Geo-Processing in QGIS
I’m going to look at the geo-processing tools. The geo-processing tools are found on the Vector menu under Geo-processing tools. These tools do not edit the input tables; instead you are prompted to create a new layer for the results. Therefore the input layers don’t need to be editable. You can choose to carry out the operation on every feature in the chosen layers, or just the selected features. These functions can be combined with attribute updates and calculations to carry out more complex analysis (e.g. calculate proportional overlap) or to count the address points within set distances of proposed new roads.
I will carry out most of the operations on the green square and red circle shown below:-
QGIS map window
Intersect creates a new feature based on the area of overlap (the intersection) between the two layers. The attributes from both source layers are copied to the new feature:-
To calculate the area of overlap, update the newly created feature’s attribute table with its area.
Union creates a new layer that covers the combined features
Symmetrical Difference creates new shapes based on the non overlapping areas of the original features:-
Clip creates a new shape based on the area of the input layer that is overlapped by the clipping layer. It is similar to the intersection but differs in that the attributes of the chosen layer only are copied to the new feature. It is similar to MapInfo’s Erase Outside function.
Difference creates a new feature based on the area of the input layer that isn’t overlapped by the clipping layer. It is similar to MapInfo’s Erase function.
Dissolve breaks apart overlapping regions in the same layer.
Buffer creates a region around each feature in the source layer. I have used buffers to count address points within set distances of new roads, assign address points to local amenity catchment zones etc. In this case I’m going to apply a 100m buffer around overhead electricity lines. These can be downloaded from OS Open Data.:-
- Input vector Layer – the layer that contains the source objects
- Buffer Distance – the distance the buffer will extend from the source objects
- Buffer Distance Field – alternatively QGIS can use a value from a numeric field, this makes drawing variable width buffers for features in the same layer easy e.g. Sites rated High Sensitivity could be updated with a buffer distance of 1,000m, sites rated Medium Sensitivity could be updated with a buffer distance of 500m.
- Dissolve Buffer Results. The default is to combine the buffers into one region. Enabling this creates a separate region for each source object.
