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January 2007
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2007.01.17 21:08 "Re: Elevation Data", by Frank Warmerdam
Joris wrote: > I would expect you guys were writing this sort of thing as an extra > sample, tagged as Undefined, as as to accompany a colour image of the > same area or something. But the original poster seemed to imply the > elevation data he was seeking to write was covered under the MinIsBlack > photometric, thus leading me to think you write greyscale data to be > re-interpreted as elevation data by those apps that know its intention. > > Which is it? Joris, I normally write such scientific data as the whole dataset (not as some sort of addition to a display image). My normal practice is to set PHOTOMETRIC_MINISBLACK for this sample. That is, I think the default display is as a greyscale image. >> rescaling in the file to some "natural image range" loses all the >> value of the actual elevations. > > We've Celsius and Fahrenheit, and metres and foot, and as long as we're > talking floating point I take that to mean rescaling doesn't lose > anything. If values are rescaled to some arbitrary range (like 0.0 to 1.0) before writing to the TIFF file there is no way to reconstruct the original values since we can't embed the scaling information in TIFF. It is true that we also don't capture the units properly in TIFF (as a rule) and it is up to the user to keep track of those themselves (ie. meters or feet). Best regards, -- ---------------------------------------+-------------------------------------- I set the clouds in motion - turn up | Frank Warmerdam, warmerdam@pobox.com light and sound - activate the windows | http://pobox.com/~warmerdam and watch the world go round - Rush | President OSGeo, http://osgeo.org