How is georeferencing in NITF imagery handled?

The NITF format has several TREs that can contain different types of geolocation-related information: RPCs, four corner coordinates, and a traditional map projection. When writing a NITF image out to TIFF, the transfer of georeferencing information depends on which of these TREs are available in the NITF file. This help article discusses the various possibilities, and how ENVI handles them.

A NITF file can contain several different types of map info in the TREs: RPC s (RPC0A and RPC0B TREs), four corner coordinates (IGEOLO and ICORDS TREs), and a traditional map projection. One NITF image can't have all three sets of map information. It may always have RPCs (or not), and then it *may* (but doesn't have to) also have four corner coordinates OR traditional map projection. So the options are: just corners, just traditional projection, RPC and corners, RPC and traditional projection, or just RPC.

If the file does not contain a traditional map projection, then when exporting to TIFF, no geo information is included in the TIFF tags. In other words, a plain TIFF file, rather than a GeoTIFF file, is generated. RPC information and/or the four corner coordinatess are not sufficient to populate the TIFF tags, because in both of those systems, the pixel size is not fixed. It varies across the image. So if ENVI were to populate the GeoTIFF tags with whatever info it has from the RPC and/or four corner data, the GeoTIFF file would have inaccurate map info. Therefore, by default, ENVI does not populate those GeoTIFF tags.

In general, the priority for using map info from NITF files, wherever map info is used in ENVI, is this:

1. RPC takes precedence if it is there, even if a traditional map projection is also present. The assumption here is that the traditional map projection is the result of a rough georeferencing, and that the RPC info will be more accurate. So a pseudoprojection based on the RPCs is used in ENVI. This default behavior can be turned off (so that the traditional map projection is used, rather than the RPCs) by setting an item in the ENVI header for the file. One way to access this is to select Edit ENVI Header from the Toolbox, choose your image, then in the Header Info dialog, choose Edit Attributes > RPC or RSM Projection Emulation.  In the resulting dialog, clear the existing RPC projection emulation.  The RPC information will still be there, and you can turn it back on later, if you wish, by again selecting Edit ENVI Header > Edit Attributes > RPC or RSM Projection Emulation.

In Classic ENVI, to turn off the RPCs, select the Map Info for the image in the Available Bands List, then right click, and choose Edit Map Information. At the top of the resulting dialog, you will see a RPC Projection Emulation Enabled option. Set that to Off, then choose OK. You will notice that the Map Info shown in the Available Bands List for that file is no longer listed as *RPC*.  To switch back to using the RPC pseudoprojection, you need to select the image file itself in the Available Bands List, right click, and choose Edit Header. In the resulting dialog, choose Edit Attributes -> RPC or RMS Projection Emulation. Select Edit existing RPC projection emulation. In the resulting dialog, you can switch the RPC Projection Emulation Enabled option to On.

2. If RPC data is not present, then traditional map projection info is used in ENVI.

3. If neither RPC, nor traditional map projection info is present, then the four corner coordinates are used to derive kxky information (which is the number of pixels in the x and y dimensions of the image), which can then define a pseudoprojection.