# NITF – Advanced Driver Information

The NITF (National Imagery Transmission Format) driver in GDAL includes a number of advanced, and somewhat esoteric options not suitable for the general end user documentation for the driver. This information is collected here, and is primarily aimed at developers and advanced users.

## CGM Segments

NITF files that have CGM data (that is segment type GR - graphics, or SY with an STYPE value of ‘C’) will make that information available as metadata in the CGM domain. The returned metadata will look something like:

    SEGMENT_COUNT=1
    SEGMENT_0_SLOC_ROW=25
    SEGMENT_0_SLOC_COL=25
    SEGMENT_0_SDLVL=2
    SEGMENT_0_SALVL=1
    SEGMENT_0_CCS_ROW=00025
    SEGMENT_0_CCS_COL=00025
    SEGMENT_0_DATA=\0!\0...

The SLOC_ROW and SLOC_COL values are the placement of the CGM object relative to the base (SALVL) image. The CCS_ROW/COL values are relative to the common coordinate system. The _SDLVL is the display level. The DATA is the raw CGM data with “backslash quotable” escaping applied. All occurrences of ASCII zero will be translated to ‘0’, and all backslashes and double quotes will be backslashed escaped. The CPLUnescapeString() function can be used to unescape the data into binary format using scheme CPLES_BackslashQuotable.

To add CGM data to a NITF image, you can pass creation options in the following format:

    CGM=SEGMENT_COUNT=1
    CGM=SEGMENT_0_SLOC_ROW=25
    CGM=SEGMENT_0_SLOC_COL=25
    CGM=SEGMENT_0_SDLVL=2
    CGM=SEGMENT_0_SALVL=1
    CGM=SEGMENT_0_DATA=\0!\0...

Notice that passing CGM as creation options will overwrite existing CGM segment read in the CGM metadata domain.

While GDAL does not support parsing or rendering CGM data, at least one user has found the UniConverter library useful for this purpose.

## Multi-Image NITF Files

NITF files with more than one image segment (IM) will present the image segments as subdatasets. Opening a multiple NITF file by filename will provide access to the first image segment. The subdataset metadata for a 3 image NITF file might look like:

    Subdatasets:
      SUBDATASET_1_NAME=NITF_IM:0:multi_image_jpeg_2.0.ntf
      SUBDATASET_1_DESC=Image 1 of multi_image_jpeg_2.0.ntf
      SUBDATASET_2_NAME=NITF_IM:1:multi_image_jpeg_2.0.ntf
      SUBDATASET_2_DESC=Image 2 of multi_image_jpeg_2.0.ntf
      SUBDATASET_3_NAME=NITF_IM:2:multi_image_jpeg_2.0.ntf
      SUBDATASET_3_DESC=Image 3 of multi_image_jpeg_2.0.ntf

In this case opening “multi_image_jpeg_2.0.ntf” directly will give access to “NITF_IM:0:multi_image_jpeg_2.0.ntf”. To open the others use the corresponding subdataset names. The Subdataset mechanism is generic GDAL concept discussed in the Raster Data Model document.

## Text Segments

NITF files that have text segments (that is segment type TX) will make that information available as metadata in the TEXT domain. The returned metadata will look something like:

    HEADER_0=TE       00020021216151629xxxxxxxxxxxxxxxxxxxxxxxxxxx
    DATA_0=This is test text file 01.

    HEADER_1=TE       00020021216151629xxxxxxxxxxxxxxxxxxxxxxxxxxx
    DATA_1=This is test text file 02.

    HEADER_2=TE       00020021216151629xxxxxxxxxxxxxxxxxxxxxxxxxxx
    DATA_2=This is test text file 03.

    HEADER_3=TE       00020021216151629xxxxxxxxxxxxxxxxxxxxxxxxxxx
    DATA_3=This is test text file 04.

    HEADER_4=TE       00020021216151629xxxxxxxxxxxxxxxxxxxxxxxxxxx
    DATA_4=This is test text file 05.

The argument to DATA_n is the raw text of the n’th (zero based) text segment with no escaping of any kind applied.

The TEXT segment header data is preserved in HEADER_n metadata item. The CreateCopy() method on the NITF driver also supports creating text segments on the output file as long as the input file has metadata in the TEXT domain as defined above.

To add TEXT data to a NITF image, you can also pass creation options in the following format:

    TEXT=HEADER_0=TE       00020021216151629xxxxxxxxxxxxxxxxxxxxxxxxxxx
    TEXT=DATA_0=This is test text file 01.
    TEXT=HEADER_1=TE       00020021216151629xxxxxxxxxxxxxxxxxxxxxxxxxxx
    TEXT=DATA_1=This is test text file 02.

Notice that passing TEXT as creation options will overwrite existing text segment read in the TEXT metadata domain.

## TREs

NITF files with registered (or unregistered?) extensions on the file header, or the referenced image header will make them available in a raw form in metadata via the TRE domain. The TRE domain will hold one metadata item per TRE which will have the name of the TRE as the name, and the data of the TRE as the contents. The data contents will be “backslash escaped” like CGM data above.

In case of multiple occurrences of the same TRE, the second occurrence will be named “TRENAME_2”, the third “TRENAME_3” where TRENAME is the TRE name.

