Using images as a data source for a GIS: Two different approaches
Limitations of the vector approach
The vector modification approach works well with simple images. However consider typical classified image which purports to show, for example, land use. There are likely to be several thousand polygons even if as few as only five land use classes are identified. Using the vector approach each polygon must be examined separately and the operator must make a decision on boundary definition for each area independently. Nor can the class of each polygon be automatically assigned. The operator must subsequently edit the vector file to add polygon identification and create a database describing each polygon. This can become an extraordinarily tedious process, which frequently takes longer than conventional cartographic methods.
The problems of the vector approach are:
- Conversion from a classified raster to a vector;
- defining polygon boundaries;
- assigning identification to polygons and
- buileding an associated data base.
A Raster Alternative
Consider the possibilities if the user is free of the necessity to amena maps created in vector format. The main objective is conversion of data obtained from imagery into useful maps. A raster based GIS allows direct use of raster images as maps without the intermediate step of creation of vector maps. Without the intermediate step of creation of vector maps. A GIS based on a raster data model gives users a previously unavailable flexibility in integrating imagery and all other digital maps.
First the geometrically corrected, classified image is imported to the GIS as a map. The cell size selected for the raster map should if possible be identical to that of the pixels of which the image is composed. Image classes can be converted directly to map legend items. As classified images usually represent only a single theme this approach is sufficient, but if necessary a database can also be appended containing multiple fields of data for each class. All existing digital maps of the study area can also be contained in a library of raster maps. It is then possible to directly compare, using overly techniques, the image and any of the maps in this library.
Second, if the objective is amendment of an older for example land use map with more current information contained in an image it is possible to use GIS functions to automate this procedure. A two map, or matrix, overlay can be used to identify all locations in which the classes on the existing map and the image are different. Some changes are logically impossible and these can be identified so that the existing map class prevails over the image class. Some images still contain noisy data even after filtering; these areas can be eliminated by specifying size thresholds below which areas will not be recognized. The remaining actual change areas, which will be used to amend the land use map, can then be preserved as a new and separate map and\or integrated with the old map.
Third, if vector output is required, the raster maps can be converted, with or without an associated database, as a last step, to vector format.
