Spatial joins enable you to analyse multiple sources of spatial data together and answer questions based on the geographic relationships between those data sources.
For example, let’s say we have two spatial files:
- One file containing two polygons, A and B, that exist within the boundaries of a particular area of land – defined by the blue, rectangular border in the images below
- A second file containing a number of spatial points that are located within the same area of land
Graphical Representation of Spatial Joins?
If we want to determine which, and how many, points are located within each polygon’s boundary, we can perform a spatial join. A graphical result of which is shown below:
We can now confirm that:
- 5 points are within the boundary of polygon A
- 7 points are within the boundary of polygon B
In the below video I step through how to answer a similar question: how many educational facilities (spatial points) are there within each administrative area (polygons) of Mauritius?
I hope the above helps clarify how and why you would use spatial joins in Tableau. You can learn more about this feature in Tableau’s online help here.
Thanks for reading!