The values can be of any length, and contain anything except an embedded null (ASCII zero). The names are required to be well behaved tokens (no spaces, or odd characters). GDAL metadata is auxiliary format and application specific textual data kept as a list of name/value pairs. It is uncommon to have both, and it is undefined which is authoritative. Normally a dataset will contain either an affine geotransform, GCPs or neither. However 1st to 5th order polynomials are common. The GDAL data model does not imply a transformation mechanism that must be generated from the GCPs. The (X,Y,Z) position is the associated georeferenced location with the Z often being zero. The (Pixel,Line) position is the GCP location on the raster. Potentially this can also contain machine parsable information on GCP status though that isn't done at this time. The pszInfo is usually an empty string, but can contain any user defined text associated with the 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 affine transform consists of six coefficients returned by GDALDataset::GetGeoTransform() which map pixel/line coordinates into georeferenced space using the following relationship: The first, and most commonly used is the affine transform (the other is GCPs). GDAL datasets have two ways of describing the relationship between raster positions (in pixel/line coordinates) and georeferenced coordinates. Note that a returned coordinate system strings of "" indicates nothing is known about the georeferencing coordinate system. The coordinate system returned by GDALDataset::GetGCPProjection() describes the georeferenced coordinates of the GCPs returned by GDALDataset::GetGCPs(). The coordinate system returned by GDALDataset::GetProjectionRef() describes the georeferenced coordinates implied by the affine georeferencing transform returned by GDALDataset::GetGeoTransform(). central_meridian).Ī units name, and conversion factor to meters or radians.Ĭodes for most of the above in terms of predefined coordinate systems from authorities such as EPSG.įor 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. Transverse Mercator).Ī list of projection parameters (i.e. This can contain:Īn ellipsoid name, semi-major axis, and inverse flattening.Ī prime meridian name and offset from Greenwich.Ī projection method type (i.e. Coordinate System ĭataset coordinate systems are represented as OpenGIS Well Known Text strings. Note that the GDAL dataset, and raster band data model is loosely based on the OpenGIS Grid Coverages specification. The dataset itself can also have associated metadata, a list of name/value pairs in string form. The dataset is also responsible for the georeferencing transform and coordinate system definition of all bands. In particular the dataset has a concept of the raster size (in pixels and lines) that applies to all the bands. Dataset Ī dataset (represented by the GDALDataset class) is an assembly of related raster bands and some information common to them all. That is the types of information that a GDAL data store can contain, and their semantics. This document attempts to describe the GDAL data model.
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