Data Overlays GIS data overlays can be conducted for visual purposes only or in order to conduct numaric or spatial analysis. The visual overlays are used to produce maps and other visual presentations, hence general graphic and cartographic criteria must be applied to achieve a clear, well balanced outcome. The GIS overlays that produce a new data outcome are more complex and require a range of spatial operations that depend upon the data structure and methodological design used in the exercise. The data overlays are of the interest in this lecture and are described below. Vector overlay - methodologically and technically vector overlays are more complex than raster overlays.
Geometry is used to define new objects in a topological sense.
Vector overlays usually produce complex output files with more nodes, arcs and polygons than the original files. When conducting overlays it is often important whether the point of the first layer lies within a polygon of the the second layer. Half line method is often used to solve this problem. The test result shows that if the half line drawn from the point crosses the polygon boundaries odd number of times the point is inside the polygon. If there are even number of crossings the point is outside the polygon.