RFC 33: GTiff - Fixing PixelIsPoint? Interpretation
Authors: Frank Warmerdam
This document proposes changes in the GDAL GTiff (GeoTIFF) driver's interpretation of PixelIsPoint when constructing the geotransform and interpreting control points. An RFC is used due to the fundamental role of GeoTIFF in GDAL and the GDAL user community and the risk for significant backward compatibility problems with this adjustment.
The GeoTIFF specification includes a data item, GTRasterTypeGeoKey, which may be set to either RasterPixelIsArea (the default), or RasterPixelIsPoint. RasterPixelIsArea defines that a pixel represents an area in the real world, while RasterPixelIsPoint defines a pixel to represent a point in the real world. Often this is useful to distinguish the behavior of optical sensors that average light values over an area vs. raster data which is point oriented like an elevation sample at a point.
Traditionally GDAL has treated this flag as having no relevance to the georeferencing of the image despite disputes from a variety of other software developers and data producers. This was based on the authors interpretation of something said once by the GeoTIFF author. However, a recent review of section [ section 126.96.36.199] of the GeoTIFF specificaiton has made it clear that GDAL behavior is incorrect and that PixelIsPoint georeferencing needs to be offset by a half a pixel when transformed to the GDAL georeferencing model. This issue is documented in the following tickets including #3837, #3838, ....
This RFC attempts to manage this transition with a minimum of disruption for the users of GDAL/OGR.
Interpretation of the raster space from the GeoTIFF tie points will be offset by half a pixel in the PixelIsPoint case in gdal/frmts/gtiff/geotiff.cpp. This will impact the formation of the geotransform and the formation of GCPs when there are multiple tie points. geotransmatrix conversion to geotransform will also be affected.
Conversely if writing files with PixelIsPoint (as driven by the "AREA_OR_POINT" metadata item being set to "POINT") the written raster space coordinates would be offset by half a pixel.
In trunk the above behavior may be disabled by setting the GTIFF_POINT_GEO_IGNORE configuration option to TRUE (it will default to FALSE).
In GDAL 1.7 and 1.6 branch the same changes will be applied, except the GTIFF_POINT_GEO_IGNORE configuration option will default to TRUE.
This change will alter the apparent georeferencing of all GeoTIFF files with PixelIsPoint set. It is not clear how large a proportion of GeoTIFF files this will apply to, but it is significant. This isn't too bad for files coming from non-GDAL sources as most other produces have made the correct interpretation of PixelIsPoint for years. However, unfortunately, files produced in the past by GDAL with PixelIsPoint will now be interpreted differently and the values will be off by half a pixel.
In practice it was not particularly convenient or well documented how to produce PixelIsPoint GeoTIFF files with GDAL, so these files should be fairly rare. Thee easiest way to produce them was by copying from another PixelIsPoint GeoTIFF file in which the error on write just undid the error when reading the source GeoTIFF file.
Folks have at various points in the past requested that we report the extents differently for files with an AREA_OR_POINT value of POINT, much as listgeo does for GeoTIFF files that have a PixelIsPoint interpretation. I do *not* plan to do this, and for the purpose of GDAL the GCPs, RPCs and GeoTransform will always be based on an area interpretation of pixels. The AREA_OR_POINT will *only* be used to control setting of the PixelIsPoint value in GeoTIFF files, and as metadata about the physical interpretation of pixels.
These changes will have no impact on how world files are treated or written. They are always based on the assumption of a area based pixel, but with the origin at the center of the top left pixel. This is effectively the same as the values for PixelIsPoint?, but is not in any way tried to this metadata.
The 1.6 and 1.7 branch test suites will not be altered.
The trunk branch test suite will be altered to check for the updated values and will be extended with a test to confirm that setting the config option GTIFF_POINT_GEO_IGNORE to TRUE suppresses the altered behavior.
The situation will be noted in the 1.8.0, 1.7.4 and 1.6.4 release notes as well as in the GeoTIFF driver documentation in trunk.
The GeoTIFF web site GeoTIFF FAQ will be updated to clarify the interpretation of PixelIsPoint and note that up to GDAL 1.8 it was improperly interpreted by GDAL.
All code implementation will be by Frank Warmerdam in the next few weeks.