GTI -- GDAL Raster Tile Index

New in version 3.9.

Driver short name

GTI

Driver built-in by default

This driver is built-in by default

Introduction

The GTI driver is a driver that allows to handle catalogs with a large number of raster files (called "tiles" in the rest of this document, even if a regular tiling is not required by the driver), and build a virtual mosaic from them. Each tile may be in any GDAL supported raster format, and be a file stored on a regular filesystem, or any GDAL supported virtual filesystem (for raster drivers that support such files).

This driver offers similar functionality as the VRT driver with the following main differences:

The tiles are listed as features of any GDAL supported vector format. Use of formats with efficient spatial filtering is recommended, such as GeoPackage, FlatGeoBuf or PostGIS. The GTI driver can thus use a larger number of tiles than the VRT driver (hundreds of thousands or more), provided the underlying vector format is efficient.
The tiles may have different SRS. The GTI driver is capable of on-the-fly reprojection
The GTI driver offers control on the order in which tiles are composited, when they overlap (z-order)
The GTI driver honours the mask/alpha band when compositing together overlapping tiles.
Contrary to the VRT driver, the GTI driver does not enable to alter characteristics of referenced tiles, such as their georeferencing, nodata value, etc. If such behavior is desired, the tiles must be for example wrapped individually in a VRT file before being referenced in the GTI index.

Connection strings

The GTI driver accepts different types of connection strings:

a vector file in GeoPackage format with a .gti.gpkg extension, or in FlatGeoBuf format with a .gti.fgb extension, meeting the minimum requirements for a GTI compatible tile index, detailed later.

For example: tileindex.gti.gpkg
any vector file in a GDAL supported format, with its filename (or connection string prefixed with GTI:

For example: GTI:tileindex.shp or GTI:PG:database=my_db schema=tileindex
a XML file, following the below GTI XML format, generally with the recommended .gti extension, referencing a vector file. Using such XML file may be more practical for tile indexes not stored in a file, or if some additional metadata must be defined at the dataset or band level of the virtual mosaic.

For example: tileindex.gti

Tile index requirements

The minimum requirements for a GTI compatible tile index is to be a vector format supported by GDAL, with a geometry column storing polygons with the extent of the tiles, and an attribute field of type string, storing the path to each tile. The default name for this attribute field is location. If relative filenames are stored in the tile index, they are considered to be relative to the path of the tile index.

In addition, for formats that can store layer metadata (GeoPackage, FlatGeoBuf, PostGIS, ...), the following layer metadata items may be set:

RESX=<float> and RESY=<float>: resolution along X and Y axis, in SRS units / pixel.

Setting those metadata items is recommended, otherwise the driver will try to open one of the tiles referenced in the tile index, and use its resolution as the resolution for the mosaic.
BAND_COUNT=<int>: number of bands of the virtual mosaic. The tiles stored in an index should generally have the same number of bands.

Setting that metadata item is recommended, otherwise the driver will try to open one of the tiles referenced in the tile index, and use it as the number of bands for the mosaic.

A mix of tiles with N and N+1 bands is allowed, provided that the color interpretation of the (N+1)th band is alpha. The N+1 value must be written as the band count in that situation.

If tiles contains a single band with a color table, and that the color table may differ among tiles, BAND_COUNT should be set to 3 (resp. 4) to perform expansion to Red, Green, Blue components (resp. Red, Green, Blue, Alpha). If the color table is identical between the tiles, and it is desired to preserve it, the VRRTI XML file format may be used to include the ColorTable element.
DATA_TYPE=<val>: data type of the tiles of the tile index Byte, Int8, UInt16, Int16, UInt32, Int32, UInt64, Int64, Float32, Float64, CInt16, CInt32, CFloat32 or CFloat64

Setting that metadata item is recommended, otherwise the driver will try to open one of the tiles referenced in the tile index, and use it as the data type for the mosaic.
NODATA=<val>[,<val]...: nodata value of the bands of the virtual mosaic.

Note that source tiles may have or may not have a nodata value themselves, and it may be different than the nodata value of the virtual mosaic.
MINX=<float>, MINY=<float>, MAXX=<float> and MAXY=<float>: defines the extent of the virtual mosaic.

For vector formats that have efficient retrieval of the layer extent, setting those items is not needed.
GEOTRANSFORM=<gt0>,<gt1>,<gt2>,<gt3>,<gt4>,<gt5>: defines the GeoTransform. Used together with XSIZE and YSIZE, this is an alternate way of defining the extent and resolution os the virtual mosaic.

It is not necessary to define this item if RESX= and RESY are set (potentially accompanied with MINX, MINY, MAXX and MAXY)
XSIZE=<int>, YSIZE=<int>: size of the virtual mosaic in pixel.
COLOR_INTERPRETATION=<val>[,<val]...: color interpretation of the bands of the mosaic. Possible values are red, green, blue, alpha, undefined
SRS=<string>: defines the SRS of the virtual mosaic, using any value supported by the OGRSpatialReference::SetFromUserInput() call, which includes EPSG Projected, Geographic or Compound CRS (i.e. EPSG:4296), a well known text (WKT) CRS definition, PROJ.4 declarations, etc.

