[OpenLayers-Users] OpenLayers.Control.ScaleLine accuracy issue

[plen]

Hello,

I have a scenario where I need to measure the distance of my map's
viewport's diagonal by measuring the distance between the SW viewport
coordinate and the NE viewport coordinate.  What I have found is that the
distance will be different depending on where in the world the viewport is
looking.  For example, the distance between the SW and NE points might be
500 km in the middle of North America but might be 325 km in Ecuador.  
Visually, it looks the same because I have not zoomed in or out.  I am
starting to find out that the difference is due to the curvature of the
earth as a sphere vs a flat map.  I added the control
OpenLayers.Control.ScaleLine to my map to see the scale.  The scale remains
the same at a given zoom level regardless of where I am in the world. 
Because I have found that the viewport distance will be different depending
on where you are, I am now finding that the map scale does not reflect the
proper measurement.  I went to Google Maps to see if they had a scale and
found that the Google Maps scale changes as the map is panned around the
world.  This seemed to verify my earlier findings.

My question is to see why the OpenLayers scale remains the same, as if it
were placed on a flat map.  Maybe I am missing some key piece of information
and that is what I am trying to find out.  Any thoughts would be greatly
appreciated.

Thanks - Peter Len
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