Golang Code
Is Long/Lat within Polygon from GeoJson
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282 words
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2 minutes read
This is an example of checking if a pair of long/lat coordinates lie within a polygon or multipolygon when working with geojson. It's often useful when working with geo-special data and maps to determine if the point your looking is within an area - or which area it's within.
We use the paulmach/orb package (see on github), which is deep and precise library for dealing with all sorts of spacial and geometric data. To begin with we load in our geojson file, we're using an example file, into a feature collection. Then we're looping through each polygon to check if the point exists within it.
package main
import (
"fmt"
"io/ioutil"
"github.com/paulmach/orb"
"github.com/paulmach/orb/geojson"
"github.com/paulmach/orb/planar"
)
const (
GEO_FILE = "points.geojson"
)
func main() {
// Load in our geojson file into a feature collection
b, _ := ioutil.ReadFile(GEO_FILE)
featureCollection, _ := geojson.UnmarshalFeatureCollection(b)
// Pass in the feature collection + a point of Long/Lat
if isPointInsidePolygon(featureCollection, orb.Point{100.5, 0.5}) {
fmt.Println("Point 1 is inside a Polygon")
} else {
fmt.Println("Point 1 is not found inside Polygon")
}
if isPointInsidePolygon(featureCollection, orb.Point{105.5, 2.5}) {
fmt.Println("Point 2 is inside a Polygon")
} else {
fmt.Println("Point 2 is not found inside Polygon")
}
}
// isPointInsidePolygon runs through the MultiPolygon and Polygons within a
// feature collection and checks if a point (long/lat) lies within it.
func isPointInsidePolygon(fc *geojson.FeatureCollection, point orb.Point) bool {
for _, feature := range fc.Features {
// Try on a MultiPolygon to begin
multiPoly, isMulti := feature.Geometry.(orb.MultiPolygon)
if isMulti {
if planar.MultiPolygonContains(multiPoly, point) {
return true
}
} else {
// Fallback to Polygon
polygon, isPoly := feature.Geometry.(orb.Polygon)
if isPoly {
if planar.PolygonContains(polygon, point) {
return true
}
}
}
}
return false
}
