Using images as a data source for a GIS: Two different approaches
Quadtrees for more efficient integration of images and maps
SPANS is a raster based GIS, in which the implementation uses the quadtree data model. The quadtree approach is a data reduction technique which has reduced the size of raster image files by up to ONE HUNDRED times, and typically will reduce raster files of thematic maps by 3-10 times. The quadtree data structure allows the ease and speed of multiple maps overly associated with the raster approach. But it avoids the main disadvantage of rasters which is the very large files created when high resolution is required.
At a Quadtree level of 15, images of any size up to in excess of 32,000 by 32,000 can be converted. Any pixel size can be specified and this dimension can be set to the same size can be specified and this dimension can be set to the same size as the smallest quadtree cell. Any number of classes can ocur on the image, also to a maximum of 32000. Images processed on any image analysis system can be read by SPANS.
Ideally users would process images on any image analysis system which they prefer and then imaport a geometrically corrected, classified image maps into the GIS. The approach does require as a minimum that imagery has been corretly positioned on the Earth's surface and is expressed in a specified projection.
Identification of training areas is crucial to accureate interpretation of imagery. In SPANS the operator can use various map analysis functions or screen drawing techniques to isolate training areas from existing maps. These areas can be transferred to an image analysis system as either vector or raster files. The advantage of using a GIS for this step is that the precise nature of conditions on the ground is known.
SPANS is able to freely convert between vector, raster and quadtree file types. By allowing the operator this freedom, map images can be properly and fully utilized as data input to GIS. Spatial analysis functions can be used to assist in the interpretation of the map image; the need for manual intervention is minimised and full advantage is taken of the digital products available in both raster and vector modes
Summary
Vector and raster data structures in GIS have produced significantly different approaches to the use of images, perhaps much more than is commonly realised.
The image, once geometrically corrected, is a primary geographic data source, Image analysis systems are also required to classify, and position the image, the two essential steps in the conversion of an image to a map.
Vector based GIS, which are widely used for cartographic purposes, use the image map as a back drop or visual reference for amendment of digital vector maps. Raster based GIS offer much greater versatility in the use of image maps. The classified image can be directly imported as a map for use with other digital maps of the same area.
The Quaadtree implementation of raster GIS produces any advantages particularly in file size reduction, very fast operating times for multiple map overlay operations and almost instantaneous data base queries. For cartographic production rasters can be transformed to vector format.
