The raster layers in a database, including bitmap segments, are considered to have a single shared georeferencing definition, sometimes referred to as a georeferencing segment. This definition contains the map units string, a transformation from pixel/line coordinates on the raster to georeferenced space, and optionally additional projection parameters related to the particular georeferencing system.
Many of the possible georeferencing map unit values are related to true projections such as Universal Transverse Mercator, or Lambert Conformal Conic. A fully defined projection contains enough information to relate a pixel in the file to a real world location on the Earth.
Other georeferencing systems are not sufficient to relate things to a real world location, but still provide a useful coordinate system. For instance, a database may just be georeferenced in METRES relative to some implied origin. This may be sufficient to relate different databases and perform calculations of area and perimeter, but are not sufficient to establish a location in the world.
A georeferenced image may be georeferenced to a proper projection or to an arbitrary coordinate system. Images can also be ungeoreferenced, which is treated by GeoGateway using the special coordinate system ``PIXEL'', with a unity transformation. The full list of georeferencing systems, including projections, can be found in the Projections documentation. This includes information about the set of projections and their associated parameters, and also discusses the topic of earth models and data.
The transformation between pixel/line coordinates and georeferenced coordinates is stored in terms of the top left corner of the image, and an x and y pixel size. This implies that raster pixels do not have to be square in georeferenced space. However, there is no concept of a rotational coefficient; therefore georeferenced images in a projection are always stored with north at the top.
When accessing georeferenced images that are not north up, GeoGateway must discard the georeferencing information. In some cases, the corner coordinates are represented as a ground control point segment, in order that the image may be warped to north-up representation, and then treated as a georeferenced database. It is intended that this restriction be relaxed in a future release of the GeoGateway library.
Vector layers and ground control point segments also contain projection definitions local to those layers. In these projection definitions there is no transform; however, the map units and projection parameters are kept, in order that the relation between the georeferenced coordinates of the points may be related to real world locations.
See Also: Projections