Geodetic TIFF grids (GTG)¶

New in version 7.0.

Introduction¶

The Geodetic TIFF grid format has been introduced per PROJ RFC 4: Remote access to grids and GeoTIFF grids. It is a profile of the TIFF and GeoTIFF formats that addresses the specific requirements of geodetic grids: horizontal shifts, vertical shifts, velocity grids, etc… It also follows the Cloud Optimized GeoTIFF principles.

Such grids are available on a CDN of GeoTIFF grids.

General description¶

The general principles that guide the following requirements and recommendations are such that files will be properly recognized by PROJ, and also by GDAL which is an easy way to inspect such grid files:

• TIFF 6.0 based (could possibly be BigTIFF without code changes, if we ever need some day to handle grids larger than 4GB)

• GeoTIFF 1.1 for the georeferencing. GeoTIFF 1.1 is a recent standard, compared to the original GeoTIFF 1.0 version, but its backward compatibility is excellent, so that should not cause much trouble to readers that are not official GeoTIFF 1.1 compliant.

• Files hosted on the CDN will use a Geographic 2D CRS for the GeoTIFF GeoKeys. That CRS is intended to be the interpolation CRS as defined in OGC Abstract Specification Topic 2, that is the CRS to which grid values are referred to.

Given that they will nominally be related to the EPSG dataset, the GeodeticCRSGeoKey will be used to store the EPSG code of the CRS. If the CRS cannot be reliably encoded through that key or other geokeys, the interpolation_crs_wkt metadata item detailed afterwards should be used.

This CRS will be generally the source CRS (for geographic to geographic horizontal shift grids, or geographic to vertical shift grids), but for vertical to vertical CRS adjustment, this will be the geographic CRS to which the grid is referenced. In some very rare cases of geographic to vertical shift grids, the interpolation CRS might be a geographic CRS that is not the same as the source CRS (into which ellipsoidal height are expressed). The only instance we have in mind is for the EPSG:7001 “ETRS89 to NAP height (1)” transformation using the naptrans2008 VDatum-grid which is referenced to Amersfoort EPSG:4289 instead of ETRS89…

On the reading side, PROJ will ignore that information: the CRS is already stored in the source_crs or interpolation_crs column of the grid_transformation table.

For geographic to vertical shift files (geoid models), the GeoTIFF 1.1 convention will be used to store the value of the VerticalGeoKey So a geoid model that apply to WGS 84 EPSG:4979 will have GeodeticCRSGeoKey = 4326 and VerticalGeoKey = 4979.

• Files hosted on the CDN will use the GeoTIFF defined ModelTiepointTag and ModelPixelScaleTag TIFF tags to store the coordinates of the upper-left pixel and the resolution of the pixels. On the reading side, they will be required and ModelTransformationTag will be ignored.

Note

Regarding anti-meridian handling, a variety of possibilities exist. We do not attempt to standardize this and filesh hosted on the CDN will use a georeferencing close to the original data producer. For example, NOAA vertical grids that apply to Conterminous USA might even have a top-left longitude beyond 180 (for consistency with Alaska grids, whose origin is < 180) Anti-meridian handling in PROJ has probably issues. This RFC does not attempt to address them in particular, as they are believed to be orthogonal to the topics it covers, and being mostly implementation issues.

• Files hosted on the CDN will use the GTRasterTypeGeoKey = PixelIsPoint convention. This is the convention used by most existing grid formats currently. Note that GDAL typically use a PixelIsArea convention (but can handle both conventions), so the georeferencing it displays when opening a .gsb or .gtx file appears to have a half-pixel shift regarding to the coordinates stored in the original grid file. On the reading side, PROJ will accept both conventions (for equivalent georeferencing, the value of the origin in a PixelIsArea convention is shifted by a half-pixel towards the upper-left direction). Unspecified behavior if this GeoKey is absent.

• Files hosted on the CDN will be tiled, presumably with 256x256 tiles (small grids that are smaller than 256x256 will use a single strip). On the reading side, PROJ will accept TIFF files with any strip or tile organization. Tiling is expressed by specifying the TileWidth, TileHeight, TileOffsets and TileByteCounts tags. Strip organization is expressed by specifying the RowsPerStrip, StripByteCounts and StripOffsets tags.

