![]() Clearly vector processing in any GIS involves a combination of the geometery and attribute tables. Now enter GRASS in a location with the same projection as the PostGIS data base (typically EPSG:4326) This is very simple.ĪLTER TABLE chiapas.agros ADD COLUMN id int8 As a unique identifier for each polygon is needed during the overlay an index should be added to the table first if the gid is the only unique identifier for the table. The first is that the gid index is lost when the attributes are imported. There are a few issues that have to be addressed when doing this. The simplest way to achieve this result with PostGIS is to import the vector layer into GRASS, where the raster is also held. The map above shows the percentage of the total (forested + non-forested) area of each agricultural unit estimated to have been deforested between 19. Calculating the proportion of the total area of each agricultural unit deforested helps to see the pattern. ![]() Because deforestation in Chiapas has tended to take place on a rather small scale involving only a few hectares at a time, it is not at all easy to see the large scale regional pattern from an image. It overlays a map of agricultural units on a raster layer representing deforestation in the last decade derived from analysis of satelite data. This example taken from our ongoing work. The computations involved in the overlay are then simple and fast. When large raster layers derived from the analysis of satelite imagery are of interest the conventional GIS approach is to first rasterise the vector layer. However that is not the usual way to achieve the operation. ![]() If the raster layer is quite small it could be imported as points and the result achieved through a point in polygon SQL overlay. The results are then added to the attribute table of the vector map. A very common operation in GIS involves overlaying vector polygons on a raster map and calculating the area of each class in the raster map. ![]()
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