# [geos-devel] GEOSUnaryUnion performances

Pascal Leroux pa.leroux at gmail.com
Fri Oct 26 01:00:27 PDT 2012

```Hi everyone,

I want to build a planar graph with a large set of LineString geometries.
When I use OpenJump, it takes 3 or 4 seconds to determine and create the
edges of the graph. I took a look at JTS source code and this calculation
is based on the class UnaryUnionOp .

I have tried to do the same thing in Python (Shapely) with unary_union (new
in release 1.2.16, thanks Sean) and in C (geos C API) with GEOSUnaryUnion
(GEOS_MULTILINESTRING or GEOS_GEOMETRYCOLLECTION geometry types).

With the same input data, I have got the same results but it takes about 50
seconds (in C, 1 or 2 seconds more in Python).

Can anyone explain the differences ?

I use the release 3.3.3 of geos , on Mac OS X (10.6, Snow Leopard).

Here is the code I used to do tests in C :

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <geos_c.h>
#include "shapefil.h"

GEOSGeometry *SHPObject_to_LineString(SHPObject *object)
{
GEOSCoordSequence *coords = GEOSCoordSeq_create(object->nVertices,2);
int i;

assert(object->nParts == 1);
for (i=0; i<object->nVertices; i++)
{
}
return GEOSGeom_createLineString(coords);
}

{
SHPHandle shape;
int type, nobjs, i;
double minBounds[4], maxBounds[4];
GEOSGeometry **geometries;
GEOSGeometry *collection;

shape = SHPOpen(pathname,"rb");

SHPGetInfo(shape,&nobjs,&type,minBounds,maxBounds);
assert((type % 10) == SHPT_ARC);

assert(geometries = malloc(nobjs*sizeof(GEOSGeometry *)));

for (i=0; i<nobjs ;i++)
{
geometries[i] = SHPObject_to_LineString(object);
}

SHPClose(shape);

collection = GEOSGeom_createCollection(GEOS_GEOMETRYCOLLECTION,
geometries, nobjs);
// collection = GEOSGeom_createCollection(GEOS_MULTILINESTRING,
geometries, nobjs);

return collection;
}

int main(int argc,char *argv[])
{
GEOSGeometry *collection;
GEOSGeometry *unaryunion;
GEOSWKTWriter *writer;
FILE *output;
int g;

initGEOS(printf,printf);