[OpenLayers-Users] question about the measure control

[Andreas Hocevar]

Hi,

On Thu, Jul 2, 2009 at 12:33 AM, <Aaron_Gundel at sjwater.com> wrote:
> I have a question about the measure control in openlayers....  I noticed
> today that after we upgraded to Openlayers 2.8, the behavior for the control
> is somewhat different than it was in 2.7, with the primary difference being
> that if you're "in a hurry" so to speak and don't make your clicks slow and
> deliberate, an invalid distance appears on the initial click.  I checked and
> it doesn't look like its just me -- it happens on the example page also.
>
> http://openlayers.org/dev/examples/measure.html
>
> Click while you're in motion and it's fairly easy to replicate.  Your
> initial distance will vary based on...well I'm not sure, but the distance is
> not zero as you might expect.  Any thoughts on what might be causing this
> behavior?

The distance you see is the distance at the time the calculation is
done, which happens later than the click.

It is because the measure calculation is now called using setTimeout
(due to issues in IE, IIRC), and takes the geometry which already has
a length when the calculation is done. If you want to create a bug
report: the fix is easy. In the first line of the measurePartial
method in Control.Measure, you have to add

geometry = geometry.clone();

BTW, the final result of the measure calculation is still correct.

Also, you can configure the measure control to update the distance
while moving the mouse:

var control = new OpenLayers.Control.Measure(/* your config here */);
control.handler.callbacks.modify = function(point, feature) {
    control.measurePartial(point, feature.geometry);
};

Regards,
Andreas.


-- 
Andreas Hocevar
OpenGeo - http://opengeo.org/
Expert service straight from the developers.