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Implementing GeoKeys (GeoTIFF tags)

GeoTIFF files support standard TIFF comments and tags, but also contain six additional data tags describing the image location, scale, projection, and other information useful for topographic applications.

Three tags contain scalar data:

  • ModelTiepointTag = 33922 (0x8482)
  • ModelPixelScaleTag = 33550 (0x830E)
  • ModelTransformationTag = 33920 (0x8480)

Three tags store additional values, called "GeoKeys":

  • GeoKeyDirectoryTag = 34375 (0x87AF) This tag will store the all the keys and the SHORT values
  • GeoDoubleParamsTag: 34736 (0x87B0) This tag will store the DOUBLE values
  • GeoAsciiParamsTag: 34737 (0x87B1) This tag will store the ASCII values.

There are 65536 possible GeoKeys.

The GeoTIFF spec recommends the following ranges (domains) for GeoTIFF data:

0..1023Do not use; reserved for future use.
1024..2047GeoTIFF Configuration keys.
2048..3071Geographic/Geocentric CS Parameter keys.
3071..4095Projected CS Parameter keys.
4096..5119Vertical CS Parameter keys.
5120..32767Reserved.
32768..65535Private use -- use to store your own data.

Enumerate GeoKeys in an existing file by using EnumGeoKeys.

Read a specific GeoKey by using ReadGeoKey.

Use the GeoKeys property to prepare GeoKeys for file writing or saving. These GeoKeys are maintained as a per-thread array of GeoKeys.You can write the GeoKeys in two ways:

  1. When saving a new image as GeoTif , with one of the Save methods.
  2. You can update an existing file, using WriteGeoKeys.

Note: To save a region inside a TIFF file, you must have an unlocked Document, Vector, or Medical Imaging license.

For more information on the GeoTIFF tags and for the complete list of the predefined GeoKeys values, refer to the GeoTIFF specifications found at

http://www.remotesensing.org/geotiff/spec/geotiffhome.html.

GeoTIFF files also support standard TIFF comments and tags. For more information on standard TIFF tags and comments, see Implementing TIFF Comments and Tags.

For technical reasons, the following restrictions apply when you pass an Image File Directory (IFD) offset by setting the RasterCodecs.Options.Tiff.Load.ImageFileDirectoryOffset property if you set PageNumber to 1:

  • You cannot add tags, comments or GeoKeys to this IFD. You can only update existing tags, comments or GeoKeys in this IFD.
  • You cannot replace the TIFF page indicated by this IFD.
  • You cannot add a page before the this IFD.
  • You cannot delete the page indicated by this IFD.

You can, however, add tags, comments or GeoKeys to an IFD that follows the specified IFD (for example, if PageNumber is >= 2). You can also replace or delete a page that follows the specified IFD and you can insert a page after this IFD.