[OpenLayers-Users] Vector Feature Label and Geometry.getCentroid()

[Pierre Giraud]

Actually, this is not really an OpenLayers' bug.
The precision of your data is too large. You have micrometer precision
coordinates. You will not be able to compute the centroid with so much
digits with precision.
If possible, you should round the coordinates a bit before you use
them in OpenLayers.

Regards,
Pierre

On Fri, Jun 24, 2011 at 3:21 PM, Pierre Giraud
<pierre.giraud at camptocamp.com> wrote:
> You seem to be right. getCentroid isn't correct for the example you're
> giving and it has apparently nothing to do with the fact that it's a
> "multiple" geometry.
> Good catch.
>
> My first investigation showed me that the decimals are the problem. If
> I remove them I've something better.
> The following code samples can be tested in the spherical-mercator.html example.
>
> Wrong:
> var format = new OpenLayers.Format.WKT();
> var feature = format.read('MULTIPOLYGON(((-10842755.069337
> 3645931.096789,-10842790.775979 3645953.496622,-10842740.232706
> 3645955.23634,-10842755.069337 3645931.096789)))');
> var layer = map.getLayersByName('Editable Vectors')[0];
> layer.addFeatures([feature]);
> map.zoomToExtent(layer.getDataExtent());
> layer.addFeatures(new
> OpenLayers.Feature.Vector(feature.geometry.getCentroid()));
>
> Correct:
> var format = new OpenLayers.Format.WKT();
> var feature = format.read('MULTIPOLYGON(((-10842755 3645931,-10842790
> 3645953,-10842740 3645955,-10842755 3645931)))');
> var layer = map.getLayersByName('Editable Vectors')[0];
> layer.addFeatures([feature]);
> map.zoomToExtent(layer.getDataExtent());
> layer.addFeatures(new
> OpenLayers.Feature.Vector(feature.geometry.getCentroid()));
>
> Pierre
>
> On Fri, Jun 24, 2011 at 12:16 AM, B. Heath Robinson
> <heath at midnighthour.org> wrote:
>> I am using a vector layer with text features on Polygon geometries.
>>  Relatively often the text label is drawn pretty far off the center of the
>> polygon.  My polygon is basically a rectangle with a rounded corner.  Below
>> is the WKT of it.  When I do a getCentroid in OpenLayers it is different
>> than if I do it in PostGIS.  I think if the label was drawn at the PostGIS
>> centroid it would be right.  My site allows measurement, so I also tried
>> getting the centroid of a geometry I traced with the rubberband tool.  It
>> also returned a Centroid more like I expected.
>> Many of the labels also appear to be correct.  Am I missing something?  Does
>> it have to do with the geometry being Multipolygon?  Any insight would be
>> appreciated.
>> Geometry
>> MULTIPOLYGON(((-10842755.069337 3645931.096789,-10842790.775979
>> 3645953.496622,-10842772.182218 3645984.113569,-10842743.492303
>> 3645965.972283,-10842742.28656 3645964.787145,-10842741.293463
>> 3645963.414273,-10842740.538336 3645961.898144,-10842740.044376
>> 3645960.275967,-10842739.827709 3645958.592826,-10842739.889943
>> 3645956.896071,-10842740.232706 3645955.23634,-10842740.843507
>> 3645953.655201,-10842755.069337 3645931.096789)))
>> OpenLayers Centroid:
>>
>> id: "OpenLayers.Geometry.Point_492"
>> x: -10842792.853788493
>> y: 3645967.2444699085
>>
>> PostGIS Centroid
>> "POINT(-10842763.8558177 3645957.49366021)"
>>
>> Rubberband Tool Geometry:
>> POLYGON((-10842772.247212 3645983.9949693,-10842790.759305
>> 3645953.5395909,-10842754.929448 3645931.1459303,-10842740.597505
>> 3645955.0325016,-10842739.701759 3645957.7197409,-10842739.701759
>> 3645961.3027266,-10842740.298923 3645962.7956373,-10842743.284744
>> 3645965.7814587,-10842745.374819 3645967.2743694,-10842772.247212
>> 3645983.9949693))
>> Rubberband Tool Centroid:
>>
>> id: "OpenLayers.Geometry.Point_689"
>> x: -10842779.12432854
>> y: 3645962.747894722
>>
>> _______________________________________________
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>>
>>
>
>
>
> --
> Pierre GIRAUD
> Géomaticien, Analyste
>
> Camptocamp France SAS
> Savoie Technolac, BP 352
> 73377 Le Bourget du Lac, Cedex
>
> Tel : 00 33 4 79 44 44 93
> Mail : pierre.giraud at camptocamp.com
> http://www.camptocamp.com
>



-- 
Pierre GIRAUD
Géomaticien, Analyste

Camptocamp France SAS
Savoie Technolac, BP 352
73377 Le Bourget du Lac, Cedex

Tel : 00 33 4 79 44 44 93
Mail : pierre.giraud at camptocamp.com
http://www.camptocamp.com