5.6.3 Polygons Over Polygons: Frequency Analysis of Polygons
Level of expertise required for this Chapter: Advanced; general GIS training.
For a GIS crosstabulation to produce statistics how much area of one polygon map appear on each unit of another map, you overlay polygons with polygons (also called 'area crosstabulation'). You overlay a 'thematic map' (e.g. forests) on a 'unit map' (e.g. administrative map: Districts).
1. Preparation of 'thematic map'
You have to prepare your 'thematic map' source:
- Copy DRA file (e.g. forests) to temp DRA file
2. Preparation of 'unit map'
You have to prepare your 'unit map' source:
2a. Copy DRA file (e.g. Districts) 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
This is only required, if units of the 'thematic map' cross boundaries of the 'unit map':
With LUPMIS, use the LUPMIS Drawing Tools > Overlay  > Start  > Overlay polygon files to intersect  > Select 'thematic map' > Again: Overlay polygon files to intersect  > Select 'unit map' >
According to the querry: You might have to > 'Clip to the thematic map' >
> Run  > Save As  > Specify new file name > Ok
According to the complexity of the maps, this might take a long time.
You can now transfer the information of the 'unit map' to the 'thematic map' with the Inherit command:
Utilities > Vector utilities > Actions > Choose DRA file of temporary 'thematic map' (from step 1 or 3) > Open > Transform window: Spatial queries > Inherit attributes from other file > OK >
> Inherit attributes from other file window: Polygons file > Select DRA file of temporary 'unit map' (from step 2) > Open > Tick: Inherit label > OK >
> Save as window: Specify new file name (DRA file)
5. Check the inherited map layer
In Map Maker, check the id and display label of the 'inherited' map layer.
6. Export crosstabulation for all polygons
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').
7. Final frequency distribution in Excel
The next steps are carried out in Excel, explained in Annex 20.2 (Excel: Frequency) for sumation of points and in Annex 20.5 (Excel: Conditional sum) for sumation of areas.