• Files hosted on the CDN will use Compression = DEFLATE or LZW (to be determined, possibly with Predictor = 2 or 3) On the reading side, PROJ will accept TIFF files with any compression method (appropriate for the data types and PhotometricInterpretation considered) supported by the libtiff build used by PROJ. Of course uncompressed files will be supported.

• Files hosted on the CDN will use little-endian byte ordering. On the reading side, libtiff will transparently handle both little-endian and big-endian ordering.

• Files hosted on the CDN will use PlanarConfiguration=Separate. The tools described in a later section will order blocks so that blocks needed for a given location are close to each other. On the reading side, PROJ will handle also PlanarConfiguration=Contig.

• Files hosted on the CDN will generally use Float32 (BitsPerSample=32 and SampleFormat=IEEEFP) Files may be created using Signed Int 16 ( BitsPerSample =16 and SampleFormat = INT), Unsigned Int 16 (BitsPerSample=16 and SampleFormat=UINT), Signed Int 32 or Unsigned Int 32 generally with an associate scale/offset. On the reading side, only those three data types will be supported as well.

• Files hosted on the CDN will have a PhotometricInterpretation = MinIsBlack. It will be assumed, and ignored on the reading side.

• Files hosted on the CDN will nominally have:

• SamplesPerPixel = 2 for horizontal shift grid, with the first sample being the longitude offset and the second sample being the latitude offset.

• SamplesPerPixel = 1 for vertical shift grids.

• SamplesPerPixel = 3 for deformation models combining horizontal and vertical adjustments.

And even for the current identified needs of horizontal or vertical shifts, more samples may be present (to indicate for example uncertainties), but will be ignored by PROJ.

The ExtraSamples tag should be set to a value of SamplesPerPixel - 1 (given the rules that apply for PhotometricInterpretation = MinIsBlack)

• The ImageDescription tag may be used to convey extra information about the name, provenance, version and last updated date of the grid. Will be set when possible fo files hosted on the CDN. Ignored by PROJ.

• The Copyright tag may be used to convey extra information about the copyright and license of the grid. Will be set when possible fo files hosted on the CDN. Ignored by PROJ.

• The DateTime tag may be used to convey the date at which the file has been created or converted. In case of a file conversion, for example from NTv2, this will be the date at which the conversion has been performed. The ImageDescription tag however will contain the latest of the CREATED or UPDATED fields from the NTv2 file. Will be set when possible fo files hosted on the CDN. Ignored by PROJ.

• Files hosted on the CDN will use the GDAL_NODATA tag to encode the value of the nodata / missing value, when it applies to the grid.

If offset and/or scaling is used, the nodata value corresponds to the raw value, before applying offset and scaling. The value found in this tag, if present, will be honoured (to the extent to which current PROJ code makes use of nodata). For floating point data, writers are strongly discouraged to use non-finite values (+/- infinity, NaN) of nodata to maximimize interoperability. The GDAL_NODATA value applies to all samples of a given TIFF IFD.

• Files hosted on the CDN will use the GDAL_METADATA tag to encode extra metadata not supported by baseline or extended TIFF.

• The root XML node should be GDALMetadata

• Zero, one or several child XML nodes Item may be present.

• A Item should have a name attribute, and a child text node with its value. role and sample attributes may be present for attributes that have a special semantics (recognized by GDAL). The value of sample should be a integer value between 0 and number_of_samples - 1.

• Scale and offset to convert integer raw values to floating point values may be expressed with XML Item elements whose name attribute is respectively SCALE and OFFSET, and their role attribute is respectively scale and offset. The decoded value will be: {offset} + {scale} * raw_value_from_geotiff_file

For a offset value of 1 and scaling of 2, the following payload should be stored:

<GDALMetadata>
<Item name="OFFSET" sample="0" role="offset">1</Item>
<Item name="SCALE" sample="0" role="scale">2</Item>

• The type of the grid must be specified with a Item whose name is set to TYPE.

Values recognized by PROJ currently are:

• HORIZONTAL_OFFSET: implies the presence of at least two samples. The first sample must contain the latitude offset and the second sample must contain the longitude offset. Corresponds to PROJ Horizontal grid shift method.

• VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL: implies the presence of at least one sample. The first sample must contain the vertical adjustment. Must be used when the source/interpolation CRS is a Geographic CRS and the target CRS a Vertical CRS. Corresponds to PROJ Vertical grid shift method.

