# [geos-devel] GEOSIntersection returns the input

Giacomo Piva piva at meeo.it
Mon Aug 24 05:54:14 EDT 2009

```Hi all,
I'm writing because I have a problem with the GEOSIntersection function.
I'm using this function as follows:

GEOSCoordSeq coordseq = NULL, coordseq_I = NULL;
GEOSGeom area_1 = NULL, area_2 = NULL, intersection = NULL;

coordseq = (GEOSCoordSeq) GEOSCoordSeq_create(5, 2);   //5 coords to
close the ring

GEOSCoordSeq_setX(coordseq, 0, ...);
GEOSCoordSeq_setY(coordseq, 0, ...);
[...]   //Fill the Seq with the coords value for the first ring

area_1 = GEOSGeom_createLinearRing(coordseq);

//Re-fill the coords for the second ring ...
GEOSCoordSeq_setX(coordseq, 0, ...);
GEOSCoordSeq_setY(coordseq, 0, ...);
[...]

area_2 = GEOSGeom_createLinearRing(coordseq);

intersection = GEOSIntersection(area_1, area_2);
//checks on valid and non empty geometry returns no error...

Now I get the coordinates of the intersection in this way:

GEOSGeom geom;
num = GEOSGetNumGeometries(intersection);   //num is 4, because there is
4 lines

for(i=0; i < num; i++) {
geom = (GEOSGeom) GEOSGetGeometryN(intersection, i);   //Here I
get i-th line...

coordseq_I = (GEOSCoordSeq) GEOSCoordSeq_create(2, 2);   //each
line has 2 point of 2 dimensions
coordseq_I = (GEOSCoordSeq) GEOSGeom_getCoordSeq(geom);

GEOSCoordSeq_getX(coordseq_I, 0, alpha);    //the first point is
in 0-th position
GEOSCoordSeq_getY(coordseq_I, 0, beta);
printf("[%d] %.2lf %.2lf\n",i,*beta, *alpha);

GEOSCoordSeq_getX(coordseq_I, 1, alpha);    //the second in the
first
GEOSCoordSeq_getY(coordseq_I, 1, beta);
printf("[%d] %.2lf %.2lf\n\n",i,*beta, *alpha);
}

Now, if I print the coordinates of each point of the 4 geometries
(lines) composing the intersection ring, I get the same value of the
"area_2" ring.

Second Area:
[A] 43.22 -125.52
[B] 43.22 -106.47
[C] 22.71 -106.47
[D] 22.71 -125.52

Geometries: 4
[A] 43.22 -125.52
[B] 43.22 -106.47

[B] 43.22 -106.47
[C] 22.71 -106.47

[C] 22.71 -106.47
[D] 22.71 -125.52

[D] 22.71 -125.52
[A] 43.22 -125.52

Really I can't figure out what is wrong.
Someone can help me?

Thanks.

--
Giacomo

```