It is not necessary to define this element if the virtual mosaic SRS is recorded as the SRS of the vector layer of the tile index.
LOCATION_FIELD=<string>: name of the field where the tile location is stored. Defaults to location.
SORT_FIELD=<string>: name of a field to use to control the order in which tiles are composited, when they overlap (z-order). That field may be of type String, Integer, Integer64, Date or DateTime. By default, the higher the value in that field, the last the corresponding tile will be rendered in the virtual mosaic (unless SORT_FIELD_ASC=NO is set)
SORT_FIELD_ASC=YES|NO: whether the values in SORT_FIELD should be sorted in ascendent or descent order. Defaults to YES (ascendent)
BLOCKXSIZE=<int> and BLOCKYSIZE=<int>: Block size of bands of the virtual mosaic. Defaults to 256x256.
MASK_BAND=YES|NO: whether a dataset-level mask band should be exposed. Defaults to NO.
RESAMPLING=<val>: Resampling method to use for on-the-fly reprojection, or rendering of tiles whose origin coordinates are not at an offset multiple of the resolution of the mosaic compared to the origin of the mosaic. In that later case, RESAMPLING is only taken into account when requesting pixels with the default nearest resampling mode.

Possible values: nearest, cubic, cubicspline, lanczos, average, rms, gauss

Defaults to nearest
BAND_<number>_OFFSET=<val> where number is an integer index starting at 1.

Additive offset to apply to the raw numbers of the band.
BAND_<number>_SCALE=<val> where number is an integer index starting at 1.

Multiplicative factor to apply to the raw numbers of the band.
BAND_<number>_UNITTYPE=<val> where number is an integer index starting at 1.

Unit of the band.
OVERVIEW_<idx>_DATASET=<string> where idx is an integer index starting at 0.

Name of the dataset to use as the first overview level. This may be a raster dataset (for example a GeoTIFF file, or another GTI dataset). This may also be a vector dataset with a GTI compatible layer, potentially specified with OVERVIEW_<idx>_LAYER.
OVERVIEW_<idx>_OPEN_OPTIONS=<key1=value1>[,key2=value2]... where idx is an integer index starting at 0.

Open options(s) to use to open OVERVIEW_<idx>_DATASET.
OVERVIEW_<idx>_LAYER=<string> where idx is an integer index starting at 0.

Name of the vector layer to use as the first overview level, assuming OVERVIEW_<idx>_DATASET points to a vector dataset. OVERVIEW_<idx>_DATASET may also not be specified, in which case the vector dataset of the full resolution virtual mosaic is used.
OVERVIEW_<idx>_FACTOR=<int> where idx is an integer index starting at 0.

Sub-sampling factor, strictly greater than 1. If OVERVIEW_<idx>_DATASET and OVERVIEW_<idx>_LAYER are not specified, then all tiles of the full resolution virtual mosaic are used, with the specified sub-sampling factor (it is recommended, but not required, that those tiles do have a corresponding overview). OVERVIEW_<idx>_DATASET and/or OVERVIEW_<idx>_LAYER may also be specified to point to another tile index.

All overviews must have exactly the same extent as the full resolution virtual mosaic. The GTI driver does not check that, and if that condition is not met, subsampled pixel request will lead to incorrect result.

In addition to those layer metadata items, the dataset-level metadata item TILE_INDEX_LAYER may be set to indicate, for dataset with multiple layers, which one should be used as the tile index layer.

Alternatively to setting those metadata items individually, the corresponding information can be grouped together in a GTI XML document, attached in the xml:GTI metadata domain of the layer (for drivers that support alternate metadata domains such as GeoPackage)

GTI XML format

A XML schema of the GDAL GTI format is available.

The following artificial example contains all potential elements and attributes. A number of them have similar name and same semantics as layer metadata items mentioned in the previous section.

<GDALTileIndexDataset>
    <IndexDataset>PG:dbname=my_db</IndexDataset>   <!-- required for stanalone XML GTI files. Ignored if embedded in the xml:GTI metadata domain of the layer  -->
    <IndexLayer>my_layer</IndexLayer>              <!-- optional, but required if there are multiple layers in IndexDataset -->
    <Filter>pub_date >= '2023/12/01'</Filter>      <!-- optional -->
    <SortField>pub_date</SortField>                <!-- optional -->
    <SortFieldAsc>true</SortFieldAsc>              <!-- optional -->
    <SRS>EPSG:4326</SRS>                           <!-- optional -->
    <ResX>60</ResX>                                <!-- optional, but recommended -->
    <ResY>60</ResY>                                <!-- optional, but recommended -->
    <MinX>0</MinX>                                 <!-- optional -->
    <MinY>1</MinY>                                 <!-- optional -->
    <MaxX>2</MaxX>                                 <!-- optional -->
    <MaxY>3</MaxY>                                 <!-- optional -->
    <GeoTransform>2,1,0,49,0,-1</GeoTransform>     <!-- optional -->
    <XSize>2048</XSize>                            <!-- optional -->
    <YSize>1024</YSize>                            <!-- optional -->
    <BlockXSize>256</BlockXSize>                   <!-- optional -->
    <BlockYSize>256</BlockYSize>                   <!-- optional -->
    <Resampling>Cubic</Resampling>                 <!-- optional -->
    <BandCount>1</BandCount>                       <!-- optional, not needed if Band elements are defined -->

