[OpenLayers-Users] Measure tool in 2.7 RC1

[David Fawcett]

I am definitely not an expert in projections, but my understanding of
the Mercator projection is that it is accurate for direction and
shape, but not for size.  The distortion increasing as you move away
from the equator.

There is actually some pretty good info at Wikipedia:
http://en.wikipedia.org/wiki/Mercator_projection

If you want to do area calculations, you should likely look at using a
different data source that is in a different projection.  I don't know
about California, or any of your local governments, but here in
Minnesota, a couple of state agencies have free WMS services with
imagery data that are available in Universal Transverse Mercator.
There is still some distortion, but because UTM uses smaller zones,
the scale distortion is significantly less.

David.

On Sun, Sep 7, 2008 at 1:03 PM, Jennifer Strahan <jennifer at greeninfo.org> wrote:
> Hello,
>
> Thanks for including the measuring tool!  I'm wondering if there are any
> guidelines for use that I should follow.  I added a Google map layer to
> the measure example and am finding some error with the measurements.  Is
> it possible that my map settings are wrong?
>
> I measured Alamo Square in San Francisco and the area result is around
> 92557 square meters (nearly 23 acres).  The actual measurement should be
> around 13 acres.
>
> My test map is here:
> http://www.parkinfo.org/review/measure.html
>
> Any suggestions as to how I can adjust my settings to get the correct
> results?
>
> Thanks,
> Jennifer
>
>
>
>
>
> _______________________________________________
> Users mailing list
> Users at openlayers.org
> http://openlayers.org/mailman/listinfo/users
>