RE: Geotiff problem with "small" images???

by Max Martinez :: Rate this Message:

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Duncan,

There isn’t a limit that is specific to GeoTIFF but computers in general have limitations in their standard floating point representations. In the small image, the UTM projection in combination with the small image area means that the difference between the coordinates in adjacent pixels doesn’t show up until the tenth decimal place. This is not a problem if double precision floating point numbers (64-bit quantities) are used (where 17 decimal digits of precision can be accommodated), but if floats (32-bit quantities) are used anywhere in these software products, you may see funny behavior.

ERDAS IMAGINE seems to display the images OK ;-)

Max

From: geotiff-bounces@... [mailto:geotiff-bounces@...] On Behalf Of Duncan Mercer
Sent: Tuesday, July 22, 2008 8:51 PM
To: geotiff@...
Subject: [Geotiff] Geotiff problem with "small" images???

Hi

I'm new to the world of geotiff and I'm running
into a problem when generating small area coverage
geotiffs

I'm collecting images using a robotic submarine (an AUV)
and want to use geotiff to locate these images for use in a
GIS etc. Typical footprint for the images is 1m x 1.5m

If I write the geotiff tags using the correct size then the
image appears badly distorted when viewed in openev and
in arcgis. If however I keep everything else the same and
pretend that the image is 1Km x 1.5Km the image will appear
undistorted.

Has anyone come across this issue before, is there some fundamental
lower limit on the image size that I've not picked up on? Or can someone
tell me how to search the mailing list archives for similar issues?

Example images are available at.
http://www-personal.acfr.usyd.edu.au/d.mercer/geotiff
Large image appears ok, but oversized. Small image is distorted
(at least in openev. I haven't been able to check this one in arcgis
but I have seen the issue with other images I have generated in the past)

FYI.
I am using libgeotiff and writing the following geotags...

TIFFSetField(tif_ptr, TIFFTAG_GEOTRANSMATRIX, 16, position.transform);

GTIFKeySet(gtif_ptr, GTModelTypeGeoKey, TYPE_SHORT, 1, ModelTypeProjected);
GTIFKeySet(gtif_ptr, GTRasterTypeGeoKey, TYPE_SHORT, 1, RasterPixelIsArea);

GTIFKeySet(gtif_ptr, ProjectedCSTypeGeoKey, TYPE_SHORT, 1, utm_zone);
GTIFKeySet(gtif_ptr, PCSCitationGeoKey, TYPE_ASCII, PCS_Zone.str().length(), PCS_Zone.str().c_str());

Any help or suggestions gratefully received.
Best regards
Duncan

--
Dr Duncan Mercer
Australian Centre for Field Robotics
The Rose Street Building, J04
The University of Sydney.
2006 NSW, Australia
Tel: +61 (0) 2 9036 7178
Fax: +61 (0) 2 9351 7474

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