    <!-- Band is optional, but recommended. Repeated as many times as there are bands -->
    <!-- The "band" attribute is required -->
    <!-- The "dataType" attribute is optional, but recommended -->
    <Band band="1" dataType="Byte">
        <Description>my band</Description>         <!-- optional -->
        <Offset>2</Offset>                         <!-- optional -->
        <Scale>3</Scale>                           <!-- optional -->
        <NoDataValue>4</NoDataValue>               <!-- optional -->
        <UnitType>dn</UnitType>                    <!-- optional -->
        <ColorInterp>Gray</ColorInterp>            <!-- optional -->
        <ColorTable>                               <!-- optional -->
            <Entry c1="1" c2="2" c3="3" c4="255"/>
        </ColorTable>
        <CategoryNames>                            <!-- optional -->
            <Category>cat</Category>
        </CategoryNames>
        <GDALRasterAttributeTable><!--... --></GDALRasterAttributeTable>  <!-- optional -->
        <Metadata>                                 <!-- optional -->
            <MDI key="FOO">BAR</MDI>
        </Metadata>
        <Metadata domain="other_domain">           <!-- optional -->
            <MDI key="FOO">BAR</MDI>
        </Metadata>
    </Band>

    <Metadata>                                     <!-- optional -->
        <MDI key="FOO">BAR</MDI>
    </Metadata>
    <Metadata domain="other_domain">               <!-- optional -->
        <MDI key="FOO">BAR</MDI>
    </Metadata>

    <Overview>                                     <!-- optional -->
        <!-- 1st overview level will reuse the tile index of the
             IndexDataset and IndexLayer elements, with all tiles considered
             downsampled by a factor of 2 -->
        <Factor>2</Factor>
    </Overview>
    <Overview>                                     <!-- optional -->
        <!-- 2nd overview level will reuse the tile index of the
             IndexDataset and IndexLayer elements, with all tiles considered
             downsampled by a factor of 4 -->
        <Factor>4</Factor>
    </Overview>
    <Overview>                                     <!-- optional -->
        <!-- 3rd overview level (and potentially 4th, 5th... depending on
             the number of overview levels in the pointed GeoTIFF file)
        -->
        <Dataset>some.tif</Dataset>
    </Overview>
    <Overview>                                     <!-- optional -->
        <!-- Last overview level points to another GTI dataset -->
        <Dataset>other.gti.gpkg</Dataset>
        <Layer>other_layer</Layer>
        <OpenOptions>                              <!-- optional -->
            <OOI key="XMIN">0</OOI>
            <OOI key="YMIN">1</OOI>
            <OOI key="XMAX">2</OOI>
            <OOI key="YMAX">3</OOI>
        </OpenOptions>
    </Overview>

</GDALTileIndexDataset>

At the GDALTileIndexDataset level, the elements specific to GTI XML are:

Filter: value of a SQL WHERE clause, used to select a subset of the features of the index.
BlockXSize / BlockYSize: dimension of the block size of bands. Defaults to 256x256
Metadata: defines dataset-level metadata. You can refer to the documentation of the VRT driver for its syntax.

At the Band level, the elements specific to GTI XML are: Description, Offset, Scale, UnitType, ColorTable, CategoryNames, GDALRasterAttributeTable, Metadata. You can refer to the documentation of the VRT driver for their syntax and semantics.

How to build a GTI comptatible index ?

The gdaltindex program may be used to generate both a vector tile index, and optionally a wrapping .gti XML file.

A GTI comptatible index may also be created by any programmatic means, provided the above format specifications are met.

Open options

The following open options are available. Most of them can be also defined as layer metadata items or in the .gti XML file

LAYER=<string>: For dataset with multiple layers, indicates which one should be used as the tile index layer. Same role as the TILE_INDEX_LAYER dataset level metadata item
LOCATION_FIELD=<string>: Defaults to location. Name of the field where the tile location is stored.
SORT_FIELD=<string>: Name of a field to use to control the order in which tiles are composited, when they overlap (z-order). That field may be of type String, Integer, Integer64, Date or DateTime. By default, the higher the value in that field, the last the corresponding tile will be rendered in the virtual mosaic (unless SORT_FIELD_ASC=NO is set)
SORT_FIELD_ASC=[YES/NO]: Defaults to YES. Whether the values in SORT_FIELD should be sorted in ascendent or descent order
FILTER=<string>: Value of a SQL WHERE clause, used to select a subset of the features of the index.
RESX=<float>: Resolution along X axis in SRS units / pixel.
RESY=<float>: Resolution along Y axis in SRS units / pixel.
MINX=<float>: Minimum X value for the virtual mosaic extent
MINY=<float>: Minimum Y value for the virtual mosaic extent
MAXX=<float>: Maximum X value for the virtual mosaic extent
MAXY=<float>: Maximum Y value for the virtual mosaic extent