- your TK25 quadrants are expressed as a set of polygons in a vector layer
- your occurrences (observations) are expressed as a point layer
- you want to effectively turn your TK25 map into a choropleth
generally, there are many ways to accomplish #3 above. if you’re trying to create just a few maps (for just a few taxa), below is the way i would do it. (for a lot of taxa, you’d probably want to find a more efficient and automated way to do it.)
just to illustrate the general process, i’m going to use Rudbeckia amplexicaulis observations (taxon 200073) from an old copy of the AWS Open Data Set for my point layer, and US counties from the US Census Bureau for my vector layer. here, i’ve already loaded these into a new QGIS project:
at this point, i can filter down my point layer, if needed. (for example, if you had many taxa represented, you could filter for just observations of a specific taxon.) in my case, i have just one taxon in my layer, but i see that it includes both non-research-grade observations. (there shouldn’t be any naturally occurring observation out in California.) so i’d like to filter for just research grade.
in the layers pane in the left side of the QGIS window, right click on your point layer, and select Filter… in the Query Builder window, apply the filters you need. in my case, i’ll add
"quality_grade" = "research" and then click OK.
that should give me a filtered set of observations (and also note the little filter symbol that now shows up next to my point layer):
now i want to figure out which of my polygons in my vector layer contain the observation points. the easiest way to do this is to use Vector > Analysis Tools > Count Points in Polygon… in the pop-up window, i’ll fill in my polygon layer and my points layer, like so:
clicking on Run will give me a new Count vector layer. it’s basically the same as my original vector layer, except it’ll have an additional NUMPOINTS field that represents the count of points in each polygon.
to visualize the count (NUMPOINTS), you’ll want to right-click on the new layer, and select Properties…, and then go to the Symbology tab. at the top of the tab, you’ll initially see Single Symbol. to do a choropleth, you’ll probably want to select Graduated instead. then in the Value field, select the NUMPOINTS field. select a color ramp. if you just want to show absence/presence, set Mode to Equal Counts, with 2 classes:
that will produce a map that looks like this:
you can also produce a more typical choropleth that shows color variation based on number of observations. for example, Gradated, Mode = Equal Interval, and Classes = 5, with a few custom tweaks to the intervals (in the histogram tab), produces a map like this:
when you have a map you like, you can go to Project > Export, and then export the result as you like.