• VERTICAL_OFFSET_VERTICAL_TO_VERTICAL: implies the presence of at least one sample. The first sample must contain the vertical adjustment. Must be used when the source and target CRS are Vertical CRS. Corresponds to PROJ Vertical grid shift method.

• GEOCENTRIC_TRANSLATION: implies the presence of at least 3 samples. The first 3 samples must be respectively the geocentric adjustments along the X, Y and Z axis. Must be used when the source and target CRS are geocentric CRS. The interpolation CRS must be a geographic CRS. Corresponds to PROJ Geocentric grid shift method.

• VELOCITY: implies the presence of at least 3 samples. The first 3 samples must be respectively the velocities along the E(ast), N(orth), U(p) axis in the local topocentric coordinate system. Corresponds to PROJ Kinematic datum shifting utilizing a deformation model method.

• DEFORMATION_MODEL: implies the presence of the DISPLACEMENT_TYPE and UNCERTAINTY_TYPE metadata items. Corresponds to PROJ Multi-component time-based deformation model method.

For example:

<Item name="TYPE">HORIZONTAL_OFFSET</Item>

• The description of each sample must be specified with a Item whose name attribute is set to DESCRIPTION and role attribute to description.

Values recognized by PROJ for this Item are currently:

• latitude_offset: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be the value to add a latitude expressed in the CRS encoded in the GeoKeys to obtain a latitude value expressed in the target CRS.

• longitude_offset: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be the value to add a longitude expressed in the CRS encoded in the GeoKeys to obtain a longitude value expressed in the target CRS.

• geoid_undulation: valid for TYPE=VERTICAL_OFFSET_GEOGRAPHIC_TO_VERTICAL. For a source CRS being a geographic CRS and a target CRS being a vertical CRS, sample values should be the value to add to a geoid-related height (that is expressed in the target CRS) to get an ellipsoidal height (that is expressed in the source CRS), also called the geoid undulation. Note the possible confusion related to what is the source CRS and target CRS and the semantics of the value stored (to convert from the source to the target, one must subtract the value contained in the grid). This is the convention used by the EPSG:9665 operation method.

• vertical_offset: valid for TYPE=VERTICAL_OFFSET_VERTICAL_TO_VERTICAL. For a source and target CRS being vertical CRS, sample values should be the value to add to an elevation expressed in the source CRS to obtain a longitude value expressed in the target CRS.

• x_translation / y_translation / z_translation: valid for TYPE=GEOCENTRIC_TRANSLATION. Sample values should be the value to add to the input geocentric coordinates expressed in the source CRS to geocentric coordinates expressed in the target CRS.

• east_velocity / north_velocity / up_velocity: valid for TYPE=VELOCITY. Sample values should be the velocity in a linear/time unit in a ENU local topocentric coordinate system.

• east_offset / north_offset / vertical_offset: valid for TYPE=DEFORMATION_MODEL. For east_offset and north_offset, the unit might be degree or metre. For vertical_offset, the unit must be metre.

For example:

<Item name="DESCRIPTION" sample="0" role="description">latitude_offset</Item>
<Item name="DESCRIPTION" sample="1" role="description">longitude_offset</Item>


Other values may be used (not used by PROJ):

• latitude_offset_accuracy: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be the accuracy of corresponding latitude_offset samples. Generally in metre (if converted from NTv2)

• longitude_offset_accuracy: valid for TYPE=HORIZONTAL_OFFSET. Sample values should be the accuracy of corresponding longitude_offset samples. Generally in metre (if converted from NTv2)

• The sign convention for the values of the longitude_offset channel should be indicated with an Item named positive_value whose value can be west or east. NTv2 products originally use a west convention, but when converting from them to GeoTIFF, the sign of those samples will be inverted so they use a more natural east convention. If this item is absent, the default value is east.

• The unit of the values stored in the grid must be specified for each sample through an Item of name UNITTYPE and role unittype Valid values should be the name of entries from the EPSG unitofmeasure table. To maximize interoperability, writers are strongly encouraged to limit themselves to the following values:

For linear units:

• metre (default value assumed if absent for vertical shift grid files, and value used for files stored on PROJ CDN)

• US survey foot

For angular units:

• degree

• arc-second (default value assumed if absent for longitude and latitude offset samples of horizontal shift grid files, and value used for files stored on PROJ CDN)

For velocity units:

• millimetres per year

The longitude and latitude offset samples should use the same unit. The geocentric translation samples should use the same unit. The velocity samples should use the same unit.

