Length of shared borders between polygons using Virtual Layers

Question

I have a map with many counties, and I estimated the length of the common borders using one of the answers from this question posted a year ago.

The properties of my layer look like this:

I did the following:

Layer > Add Layer > Add/Edit Virtual Layer

And then entered this query:

select a.id as poly1_id,
       b.id as poly2_id,
       st_intersection(a.geometry, b.geometry) as geometry,
       st_length(st_intersection(a.geometry, b.geometry)) as border_length
from units_1 a
join units_1 b on st_intersects(a.geometry, b.geometry)
              and a.id < b.id 

I got the following results, which don't seem right:

I believe I am getting the length in the map's units which are probably degrees but I am not sure. Does anyone know what is going on and how to get the results in kilometers?

Answer 1

Your coordinates are in degrees (4326) so the length is also in degrees - which is meaningless.

You can use the version of st_length that accept a boolean parameter to use (or not) the elipsoid when computing distances. It will return a distance in meters

st_length(st_intersection(a.geometry, b.geometry),true) as border_length