2

Context

I am essentially trying to find the overlapping area between various categories of circles and various categories of polygons.

I looked for a good while at lots of posts on stack exchange and other websites but can't seem to find one that helps with the caveat I have below and the fact that I have categories.

And just as an FYI, I am extremely new to GIS in general and have been going at this problem for a couple days now. I am using QGIS 2.14.15.

Details

First, I have a vector layer with all the counties in Texas. Each county polygon has been categorized into 4 Tiers.

Second, I have 3 vector layers (could be combined later) that are essentially point coordinates that I buffered. Each resulting circle is categorized into 3 Coverage Types.

Just in case I did not explain that well, here is a picture:

Figure 1

I would like to end up with the data to answer the following questions:

  • How much Tier 1 area is covered by Coverage Type 1?
  • How much Tier 2 area is covered by Coverage Type 1?
  • How much Tier 3 area is covered by Coverage Type 1?
  • How much Tier 1 area is covered by Coverage Type 2?
  • Etc... for all Tiers and Coverage Types

And also:

  • What percentage of Coverage Type 1 Area (total) is Tier 1?
  • What percentage of Coverage Type 1 Area (total) is Tier 2?
  • Etc... for all Tiers and Coverage Types

I imagine the output will be in some sort of matrix/table format?

CAVEAT - If circles of the same type overlap, I want to make sure I do not double count (maybe there is a way to combine them temporarily?)

2

You do need several geoprocessing steps and there is more than one way to get the needed outcome. All tools mentioned here can be found in the processing toolbox when searching for the tool name. I do not use the tools from the "vector" menue as the newer ones are a bit better and are exchanged in the 2.18 Version of QGIS anyway.

One way is to first using the tool "Merge". When each cirlce has its categories in a column the result will be a new file with the combined geometries and the values from each cirlce in your categories column (if column names match).

enter image description here

You still have overlapping geometries. Therfore you use the tool "Dissolve" and use your categories column. All geometries with the same category are made into one.

Then you use the tool "Union" for the merged circles and your counties. The outcome is a combination of both geometries. In the attribut table you get all column from both layers. You can select all geometry parts based on your categories column. The outcome should look like the following picture. You now have much more geometries then before as you now have all the small pieces. When the column is empty there was no circle in that county.

enter image description here

To get your combination table you use the tool "Statistics from Categories".

  • Thanks!! I am having trouble finding the "Statistics from Categories" tool. Is that a plugin? – jaspen May 24 '17 at 22:32
  • Its called statistics by categories. Sorry for the wrong name. It is a standard QGIS geoalgorhitm and can be found in the processing toolbox - QGIS geoalgorhitms - Vector table tools. – Matte May 25 '17 at 6:52
  • I still cant get the data to answer the questions above.. maybe I am using the tool wrong? I need to get the area of the intersection b/w all combinations of circle categories and tier categories (i.e. how much tier 1 land area do all the type 2 circles cover?) – jaspen May 25 '17 at 20:24
1

If I understand your question correctly you are looking for the area of the counties which intersect your areas of interest(Circles). In this case the Intersect tool should do the trick.

  • Thanks! I will try it out shortly. Do you know if there is a way to do this iteratively so that I do not need to manually do it for each combination of Coverage Type (Area of Interest) and County Tier? I'm asking because I will be scaling this up later and it may end up having many possible category combinations. – jaspen May 24 '17 at 14:17
  • You can use model builder or build a script in python. However, these can be tricky without previous experience. I think for your case the merge tool will work best. First merge all your areas of interest (Circles) and then intersect the new merged layer with the county polygons. – Reisenrich May 24 '17 at 14:35
  • I would suggest dissolving the circles by categories rather than merging them. – mgri May 24 '17 at 17:34

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