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FAQ: Why is there a half-cell shift between georeferenced TIFF images when compared in ArcMap and Workstation ArcPlot?

Question

Why is there a half-cell shift between georeferenced TIFF images when compared in ArcMap and Workstation ArcPlot?

Answer

There may be a half-cell shift when a TIFF image is displayed with the world file georeferencing information versus that contained in the GeoTIFF tags. This difference is most commonly noticed when comparing the location of the image between ArcMap and Workstation ArcPlot.

If a georeferenced TIFF image is generated from the GRIDIMAGE or CONVERTIMAGE commands in ArcInfo Workstation, the georeferencing information is recorded in the world file and the internal GeoTIFF tags. The world file records the spatial location of the center of the upper left (UL) pixel of the image. The GeoTIFF tags also record the spatial location of the UL pixel. However, a particular tag is not being written in Workstation that would identify that coordinate to be the center of the UL cell. Instead, the coordinate is being interpreted to be the UL corner of the UL cell, which results in the half cell width offset.

Detail:

GeoKey 1025 (GTRasterTypeGeoKey) dictates whether the reference coordinate is the UL (RasterPixelIsArea, code 1) or center (RasterPixelIsPoint, code 2) of the UL pixel. If this key is missing, the default (as defined by the specification) is to be RasterPixelIsArea, which is the UL of the UL pixel.

ArcInfo does not write GeoKeyDirectoryTag to the output file. This means the GTRasterTypeGeoKey is not present, and thus RasterPixelIsArea should be used as the default case. However, ArcInfo Workstation is using RasterPixelIsPoint.

The net effect is that the GeoTIFF tags in ArcInfo-generated Tiff files are reporting the origin of the image as the center of the UL pixel, when the UL corner of the UL pixel should be used.