RFC 33: GTiff - Fixing PixelIsPoint Interpretation

Authors: Frank Warmerdam
Contact: warmerdam@…
Status: Development

Summary

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.

Rationale

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 RasterPixelIsArea 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 ​http://www.remotesensing.org/geotiff/spec/geotiff2.5.html#2.5.2.2 section 2.5.2.2 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.

Test Suite

Documentation

Implementation