GDAL Data Model

This document attempts to describe the GDAL data model. That is the types of information that a GDAL data store can contain, and their semantics.

A dataset (represented by the GDALDataset class) is an assembly of related raster bands and some information common to them all. In particular the dataset has a concept of the raster size (in pixels and lines) that applies to all the bands. The dataset is also responsible for the georeferencing transform and coordinate system definition of all bands. The dataset itself can also have associated metadata, a list of name/value pairs in string form.

Note that the GDAL dataset, and raster band data model is loosely based on the OpenGIS Grid Coverages specification.

Dataset coordinate systems are represented as OpenGIS Well Known Text strings. This can contain:

• An overall coordinate system name.
• A geographic coordinate system name.
• A datum identifier.
• An ellipsoid name, semi-major axis, and inverse flattening.
• A prime meridian name and offset from Greenwich.
• A projection method type (ie. Transverse Mercator).
• A list of projection parameters (ie. central_meridian).
• A units name, and conversion factor to meters or radians.
• Names and ordering for the axes.
• Codes for most of the above in terms of predefined coordinate systems from authorities such as EPSG.

For more information on OpenGIS WKT coordinate system definitions, and mechanisms to manipulate them, refer to the osr_tutorial document and/or the OGRSpatialReference class documentation.

The coordinate system returned by GDALDataset::GetProjectionRef() describes the georeferenced coordinates implied by the affine georeferencing transform returned by GDALDataset::GetGeoTransform(). The coordinate system returned by GDALDataset::GetGCPProjection() describes the georeferenced coordinates of the GCPs returned by GDALDataset::GetGCPs().

Note that a returned coordinate system strings of "" indicates nothing is known about the georeferencing coordinate system.

GDAL datasets have two ways of describing the relationship between raster positions (in pixel/line coordinates) and georeferenced coordinates. The first, and most commonly used is the affine transform (the other is GCPs).

The affine transform consists of six coefficients returned by GDALDataset::GetGeoTransform() which map pixel/line coordinates into georeferenced space using the following relationship:

    Xgeo = GT(0) + Xpixel*GT(1) + Yline*GT(2)
    Ygeo = GT(3) + Xpixel*GT(4) + Yline*GT(5)

In case of north up images, the GT(2) and GT(4) coefficients are zero, and the GT(1) is pixel width, and GT(5) is pixel height. The (GT(0),GT(3)) position is the top left corner of the top left pixel of the raster.

Note that the pixel/line coordinates in the above are from (0.0,0.0) at the top left corner of the top left pixel to (width_in_pixels,height_in_pixels) at the bottom right corner of the bottom right pixel. The pixel/line location of the center of the top left pixel would therefore be (0.5,0.5).

A dataset can have a set of control points relating one or more positions on the raster to georeferenced coordinates. All GCPs share a georeferencing coordinate system (returned by GDALDataset::GetGCPProjection()). Each GCP (represented as the GDAL_GCP class) contains the following:

typedef struct
{
    char *pszId; 
    char *pszInfo;
    double  dfGCPPixel;
    double dfGCPLine;
    double dfGCPX;
    double dfGCPY;
    double dfGCPZ;
} GDAL_GCP;

The pszId string is intended to be a unique (and often, but not always numerical) identifier for the GCP within the set of GCPs on this dataset. The pszInfo is usually an empty string, but can contain any user defined text associated with the GCP. Potentially this can also contain machine parsable information on GCP status though that isn't done at this time.

The (Pixel,Line) position is the GCP location on the raster. The (X,Y,Z) position is the associated georeferenced location with the Z often being zero.

The GDAL data model does not imply a transformation mechanism that must be generated from the GCPs ... this is left to the application. However 1st to 5th order polynomials are common.

Normally a dataset will contain either an affine geotransform, GCPs or neither. It is uncommon to have both, and it is undefined which is authoritative.

GDAL metadata is auxiliary format and application specific textual data kept as a list of name/value pairs. The names are required to be well behaved tokens (no spaces, or odd characters). The values can be of any length, and contain anything except an embedded null (ASCII zero).

The metadata handling system is not well tuned to handling very large bodies of metadata. Handling of more than 100K of metadata for a dataset is likely to lead to performance degradation.

Over time there will be some well known names defined with established semantics; however, that has not occurred at this time.

Some formats will support generic (user defined) metadata, while other format drivers will map specific format fields to metadata names. For instance the TIFF driver returns a few information tags as metadata including the date/time field which is returned as:

TIFFTAG_DATETIME=1999:05:11 11:29:56

Metadata is split into named groups called domains, with the default domain having no name (NULL or ""). Some specific domains exist for special purposes. Note that currently there is no way to enumerate all the domains available for a given object, but applications can "test" for any domains they know how to interprete.

The SUBDATASETS domain holds a list of child datasets. Normally this is used to provide pointers to a list of images stored within a single multi image file (such as HDF or NITF). For instance, an NITF with four images might have the following subdataset list.

