Calculate Distance (in miles) from Point to Polygon Edge in QGIS

Question

I am attempting to calculate the distance from a point to the nearest polygon edge (county boundary). I have approximately 80 points and want to find the closest county edge for all of the points and calculate the distance in miles. I am using QGIS because I only have a basic license in ArcGIS Pro. I have tried all of the distance tools with no luck. I read about the NNJoin plug-in, but I can't seem to get that to work either.

Can you advise?

Answer 1

You can:

1. Convert your polygons to lines with "Polygons to Lines"

2. Calculate the distance to nearest line with Field Calculator. Change Lines to the name of your line layer:

distance($geometry, array_first(overlay_nearest(layer:='Lines', expression:=$geometry)))*0.000621371192

which measures the distance from each points geometry to the nearest lines geometry.

My coordinate system is in meters so I multiply the result with 0.0006... to get miles

(Field type should be Decimal number (Real), not integer)

Answer 2

To create a line from the point to the closest point on the boundary of the nearest polygon and then measure the length of that line, use this expression:

length(
    make_line (
        $geometry,
        closest_point (
            boundary(
                overlay_nearest (
                    'polygon',  -- change to the name of your polygon layer
                    $geometry 
                 )[0]
            ),
       $geometry
        )
    )
)

A modofied version of the expression (to convert m to km and round the result) labels to point layer with the distance to the nearest point on the polygon's boundary - thus the length of the black lines that are added for visualiziation purpose only: