[geos-devel] Minimun distance between two polygons

[Paul Ramsey]

On Fri, Jan 20, 2017 at 2:28 AM, alb.sil.81 at virgilio.it <
alb.sil.81 at virgilio.it> wrote:

> Hi everybody,
>
>
>
> using GEOS 3.5.0 and PHP my aim is to measure the minimum distance in
> meters between two polygons in Google Maps coordinate (Latitude/Longitude
> WGS84).
>

Since your inputs are in lon/lat, you're fundamentally out of luck, even if
you solve your problem below. The nearest points in lon/lat space are not
the actual nearest points, because the lines in lon/lat space are not the
same as the great circles that connect points in spherical space. You
really need to either project your objects into a planar space first, then
use GEOS, or move to some completely different tool set, like a port of s2
(spherical geometry) algorithms into php
https://github.com/eelf/s2-geometry-library-php


>
>
> Using the “Distance method” I obtain an angle you can’t convert in meter
> (with an acceptable error) without the information about the orientation
> and the longitude of the two point (1 grad on latitude is in meter
> different from 1 grad in longitude).
>
>
>
> I find the “nearestPoints method” that return the nearest Points of two
> polygons, with this information I will create a function able to calculate
> the right distance.
>
>
>
> This function isn’t usable from PHP.
>
>
>
> In order to use that method from PHP I tried to modify the php/geos.c file
> adding the following code:
>
>
>
> ….
>
> PHP_METHOD(Geometry, nearestPoints);
>
>
>
> ….
>
>
>
> PHP_ME(Geometry, nearestPoints, NULL, 0)
>
>
>
> ….
>
>
>
> /**
>
>   * GEOSCoordSequence::nearestPoints(GEOSGeometry)
>
>   */
>
> PHP_METHOD(Geometry, nearestPoints)
>
> {
>
>      GEOSGeometry *this;
>
>      GEOSGeometry *other;
>
>      zval *zobj;
>
>      GEOSCoordSequence *ret;
>
>
>
>      this = (GEOSGeometry*)getRelay(getThis(), Geometry_ce_ptr);
>
>
>
>      if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "o",
>
>          &zobj) == FAILURE)
>
>      {
>
>          RETURN_NULL();
>
>      }
>
>
>
>      other = getRelay(zobj, Geometry_ce_ptr);
>
>
>
>      ret = GEOSNearestPoints(this, other);
>
>      if ( ! ret ) RETURN_NULL(); /* should get an exception first */
>
>
>
>      /* return_value is a zval */
>
>      object_init_ex(return_value, Geometry_ce_ptr);
>
>      setRelay(return_value, ret);
>
> }
>
>
>
> The result obtained using this function with PHP is always an
> empty GEOSGeometry object.
>
>
>
> I would like to ask you:
>
>
>
> 1)      Is possible to obtain the distance in meter using the distance
> method?
>
> 2)      Where is the error in the code above? Some of you find errors in
> the code above?
>
> 3)      Could you suggest me another way to obtain the minimum distance
> between two polygon using GEOS :-D ?
>
>
>
> Thank you very much for your help.
>
>
>
>  AS
>
>
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> geos-devel at lists.osgeo.org
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>
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