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GML for Geographic Imagery

Perhaps you thought GML was only for vectors? Well think again. A new specification recently endorsed at the OGC (Open Geospatial Consortium) takes GML solidly into the world of imagery.

GML Coverages:

Of course, you may already be aware that GML provides imaging support via GML coverages. Loosely speaking a coverage is a sort of generalization of a geographc image – meaning it specifies the geometry of the coverage in some geographic space (e.g. some part of the earth's surface) and defines a function on that space. In image terms you can think in terms of the geometry as the image "grid" and the function as the radiometry or pixel values. The geometry can be a gridded structure (like most raster images) or can be a tesselated field, collections of polygons, a set of curves or curve segments or just a random collection of points. The function provides values at these points – such as soil samples (random point collection), road surface type (curve segments), crop type (polygons) and brightness (gridded image).

How does this work in GML? Let us consider the gridded case. Pretty much as you would expect. GML just describes the structure of the GRID – the origin, deltaX, deltaY type of thing – actual points of the grid do not appear in GML. So the GML part of the geometry description is TINY.

GML also describes the function or value part of the coverage – in gridded terms – this means what the "pixels" mean – e.g. are they elevation (in meters)? reflectivity? etc. So this part is also TINY.

GML can allow you to define ANY parameters you like including their units of measure. So you can easily construct image descriptions for fancy satellite images with multiple bands or image structures like stereo pairs or triangulation blocks – in fact these specific image types will emerge from the next phase of the specification.

GML in JPEG 2000: (GMLJP2)

So how does this all work with JPEG 2000? What is JPEG 2000 anyways? JPEG 2000 ( is a powerful image specifcation that enables both lossy and lossless compression and enables the handling of multi-gigabyte images – even via the Internet.

A neat thing about JPEG 2000 is that it allows the use of XML as part of the image description. For Geographic Images this means using GML as the natural vehicle. The idea, originally suggested by Lucio Colaiacomo has now been developed into a specification. With GML in JPEG 2000 (now called GMLJP2) we can stuff the GML description INSIDE the JPEG 2000 image to create a JPEG 2000 geographic image. This is like GeoTiff but enhanced, since not only is the geometry handled, but we can also describe the value side (radiometry for you image types) – moreover you can embedd vector annotation, coordinate reference system descriptions, units of measure and even extracted or critical features INTO the image. Never again need an image be lost for the want of data about the image – it is right there INSIDE it.

Of course this would be no good without the software and LizardTech, ERM and others have stepped up to the plate already with supporting software. Also the imaging vendors have gotten into the act – with participation by SPOT Image and others. Once you can read ONE of these "formats" (see article below why GML is NOT a format) – you can read them ALL.

This is a major step forward for geographic imaging.

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