Union
Introduction
This geoprocess is similar to the “Intersection” and "Difference" geoprocesses in that it operates on two polygon, line or point layers to obtain their intersections (this is why these three geoprocesses are known as “overlay geoprocesses”).
The "Union" geoprocess is known as "Spatial OR", because the result layer is made up of the geometries which appear in the two layers (intersections between the polygons, lines or points), plus the geometries which only appear in one of the two associated layers.
This means that the geoprocess carries out three analyses:
the first time it calculates the intersection of both layers, the second time it calculates differences between the first layer and the second, and the third time it calculates the differences between the second layer and the first.
This geoprocess may be of interest if you wish to generate new layers which show the occurrence of two phenomena so that the occurrence of one of the two or of both is highlighted.
Running the 'Union' geoprocess
If you select the “Union” option, the following dialogue appears:
When you have selected the input layer, the clip layer and an output layer, click on "Ok".
The result layer will have all the intersections and differences between the two layers. If you click on the “Information” button and then on the different polygons in the result layer, you will see that the intersections have all the attributes, whilst the differences only have the attributes of the layer that created them.
