I want to overlay two grids that show the degree of imperviousness for two different cities in order to see if I can identify any similarities in the spatial distribution. Is there any way to do that? Thanks in advance. I hope I was clear enough .
What are you using to represent impeviousness? Do you have a layer of impervious surfaces? Is this a raster or vector (polygon) file. All GIS will do similar things. If you have access to it, I recommend ArcGIS 10, If not QGIS is a good free alternative. The software's will do what you tell it to do. You just need to know what questions to ask in Google and there will be a forum or tutorial with steps. Are you trying to get a way to measure the surfaces, Area? Spatial arrangements ?
I have a vector file with the the built-up areas. I want to overlay the grid which shows the degree of imperviousness for a city X with the grid which shows the degree of imperviousness for a city Y in order to see if there are any spatial similiarities in what concerns the pattern of urban sprawl. I have acces to ArcGIS but I do not know how to do this, maybe I do not know what to search...
One option would be to load both (shape)files into arcgis.com and use a Compare Web Maps option. By using the slider one can do comparison of the two maps. See for example the map I created (though not eactly the same application as you are dealing with) that shows how a comparison can be made:
If you have vector polygon shape files for built areas X and Y you have to convert the shape files to grids where built up areas have value of 1 and non built up area have a value of 0. Assume that the grid which shows the degree of imperviousness (Im) has values from 1 to 3 then multiplying Grid X with Im give us another new grid (X_Im) do the same for Grid Y give us grid (Y_Im). In that case you can subtract the Grids X_Im and Y_Im areas with zero values are similar areas in degree of imperviousness.
I forgot to say that the above procedure assumes that both areas X and Y have the same geographic location.Otherwise, you have to compare X_Im and Y_Im using statistical procedures using the attributes in the table of each Grid.