Example:

<Item name="UNITTYPE" sample="0" role="unittype">arc-second</Item>
<Item name="UNITTYPE" sample="1" role="unittype">arc-second</Item>

• For TYPE=DEFORMATION_MODEL, the type of the displacement must be specified with a Item whose name is set to DISPLACEMENT_TYPE.

The accepted values are: HORIZONTAL, VERTICAL, 3D or NONE

• For TYPE=DEFORMATION_MODEL, the type of the uncertainty must be specified with a Item whose name is set to UNCERTAINTY_TYPE.

The accepted values are: HORIZONTAL, VERTICAL, 3D or NONE

• The target_crs_epsg_code metadata item should be present. For a horizontal shift grid, this is the EPSG code of the target geographic CRS. For a vertical shift grid, this is the EPSG code of a the target vertical CRS. If the target CRS has no associated EPSG code, target_crs_wkt must be used. Ignored by PROJ currently.

• The target_crs_wkt metadata item must be present if the target_crs_epsg_code cannot be used. Its value should be a valid WKT string according to WKT:2015 or WKT:2019 Ignored by PROJ currently.

• The source_crs_epsg_code metadata item must be present if the source and interpolation CRS are not the same (typical use case is vertical CRS to vertical CRS transformation), because the GeoKeys encode the interpolation CRS and not the source CRS. If the source CRS has no associated EPSG code, source_crs_wkt must be used. Ignored by PROJ currently.

• The source_crs_wkt metadata item must be present if the source_crs_epsg_code cannot be used. Its value should be a valid WKT string according to WKT:2015 or WKT:2019. Ignored by PROJ currently.

• The interpolation_crs_wkt metadata item may be present if the GeoKeys cannot be used to express reliably the interpolation CRS. Its value should be a valid WKT string according to WKT:2015 or WKT:2019. Ignored by PROJ currently.

• The recommended_interpolation_method metadata item may be present to describe the method to use to interpolation values at locations not coincident with nodes stored in the grid file. Potential values: bilinear, bicubic. Ignored by PROJ currently.

• The area_of_use metadata item can be used to indicate plain text information about the area of use of the grid (like “USA - Wisconsin”). In case of multiple subgrids, it should be set only on the first one, but applies to the whole set of grids, not just the first one.

• The grid_name metadata item should be present if there are subgrids for this grid (that is grids whose extent is contained in the extent of this grid), or if this is a subgrid. It is intended to be a relatively short identifier Will be ignored by PROJ (this information can be inferred by the grids extent)

• The parent_grid_name metadata item should be present if this is a subgrid and its value should be equal to the paren’s grid_name Will be ignored by PROJ (this information can be inferred by the grids extent)

• The number_of_nested_grids metadata item should be present if there are subgrids for this grid (that is grids whose extent is contained in the extent of this grid). Will be ignored by PROJ (this information can be inferred by the grids extent)

Example¶

$tiffinfo ntf_r93.tif TIFF Directory at offset 0x4e (78) Image Width: 156 Image Length: 111 Bits/Sample: 32 Sample Format: IEEE floating point Compression Scheme: AdobeDeflate Photometric Interpretation: min-is-black Extra Samples: 3<unspecified, unspecified, unspecified> Samples/Pixel: 4 Rows/Strip: 111 Planar Configuration: separate image planes ImageDescription: NTF (EPSG:4275) to RGF93 (EPSG:4171). Converted from ntf_r93.gsb (version IGN07_01, last updated on 2007-10-31) DateTime: 2019:12:09 00:00:00 Copyright: Derived from work by IGN France. Open License https://www.etalab.gouv.fr/wp-content/uploads/2014/05/Open_Licence.pdf Tag 33550: 0.100000,0.100000,0.000000 Tag 33922: 0.000000,0.000000,0.000000,-5.500000,52.000000,0.000000 Tag 34735: 1,1,1,3,1024,0,1,2,1025,0,1,2,2048,0,1,4275 Tag 42112: <GDALMetadata> <Item name="grid_name">FRANCE</Item> <Item name="target_crs_epsg_code">4171</Item> <Item name="TYPE">HORIZONTAL_OFFSET</Item> <Item name="UNITTYPE" sample="0" role="unittype">arc-second</Item> <Item name="DESCRIPTION" sample="0" role="description">latitude_offset</Item> <Item name="positive_value" sample="1">east</Item> <Item name="UNITTYPE" sample="1" role="unittype">arc-second</Item> <Item name="DESCRIPTION" sample="1" role="description">longitude_offset</Item> <Item name="UNITTYPE" sample="2" role="unittype">arc-second</Item> <Item name="DESCRIPTION" sample="2" role="description">latitude_offset_accuracy</Item> <Item name="UNITTYPE" sample="3" role="unittype">arc-second</Item> <Item name="DESCRIPTION" sample="3" role="description">longitude_offset_accuracy</Item> </GDALMetadata> Predictor: floating point predictor 3 (0x3)  $ listgeo ntf_r93.tif

