|
GeoTIFF - A standard image file format for GIS applications
Image File Directories (IFDs)
Most Likely, the next structure in the file after the header will be the first (or only) IFD, but not necessarily. From here on, everything is found by following pointers. So, to locate the first IFD use the header's pointer.  Fig. 1 The File Structure of a Standard TIFF File An IFD consists of 12-byte entries, typically tagged pointers. The structure of an IFD and its entries are as follows: Table 1: TIFF IFD Entry Structure
Tag : The first two bytes are tag, which , if public , may be looked up in the specification. These codes are assigned by the TIFF administrator (Aldus Developer's Desk), in blocks of five. This tag concept has been extended to support GeoTIFF. Type code : The next two bytes comprise a code indicating the type of data in the pointed field. Count field : The 4-byte field specifies the number of values in the data field, not the number of bytes. Data pointer or data field : The final four bytes are usually a pointer to the start of a data field. Sometimes, however, this field contains not a pointer, but the actual data. Brief Description of GeoTIFF The GeoTIFF specification [2] defines a set of TIFF tags provided to describe all "Cartographic" information associated with TIFF imagery that originates from satellite imaging systems, scanned aerial photography, scanned maps, digital elevation models, or as a result of geographic analyses. Its aim is to allow means for tying a raster image to a known model space or map projection. Basic Features of GeoTIFF GeoTIFF format fully complies with the TIFF 6.0 specifications, and its extensions do not in any way go against the TIFF recommendations, nor do they limit the scope of raster data supported by TIFF. It uses a small set of reserved TIFF tags to store a broad range of georeferencing information, catering to geographic as well as projected coordinate systems needs. Projections include Universal Transverse Mercator (UTM), US State Plane and National Grids, as well as the underlying projection types such as Transverse Mercator, Lambert Conformal Conic, etc. It uses a "MetaTag" (GeoKey) approach to encode dozens of information elements into just six tags. These keys are designed in a manner parallel to standard TIFF tags, and closely follow the TIFF discipline in their structure and layout. New keys may be defined as needs arise, within the current framework, and without requiring the allocation of new tags from Aldus/Adobe. GeoTIFF format uses numerical codes to describe projection types, coordinate systems, datums, ellipsoids, etc. The projection, datums and ellipsoid codes are derived from the EPSG list compiled by the Petrotechnical Open Software Corporation (POSC), and mechanisms for adding further international projections, datums and ellipsoids has been established. The GeoTIFF information content is designed to be compatible with the data decomposition approach used by the National Spatial Data Infrastructure (NSDI) of the U.S. Federal Geographic Data Committee (FGDC). |