Geofence. Check if a point is inside a polygon. JavaScript. Map

Geo-fence. Check if a point is inside a polygon.

  • We have the polygon = [[2,9],[8,6],[12,10],[15,2],[10,4],[5,1]]

  • Let's check if these points are inside or outside that polygon:

  • point = [6,5]

  • point = [7,8]

  • We will use the following algorithm in JavaScript, it is called even-odd rule.

  • The number of intersections for a ray passing from the exterior of the polygon to any point; if odd, it shows that the point lies inside the polygon. If it is even, the point lies outside the polygon. Point in polygon. Wikipedia.

cuadricula2

- JavaScript:

// var poligono = [[2,9],[8,6],[12,10],[15,2],[10,4],[5,1]];
// var punto = [6, 5];
 
 
 
function pointInPolygon(polygon, point) {
    let odd = false;
    for (let i = 0, j = polygon.length - 1; i < polygon.length; i++) {
        if (((polygon[i][1] > point[1]) !== (polygon[j][1] > point[1])) 
            && (point[0] < ((polygon[j][0] - polygon[i][0]) * (point[1] - polygon[i][1]) / (polygon[j][1] - polygon[i][1]) + polygon[i][0]))) {
            odd = !odd;
        }
        j = i;
    }
    return odd;
};

result = pointInPolygon(poligono, punto);
AppInventor.setWebViewString(result);
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2.- Check if a point is inside a polygon. JavaScript. RunJavaScript.

p251_geofenceJavaScript.aia (165.3 KB)

- Designer.

- Blocks.

- Comments.

  • The code returns a String "true" or "false".
  • Be careful with the semicolons;
  • I have used poligono and punto as variable names.

3.- Map. Check if a point is inside a polygon on a map. Geofence.

p251_geofenceJavaScript_Mapa.aia (4.4 KB)

  • We draw a polygon in a Map using LineString.PointsFromString, note that the initial and final coordinates are the same so that the polygon closes:

LineString.PointsFromString=[[36.50318,-6.27281],[36.50226,-6.27231],[36.50198,-6.27311],[36.50291,-6.27361],[36.50263,-6.27298],[36.50318,-6.27281]]

  • We set the check polygon with these coordinates:

poligono = [[36.50318,-6.27281],[36.50226,-6.27231],[36.50198,-6.27311],[36.50291,-6.27361],[36.50263,-6.27298]]

  • We move by means of a Clock, a point horizontally:

punto = [36.5027,h]

  • We can stop the Clock using "Stop" and tap on the map, using the TapAtPoint block we can see if the point is inside or outside the polygon.

- Designer.

- Blocks.

- Comments.

  • It also works if we set the query polygon by repeating the initial coordinate at the end:
    poligono = [[36.50318,-6.27281],[36.50226,-6.27231],[36.50198,-6.27311],[36.50291,-6.27361],[36.50263,-6.27298],[36.50318,-6.27281]]

  • The code returns a String "true" or "false".

4.- Extension Geofence.

  • Instead of creating the code using JavaScript, we are going to create it with Java to make an extension, we will see it in this topic:

Part of this example can be accomplished more simply without javascript using a Polygon component.

geofenceSimpleSteveJG.aia (3.2 KB)

...just another way. :slight_smile:

You present an interesting alternative.

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