In the illustration below, logging roads (lines) and vegetation types (polygons) are overlaid to create a new line feature class. Similarly, you'd overlay watershed boundaries with a vegetation layer to calculate the amount of each vegetation type in each watershed. Following is a chart illustrating the result of this calculation. The total area of each land-use type in the flood zone can be calculated by selecting all polygons within the flood zone (using the Select Layer By Attribute tool, for example) and summarizing the area by land-use type (using the Frequency tool). The FID_flood value indicates whether polygons are outside (-1) or inside the flood zone, and all polygons retain their original land-use category values. The parcels are split where they are crossed by the flood zone boundary, and new polygons created. Because overlay yields such valuable information, it was paramount to the development of GIS.Īn overlay operation is much more than a simple merging of line work all the attributes of the features taking part in the overlay are carried through, as shown in the example below, where parcels (polygons) and flood zones (polygons) are overlaid (using the Union tool) to create a new polygon dataset. To answer such questions before the days of GIS, cartographers would create maps on clear plastic sheets and overlay these sheets on a light table to create a new map of the overlaid data. What wells are within abandoned military bases?.What parcels are within the 100-year floodplain? ("Within" is just another way of saying "on top of.").What land use is on top of what soil type?. One of the most basic questions asked of a GIS is "What's on top of what?" For example:
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |