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opengeospatial / geotiff

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ModelPixelScaleTag and use of ScaleZ for elevation data #8

Closed EmDevys closed 5 years ago

EmDevys commented 8 years ago

according to http://www.remotesensing.org/geotiff/spec/geotiff2.6.html The ScaleZ is primarily used to map the pixel value of a digital elevation model into the correct Z-scale, and so for most other purposes this value should be zero (since most model spaces are 2-D, with Z=0).

Consequently, for elevation data, ScaleZ may (if I am correct) be used as a Scaling factor for elevations; this would allow to avoid the use of unnecessary floating-point values for hight resolution elevation values, that may be provided with values in cm or mm, but to be converted to meters (unit assicated in most cases to EPSG CRS codes).

In such cases, ScaleZ should therefore be 0.01 or 0.001, which seems a perfectly valid way to encode this. Unfortunately, it appears that most COTS - as well as gdal - do not handle the z dimension parameters even for elevation data.

This should (IMHO) be clarified in the revision of GeoTIFF, as well as ensured by key open-source software such as gdal.

EmDevys commented 5 years ago

Clarification added to Annex B (for Coordinate Transformations).