Iterate over points in GEOS Polygon using C API

Question

How can I iterate over the points in a GEOS Polygon without turning it into a WKT string? Currently we're doing something like this:

GEOSGeometry *geom = GEOSGeomFromWKT("POLYGON ((1 1, 2 1, 2 2, 1 2, 11))");
char geomAsWKT[900] = GEOSGeomToWKT(geom);
/* Iterate over the geomAsWKT to get the points */

All the functions I've tried (GEOSGeomGetPointN, GEOSGeom_getCoordSeq, and some others) only work on a LinearRing.

Answer 1

To iterate over the points of a polygon, you have to get the LinearRing by calling GEOSGetExteriorRing, as demonstrated below.

This example works for MultiPolygons or Polygons. This example prints all the x,y coordinates of the points in a the LinearRings that make up a multipolygon. It also works if the inputGeom is a Polygon.

GEOSGeometry *inputGeom = GEOSGeomFromWKT("MULTIPOLYGON (((30 20, 45 40, 10 40, 30 20)), ((15 5, 40 10, 10 20, 5 10, 15 5)))");
const GEOSGeometry *linearRing;
const GEOSCoordSequence *coordSeq;
int numGeom = GEOSGetNumGeometries(inputGeom);
int n;
for (n=0; n < numGeom; n++) {
    linearRing = GEOSGetExteriorRing(GEOSGetGeometryN(inputGeom, n));
    printf("%s\n", GEOSGeomToWKT(linearRing));
    unsigned int numPoints, p;
    numPoints = GEOSGeomGetNumPoints(linearRing);
    for (p=0; p < numPoints; p++) {
        double xCoord, yCoord;
        coordSeq = GEOSGeom_getCoordSeq(linearRing);
        GEOSCoordSeq_getX(coordSeq, p, &xCoord);
        GEOSCoordSeq_getY(coordSeq, p, &yCoord);

        printf("%f, %f\n", xCoord, yCoord);
    }
}

For my purposes, the Polygons never had holes in them, so this was all I needed.