I have distribution data of a set of species (plants, mosses and lichens: 13 species total) in the Netherlands. I made a grid that divides the Netherlands into 1x1 km squares. I am not interested in how many points of a single species are within a square. I used the TomBio addon to transform the species point data into 1x1 km squares, so that they match the grid (i.e. ten points of a species in a gridcell are now one 1x1 km square. Also, one species in a gridcell is now a 1x1 km square).

I want to make an estimate of the distribution of the community these species occur in. Every species has a weighing value related to how much they are bound to the community i want to estimate the distribution of. This means that some species that do occur in the community, but also occur in other communities get a value of 1. Species that occur solely in this community get a value of 10. Of course this is different for each of the 13 species.

Based on the weighing values, i determined a threshold value. i.e, if the sum of weighing values (determined by the species that occur) within a grid cell exceed this threshold, i can assume that the community occurs in that gridcell.

To make the whole story a bit more comprehensible:

Species      Weighing value 
A            2       
B            4
C            6
D            6
E            10

If the sum of the values in a grid is say twelve or more, I assume the community is present there. See below for a heads example of the attribute tables.

How can I use QGIS to calcuate within a gridcell the value of the sum of the appointed weighing values, and if the threshold is exceeded, the gridcell gets a color?

I know how to count the individual blocks (species) in a gridcell, but I havn't worked out a way to include the weighing values in this process.

attribute table example

  • So each gridcell has attribute table fields, with the count of each species in that cell?
    – csk
    Nov 2 '19 at 20:40
  • Yes, thats true. Each grid cell is a seperate polygon if you will. So i can count the numer of points in it, but not the the sum of the values connected to that point. Nov 2 '19 at 20:41
  • 2
    I'm not sure I entirely understand your data structure. Can you add a screenshot of each attribute table to your question? I don't need to see the entire table, just the names of the columns and a couple lines of data in each table.
    – csk
    Nov 2 '19 at 20:46
  • I added it in the main question. I also added a short picture of the 1x1 km grid. Species are from the excel list since qgis keeps crashing when i start the TomBio addon... Nov 3 '19 at 15:18

Use the Field Calculator to add an integer field called "present" with this expression:

"species_ct_A"*"value_A" + "species_ct_B"*"value_B" + "species_ct_C"*"value_C" + "species_ct_D"*"value_D" = 12

This expression will evaluate to either "true" (1) or "false" (0). Cells that meet the threshold value of 12 in the above example will have a "present" value of 1. Cells that don't meet the threshold value will have a "present" value of 0.

If you want cells that meet the threshold value to have a different fill color than cells that don't meet the threshold value, use a categorized style to symbolize the grid layer.

I still don't entirely understand your data setup, so this is a generic solution using placeholder field names. You will need to substitute the actual field names from your data for the placeholders.

Species Count and Weighting Values: The placeholder field name "species_ct_A" represents the counted number of species A in each grid cell. The placeholder "value_A" represents the weighing value for species A. Each species has corresponding fields. I assumed that these values are attribute table fields in the grid layer. If they are fields in a different layer, you'll need to retrieve them from that layer. The easiest way to retrieve those values would be to join the other layer to the main grid layer.

  • If both grid layers have a shared unique ID value, you can do a regular table join. (Layer properties > Joins).
  • If they don't, do a spatial join with the join attributes by location algorithm (Vector menu > Data management), with the geometry predicate equals.

Threshold value: I used a threshold value of 12, as in your example. You can substitute any numerical value here, or a field name if the threshold values are stored in that field.

  • This works for me. Thanks a lot! Nov 5 '19 at 19:32

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