5.6.2 Points Over Polygons: Frequency Analysis of Points
Level of expertise required for this Chapter: Advanced; general GIS training. This Chapter is repeated in 5.1.9 (shortly to be deprecated).
For a GIS crosstabulation to produce statistics how many points occur in particular areas, you overlay points with polygons (also called 'point crosstabulation').
1. Preparation of point map
You have to prepare your point map:
- Copy point DRA file to temp DRA file
2. Preparation of polygon map
You have to prepare your polygon overlay map:
2a. Copy polygon DRA file to temp DRA file
2b. If the overlay information is in the ID field: Copy this temp DRA file to live layer, then Live layer actions > IDs and labels > Amend labels and IDs > OK > Set display labels equal to IDs > OK > Save live layer as... > Same name > Save
You can now transfer the information of the polygon map to the point map with the Inherit command:
Utilities > Vector utilities > Actions > Choose DRA file of points > Open > Transform window: Spatial queries > Inherit attributes from other file > OK >
> Inherit attributes from other file window: Polygons file > Select temp DRA file (from step 2) > Open > Tick: Inherit label > OK >
> Save as window: Specify new file name (DRA file)
4. Export crosstabulation for all points
Utilities > Database utilities > Generate database from DRA > Select file, just created > Open > Generate database window: Tick Display label > OK > Save as window: Specify new file name > Save as type: CSV > Save.
This step is also explained in Chapter 5.6.1 ('Export tabular GIS data: From DRA').
5. Final frequency distribution in Excel
The next steps are carried out in Excel, explained in Annex 20.2 (Excel: Frequency).