[Geotiff] Images vs DEMs in GeoTiff

Nagle, Gail A (US SSA) gail.nagle at baesystems.com
Wed Jun 25 22:29:03 EDT 2008

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:

Key ID = 1103
Type = SHORT (code)

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!


-----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


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

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

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.

Best wishes

Cody A. Benkelman
ASPRS Certified Mapping Scientist - Remote Sensing (#RS-144)
17 Willowbrook Close
Whitefish, Montana  59937
cell (406) 270-1176

-----Original Message-----
From: geotiff-bounces at lists.maptools.org
[mailto:geotiff-bounces at lists.maptools.org] On Behalf Of Frank Warmerdam
Sent: Wednesday, June 25, 2008 5:36 PM
To: Nagle, Gail A (US SSA)
Cc: geotiff at lists.maptools.org
Subject: Re: [Geotiff] Images vs DEMs in GeoTiff

Nagle, Gail A (US SSA) wrote:
> Hello,
> Is there currently anyway to determine if a given GeoTIFF file 
> contains pixels or posts?
> In other words, is there a tag that states that the file is an image 
> or a DEM? If not a specific tag, then perhaps a combination of tags?


Yes, the GTRasterTypeGeoKey geokey can have the value RasterPixelIsArea or

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

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