Geotiff_Information:
Version: 1
Key_Revision: 1.1
Tagged_Information:
ModelTiepointTag (2,3):
0                 0                 0
-5.5              52                0
ModelPixelScaleTag (1,3):
0.1               0.1               0
End_Of_Tags.
Keyed_Information:
GTModelTypeGeoKey (Short,1): ModelTypeGeographic
GTRasterTypeGeoKey (Short,1): RasterPixelIsPoint
GeodeticCRSGeoKey (Short,1): Code-4275 (NTF)
End_Of_Keys.
End_Of_Geotiff.

GCS: 4275/NTF
Datum: 6275/Nouvelle Triangulation Francaise
Ellipsoid: 7011/Clarke 1880 (IGN) (6378249.20,6356515.00)
Prime Meridian: 8901/Greenwich (0.000000/  0d 0' 0.00"E)
Projection Linear Units: User-Defined (1.000000m)

Corner Coordinates:
Upper Left    (  5d30' 0.00"W, 52d 0' 0.00"N)
Lower Left    (  5d30' 0.00"W, 40d54' 0.00"N)
Upper Right   ( 10d 6' 0.00"E, 52d 0' 0.00"N)
Lower Right   ( 10d 6' 0.00"E, 40d54' 0.00"N)
Center        (  2d18' 0.00"E, 46d27' 0.00"N)

\$ gdalinfo ntf_r93.tif

Driver: GTiff/GeoTIFF
Files: ntf_r93.tif
Size is 156, 111
Coordinate System is:
GEOGCRS["NTF",
DATUM["Nouvelle Triangulation Francaise",
ELLIPSOID["Clarke 1880 (IGN)",6378249.2,293.466021293627,
LENGTHUNIT["metre",1]]],
PRIMEM["Greenwich",0,
ANGLEUNIT["degree",0.0174532925199433]],
CS[ellipsoidal,2],
AXIS["geodetic latitude (Lat)",north,
ORDER[1],
ANGLEUNIT["degree",0.0174532925199433]],
AXIS["geodetic longitude (Lon)",east,
ORDER[2],
ANGLEUNIT["degree",0.0174532925199433]],
ID["EPSG",4275]]
Data axis to CRS axis mapping: 2,1
Origin = (-5.550000000000000,52.049999999999997)
Pixel Size = (0.100000000000000,-0.100000000000000)
AREA_OR_POINT=Point
grid_name=FRANCE
target_crs_epsg_code=4171
TIFFTAG_DATETIME=2019:12:09 00:00:00
TIFFTAG_IMAGEDESCRIPTION=NTF (EPSG:4275) to RGF93 (EPSG:4171). Converted from ntf_r93.gsb (version IGN07_01, last updated on 2007-10-31)
TYPE=HORIZONTAL_OFFSET
COMPRESSION=DEFLATE
INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (  -5.5500000,  52.0500000) (  5d33' 0.00"W, 52d 3' 0.00"N)
Lower Left  (  -5.5500000,  40.9500000) (  5d33' 0.00"W, 40d57' 0.00"N)
Upper Right (  10.0500000,  52.0500000) ( 10d 3' 0.00"E, 52d 3' 0.00"N)
Lower Right (  10.0500000,  40.9500000) ( 10d 3' 0.00"E, 40d57' 0.00"N)
Center      (   2.2500000,  46.5000000) (  2d15' 0.00"E, 46d30' 0.00"N)
Band 1 Block=156x111 Type=Float32, ColorInterp=Gray
Description = latitude_offset
Unit Type: arc-second
Band 2 Block=156x111 Type=Float32, ColorInterp=Undefined
Description = longitude_offset
Unit Type: arc-second
positive_value=east
Band 3 Block=156x111 Type=Float32, ColorInterp=Undefined
Description = latitude_offset_accuracy
Unit Type: arc-second
Band 4 Block=156x111 Type=Float32, ColorInterp=Undefined
Description = longitude_offset_accuracy
Unit Type: arc-second


