What's the best way to determine if a point is within a certain distance of a GEOJSON polygon?

Question

What's the best way to determine if a point is within a certain distance of a GEOJSON polygon? Should one use TurfJS buffer method (https://github.com/Turfjs/turf-buffer#turf-buffer)? Can one perform queries on the buffered polygon?

It's clear to me one could us the TurfJS' inside method (https://github.com/Turfjs/turf-inside) to determine whether a point is within a polygon. I'm just curious what the best approach would be for finding whether or not a point is inside of a buffered polygon.

For example: I have a number of neighborhoods provided as a GEOJSON polygon files. I also have a set of locations/addresses for employees (already geocoded to lat/long coordinates). What would be the best way to see whether or not my employees live within 10 miles of a given neighborhood polygon?

Thanks!

Answer 1

Yes, you can use buffer in conjunction with inside to find points within 10 miles of something else, eg, expanding on the existing examples,

var pt = point(14.616599, -90.548630)
var unit = 'miles'
var buffered = buffer(pt, 10, unit)
var ptTest = point(-1, 52)
var bIn = inside (ptIn, buffer)

which should obviously be false.

In general, though, buffering is somewhat expensive, so you would not necessarily want to do this every time you run the query. There are a couple of things you can do to speed things up:

1). Pre-buffer your search areas

2). Use some kind of R-tree type index, which will first check bounding box intersection, and avoid lots of unecessary point in polygon operations. turfjs, which I hadn't heard of until seeing your post, uses jsts under the hood for a number of operations, including buffering. This library has an implemention of R-tree indexes that you could potentially use. Here is a fun example of this being done.

In general, in situations where you have a spatial (R-tree type) index in place, such as a spatially enabled database like Postgis on top of Postgres, you would use an operator like ST_Dwithin (geom1, geom2, distance) in a where clause to find all points within some distance of another geometry, and this would be very efficient as many candidates would be rejected for failing an initial bounding box test.

Really, it depends on the size of your data and frequency of queries. There is nothing, in principle, wrong with doing contains queries on a buffer. I hope I haven't created more questions than answers.

Answer 2

I'm using GeoScript to do that sort of calculations in JavaScript. It has a distance method in the geom.Geometry class which can return the minimum distance between two geometries. You could use that, or take a look at the source on GitHub to see how they do it if you want to roll your own solution.