree with Frank Warmerdam, but I am very confident there
> are GeoTIFF images that are examples of raster images (not DEM data) out
> there in the world with the GTRasterTypeGeoKey geokey set to values of both
> RasterPixelIsArea and RasterIsPoint (not "both in one file" but "examples of
> both").
> 
> That may not be what he and others *intended* for usage of that field, but I
> believe it has (in many cases) been interpreted as "is the precise
> coordinate of the pixel in the CENTER of the pixel or at the CORNER of the
> pixel?".
> 
> Unfortunately this is a tangent and doesn't answer your question - I do not
> know if there is another tag that WILL specify if a GeoTIFF is a DEM (if so,
> Frank must have misunderstood your question, or he would have offered it),
> but I didn't want you to go too far with his response.
> 
> Having said all that, I *may* be wrong so please verify with others, but I'm
> concerned that if you assume "GTRasterTypeGeoKey = RasterIsPoint" implies
> "data = DEM" you'll run into problems.



-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | President OSGeo, http://osgeo.org