Calculate distances from points to multiple country borders in QGIS

Question

I have a shapefile containing several thousand GPS locations located in multiple countries and another shapefile with country polygons (or alternatively also one with country border polylines).

How can I calculate the shortest distance from each GPS location (point) to the borders of all countries adjacent to the country the point is located in?

Is there even a way to achieve this in QGIS?

Example of what I would like to calculate for every point:

I managed to get the distance to the (single) nearest border using the NNJoin plugin, but I have trouble identifying all adjacent borders and calculating all distances.

Ideally, the data structure would look like this, assuming that the distance is only reported for adjacent countries:

If this is difficult to achieve for adjacent countries only, a simple distance matrix with the distances to all countries in the sample would also be fine, but I have not found a way to create one for polylines rather than points.

Answer 1

You can achieve this using a virtual layer.

The idea is to read the points, join them to the polygon layer to get the polygons where the points are located, then join again these polygons to the polygon layer to get the adjacent ones and finally compute the distance between the point and the adjacent polygons.

Go to the menu Layer > Add Layer > Add/Edit Virtual Layer... and enter the following query.

SELECT  gps.id,neighbors_a.CSDUID, 
        st_distance(neighbors_a.geometry,gps.geometry) as dist
FROM gps
JOIN a gps_in_a 
 ON st_intersects(gps.geometry, gps_in_a.geometry)
JOIN a neighbors_a
 ON st_intersects(gps_in_a.geometry,neighbors_a.geometry)
  AND gps_in_a.csduid != neighbors_a.csduid

In this example my point layer is called gps and my area layer is called a -> replace with your own. Also replace with the proper id column name.

It is important to use a CRS whose unit is in meters, and that is suitable for distance computation.

Answer 2

This is not very elegant but you could try something like this:

• Convert each border polyline to a raster with a small resolution
• for each raster country outline run QGIS > Raster > Proximity (you'll end up with multiple proximity outputs, one for each outline
• for your location point file use the Point Sampling Tool (from plugins) to extract the distances from the proximity outputs. This tool allows batch processing and on-the-fly renaming of the fields that it creates in your point file so you should be able to create a table as in your original post.