    Metadata (TRE):
      GEOPSB=MAPM  World Geodetic System 1984
                   WGE World Geodetic System 1984
                       WE Geodetic
                          GEODMean Sea
                              MSL 000000000000000
                                                    0000
      PRJPSB=Polar Stereographic
             PG2-00090.00000250000039.99999884000000000000000000000000000000
      MAPLOB=M  0598005958-000003067885.8-000002163353.8

## TREs as xml:TRE

All TREs found in file and matching one of the TRE description of the nitf_spec.xml in GDAL data directory will be reported as XML content in the xml:TRE metadata domain.

    Metadata (xml:TRE):
    <tres>
      <tre name="RSMDCA" location="des TRE_OVERFLOW">
        <field name="IID" value="2_8" />
        <field name="EDITION" value="1101222272-2" />
        <field name="TID" value="1101222272-1" />
        <field name="NPAR" value="06" />
        <field name="NIMGE" value="001" />
        <field name="NPART" value="00006" />
        <repeated name="IMAGE" number="1">
          <group index="0">
            <field name="IID" value="2_8" />
            <field name="NPARI" value="06" />
          </group>
        </repeated>
        <field name="XUOL" value="-2.42965895449297E+06" />
        <field name="YUOL" value="-4.76049894293300E+06" />
        <field name="ZUOL" value="+3.46898407315533E+06" />
        <field name="XUXL" value="+8.90698769551156E-01" />
        <field name="XUYL" value="+2.48664813021570E-01" />
        <field name="XUZL" value="-3.80554217799520E-01" />
        <field name="YUXL" value="-4.54593996792805E-01" />
        <field name="YUYL" value="+4.87215943350720E-01" />
        <field name="YUZL" value="-7.45630553709282E-01" />
        <field name="ZUXL" value="+0.00000000000000E+00" />
        <field name="ZUYL" value="+8.37129879594448E-01" />
        <field name="ZUZL" value="+5.47004172461403E-01" />
    [...]
        <repeated name="DERCOV" number="21">
          <group index="0">
            <field name="DERCOV" value="+5.77388827727787E+04" />
          </group>
    [...]
          <group index="20">
            <field name="DERCOV" value="+1.14369570920252E-02" />
          </group>
        </repeated>
      </tre>
      <tre name="RSMECA" location="des TRE_OVERFLOW">
    [...]
      </tre>
      <tre name="RSMIDA" location="des TRE_OVERFLOW">
    [...]
      </tre>
      <tre name="RSMPCA" location="des TRE_OVERFLOW">
    [...]
      </tre>
    </tres>

## TRE creation from hexadecimal data

TRE data can be added to a newly created NITF file in hexadecimal format to encode binary data such as unsigned int or floating point types. The hexadecimal TRE creation option is supplied as “TRE=HEX/<tre_name>=<hex_tre_data>” or “FILE_TRE=HEX/<tre_name>=<hex_tre_data>

    # Encode "ABC" as 3 bytes of hex data, "414243"
    ds = gdal.GetDriverByName('NITF').Create('/vsimem/file.ntf', 1, 1, options=["TRE=HEX/TSTTRE=414243"])

## Data Extension Segments (xml:DES)

Data Extension Segments (DESs) are user-defined metadata extensions to the NITF format. The DES metadata is available through the xml:DES metadata domain. The xml:DES domain returns an XML string with standard and user-defined DES fields in plain text, and the user-defined DES data as Base64 encoded text. The following is an example XML structure:

    <des_list>
      <des name="TEST">
        <field name="NITF_DESVER" value="02" />
        <field name="NITF_DECLAS" value="U" />
        <field name="NITF_DESCLSY" value="" />
        [...]
        <field name="NITF_DESCTLN" value="" />
        <field name="NITF_DESSHL" value="0004" />
        <field name="NITF_DESSHF" value="ABCD" />
        <field name="NITF_DESDATA" value="MTIzNDU2Nzg5MA==" />
      </des>
    </des_list>

## Raw File / Image Headers

In some cases application may need to recover very specific information from the image or file headers that isn’t normally available as metadata. In this case it is possible to query the “NITF_METADATA” metadata domain. The complete file and image headers will be returned as metadata in base64 encoded format. Something like:

    Metadata (NITF_METADATA):
      NITFFileHeader=002213 TklURjAyLjAwMDEgICAgVTIxN0cwSjA...
      NITFImageSubheader=439 SU1NaXNzaW5nIElEMjUxNTI1NTlaTU...

Note that the ascii encoded numeric values prefixing the base64 encoded header is the length (decoded) in bytes, followed by one space.

## Writing multiple Image Segments

(GDAL >= 3.4)

Writing a NITF file with several image segments is done with the `APPEND_SUBDATASET=YES` creation option but requires some care.

The file must be created with the content of the first image segment with the `NUMI` creation option to specify the total number of image segments and optionally with the `NUMDES` creation option with the number of DE segments. TREs that apply to the whole file should also be specified at that stage. The `APPEND_SUBDATASET` creation option must NOT be specified.

Subsequent images are appended with the `APPEND_SUBDATASET=YES` creation option. The `IDLVL`, `IALVL`, `ILOCROW` and `ILOCCOL` creation option may be specified.

On the last image segment, the DES content, if any must be specified with the `DES` creation option.

Note

The file will only be valid if all NUMI images have been actually written.

Example:

    gdal_translate first_image.tif  dest.tif -co NUMI=3 -co NUMDES=1
    gdal_translate second_image.tif dest.tif -co APPEND_SUBDATASET=YES -co IC=C3 -co IDLVL=2
    gdal_translate third_image.tif  dest.tif -co APPEND_SUBDATASET=YES -co IC=C8 -co IDLVL=3 -co "DES=DES1={des_content}"