Multi-grid storage¶

Formats like NTv2 can contain multiple subgrids. This can be transposed to TIFF by using several IFD chained together with the last 4 bytes (or 8 bytes for BigTIFF) of an IFD pointing to the offset of the next one.

The first IFD should have a full description according to the General description. Subsequent IFD might have a more compact description, omitting for example, CRS information if it is identical to the main IFD (which should be the case for the currently envisionned use cases), or Copyright / ImageDescription metadata items.

Each IFD will have its NewSubfileType tag set to 0.

If a low-resolution grid is available, it should be put before subgrids of higher-resolution in the chain of IFD linking. On reading, PROJ will use the value from the highest-resoluted grid that contains the point of interest.

For efficient reading from the network, files hosted on the CDN will use a layout similar to the one described in the low level paragraph of the Cloud Optimized GeoTIFF GDAL driver page

The layout for a file converted from NTv2 will for example be:

• TIFF/BigTIFF header/signature and pointer to first IFD (Image File Directory)

• “ghost area” indicating the generated process

• IFD of the first grid, followed by TIFF tags values, excluding the TileOffsets and TileByteCounts arrays

• IFD of the last grid, followed by TIFF tags values, excluding the GDAL_METADATA tag, TileOffsets and TileByteCounts arrays

• TileOffsets and TileByteCounts arrays for first IFD

• TileOffsets and TileByteCounts arrays for last IFD

• Value of GDAL_METADATA tag for IFDs following the first IFD

• First IFD: Data corresponding to latitude offset of Block_0_0

• First IFD: Data corresponding to longitude offset of Block_0_0

• First IFD: Data corresponding to latitude offset of Block_0_1

• First IFD: Data corresponding to longitude offset of Block_0_1

• First IFD: Data corresponding to latitude offset of Block_n_m

• First IFD: Data corresponding to longitude offset of Block_n_m

• Last IFD: Data corresponding to latitude offset of Block_0_0

• Last IFD: Data corresponding to longitude offset of Block_0_0

• Last IFD: Data corresponding to latitude offset of Block_0_1

• Last IFD: Data corresponding to longitude offset of Block_0_1

• Last IFD: Data corresponding to latitude offset of Block_n_m

• Last IFD: Data corresponding to longitude offset of Block_n_m

If longitude_offset_accuracy and latitude_offset_accuracy are present, this will be followed by:

• First IFD: Data corresponding to latitude offset accuracy of Block_0_0

• First IFD: Data corresponding to longitude offset accuracy of Block_0_0

• First IFD: Data corresponding to latitude offset accuracy of Block_n_m

• First IFD: Data corresponding to longitude offset accuracy of Block_n_m

• Last IFD: Data corresponding to latitude offset accuracy of Block_0_0

• Last IFD: Data corresponding to longitude offset accuracy of Block_0_0

• Last IFD: Data corresponding to latitude offset accuracy of Block_n_m

• Last IFD: Data corresponding to longitude offset accuracy of Block_n_m

Note

TIFF has another mechanism to link IFDs, the SubIFD tag. This potentially enables to define a hierarchy of IFDs (similar to HDF5 groups). There is no support for that in most TIFF-using software, notably GDAL, and no compelling need to have a nested hierarchy, so “flat” organization with the standard IFD chaining mechanism is adopted.

Examples of multi-grid dataset¶

It contains 5 subgrids. All essential metadata to list the subgrids and their georeferencing is contained within the first 3 KB of the file.

The file size is 4.8 MB using DEFLATE compression and floating-point predictor. To be compared with the 83 MB of the original .gsb file.

It contains 114 subgrids. All essential metadata to list the subgrids and their georeferencing is contained within the first 40 KB of the file.