GeoKeyDirectoryTag

GeoKeyDirectoryTag is a metadata tag used by the GeoTIFF format to store a compact directory of geospatial keys. It serves as a central reference that describes the geographic coordinate system, map projection, and related transformation parameters used by the image data. The tag is widely supported by GIS and remote sensing software for interpreting the spatial content of GeoTIFF files. The tag is assigned the TIFF tag number 34735.

The content of GeoKeyDirectoryTag is organized as a sequence of 4-byte integers. The first four values form the header and specify the key directory version, revision, minor revision, and the number of geokeys defined (the key entries that follow). After the header, the tag contains a series of key entries. Each key entry is a four-integer group that identifies a specific geokey with four fields: KeyID (the geokey code), TIFFTagLocation (where the key’s value is stored in the TIFF structure or its deviation), Count (how many values are associated with the key), and ValueOffset (the location of the actual value(s) or an offset to them). Values may be stored directly in the directory or in additional TIFF tags, and string values can be referenced via an offset to an ASCII data block.

Commonly used geokeys include those describing the geographic type (geographic or projected coordinate system), the specific coordinate reference system, and projection parameters. Together with additional TIFF tags, GeoKeyDirectoryTag enables software to correctly map pixel coordinates to real-world locations.

In practice, software reads GeoKeyDirectoryTag to determine how to interpret the image’s geospatial context, then consults the referenced keys and values to apply the appropriate coordinate transformations.