7.9.8. Mathematical operations using the field calculator
Within most of the honey bee colony buffers there are also other land cover categories that can be calculated using the following procedure:
- Right-click on bees_colony_locations_buffer.shp in the Map Legend > Open Attribute Table.
- Toggle on the editing mode with the button .
- Open the Field calculator .
- Make sure the checkbox Create a new field is activated and specify an Output field name (e.g. “other”).
- Change the Output field type to decimal.
- Increase the Precision to 3.
- Combine the Attribute Fields and Values and different operators so that the resulting Expression equals 1-("211mean"+"312mean"+"313mean") (see Fig. 116.).
- Click OK to perform the calculation and have a look at the new field created on the right within the attribute table..
- Repeat steps 51 to 56 and add a new field (type decimal) called “sum” that summarizes all 3 land use categories 211, 312 and 313 (the Expression should equal 211mean"+"312mean"+"313mean").
- Toggle off the editing mode
with the button .
The resulting attribute field “other” represents the remaining area not covered by the CLC06 categories 211,312, and 313 relative to the total area of the polygon layer. The field “sum” summarizes the relative area covered by these three land cover categories of interest.
Fig. 116. Calculate the percentage or other landuse categories with the field calculator.