  SUBDATASET_1_NAME=NITF_IM:0:multi_1b.ntf
  SUBDATASET_1_DESC=Image 1 of multi_1b.ntf
  SUBDATASET_2_NAME=NITF_IM:1:multi_1b.ntf
  SUBDATASET_2_DESC=Image 2 of multi_1b.ntf
  SUBDATASET_3_NAME=NITF_IM:2:multi_1b.ntf
  SUBDATASET_3_DESC=Image 3 of multi_1b.ntf
  SUBDATASET_4_NAME=NITF_IM:3:multi_1b.ntf
  SUBDATASET_4_DESC=Image 4 of multi_1b.ntf
  SUBDATASET_5_NAME=NITF_IM:4:multi_1b.ntf
  SUBDATASET_5_DESC=Image 5 of multi_1b.ntf

The value of the _NAME is the string that can be passed to GDALOpen() to access the file. The _DESC value is intended to be a more user friendly string that can be displayed to the user in a selector.

Metadata in the default domain is intended to be related to the image, and not particularly related to the way the image is stored on disk. That is, it is suitable for copying with the dataset when it is copied to a new format. Some information of interest is closely tied to a particular file format and storage mechanism. In order to prevent this getting copied along with datasets it is placed in a special domain called IMAGE_STRUCTURE that should not normally be copied to new formats.

One item that appears in the IMAGE_STRUCTURE domain is the compression scheme used for a fromat. The metadata item name is COMPRESSION but the value can be format specific.

Any domain name prefixed with "xml:" is not normal name/value metadata. It is a single XML document stored in one big string.

A raster band is represented in GDAL with the GDALRasterBand class. It represents a single raster band/channel/layer. It does not necessarily represent a whole image. For instance, a 24bit RGB image would normally be represented as a dataset with three bands, one for red, one for green and one for blue.

A raster band has the following properties:

• A width and height in pixels and lines. This is the same as that defined for the dataset, if this is a full resolution band.

• A datatype (GDALDataType). One of Byte, UInt16, Int16, UInt32, Int32, Float32, Float64, and the complex types CInt16, CInt32, CFloat32, and CFloat64.

• A block size. This is a preferred (efficient) access chunk size. For tiled images this will be one tile. For scanline oriented images this will normally be one scanline.

• A list of name/value pair metadata in the same format as the dataset, but of information that is potentially specific to this band.

• An optional description string.

• An optional list of category names (effectively class names in a thematic image).

• An optional minimum and maximum value.

• An optional offset and scale for transforming raster values into meaning full values (ie translate height to meters)

• An optional raster unit name. For instance, this might indicate linear units for elevation data.

• A color interpretation for the band. This is one of:
  • GCI_Undefined: the default, nothing is known.
  • GCI_GrayIndex: this is an independent grayscale image
  • GCI_PaletteIndex: this raster acts as an index into a color table
  • GCI_RedBand: this raster is the red portion of an RGB or RGBA image
  • GCI_GreenBand: this raster is the green portion of an RGB or RGBA image
  • GCI_BlueBand: this raster is the blue portion of an RGB or RGBA image
  • GCI_AlphaBand: this raster is the alpha portion of an RGBA image
  • GCI_HueBand: this raster is the hue of an HLS image
  • GCI_SaturationBand: this raster is the saturation of an HLS image
  • GCI_LightnessBand: this raster is the hue of an HLS image
  • GCI_CyanBand: this band is the cyan portion of a CMY or CMYK image
  • GCI_MagentaBand: this band is the magenta portion of a CMY or CMYK image
  • GCI_YellowBand: this band is the yellow portion of a CMY or CMYK image
  • GCI_BlackBand: this band is the black portion of a CMYK image.

• A color table, described in more detail later.

• Knowledge of reduced resolution overviews (pyramids) if available.

A color table consists of zero or more color entries described in C by the following structure:

typedef struct
{
    /- gray, red, cyan or hue -/
    short      c1;

    /- green, magenta, or lightness -/    
    short      c2;

    /- blue, yellow, or saturation -/
    short      c3;

    /- alpha or blackband -/
    short      c4;      
} GDALColorEntry;

The color table also has a palette interpretation value (GDALPaletteInterp) which is one of the following values, and indicates how the c1/c2/c3/c4 values of a color entry should be interpreted.

• GPI_Gray: Use c1 as grayscale value.
• GPI_RGB: Use c1 as red, c2 as green, c3 as blue and c4 as alpha.
• GPI_CMYK: Use c1 as cyan, c2 as magenta, c3 as yellow and c4 as black.
• GPI_HLS: Use c1 as hue, c2 as lightness, and c3 as saturation.

To associate a color with a raster pixel, the pixel value is used as a subscript into the color table. That means that the colors are always applied starting at zero and ascending. There is no provision for indicating a prescaling mechanism before looking up in the color table.

A band may have zero or more overviews. Each overview is represented as a "free standing" GDALRasterBand. The size (in pixels and lines) of the overview will be different than the underlying raster, but the geographic region covered by overviews is the same as the full resolution band.

The overviews are used to display reduced resolution overviews more quickly than could be done by reading all the full resolution data and downsampling.

Bands also have a HasArbitraryOverviews property which is TRUE if the raster can be read at any resolution efficiently but with no distinct overview levels. This applies to some FFT encoded images, or images pulled through gateways (like OGDI) where downsampling can be done efficiently at the remote point.