[OpenLayers-Users] Square miles from Square degrees?

[cmose]

Ah good deal, thanks for the clarification. I wonder if there are some set
conversions that can be applied to various regions of the globe to get area
from square degrees with +-5% or so of accuracy? It would be nice to be able
to define 4 quadrants of the globe or something like that, and apply a
constant conversion rate to items that fall into those four segments...

If anyone has any info please chime in!

Thanks again


mpriour wrote:
> 
> It just occured to me as re-read that you are projecting to Spherical
> Mercator,
> The spherical mercator projection doesn't preserve area. It makes the
> intersection of all lat/lon lines into 90 degree intersections. While this
> is not terrible at the equator, the higher latitude you get the worse than
> warping effect is going to be.
> You could do some calculations & figure out the miles per unit degree
> latitude & longitude (they will be different) in your area of interest.
> You can also look up that information, though I can't remember where. I
> used a look up for numerous conversions of square degrees to acres in
> central Texas and it was only off by 1-2%, which was good enough for our
> calculations.
> 
> 
> cmose wrote:
>> 
>> Hmmn well the formula seems to match what I turned up online for
>> calculating the area of an irregular polygon...
>> 
>> The units thing makes sense and since I'm using a degree based map a
>> result in square degrees also makes sense. What doesn't make sense to me
>> though is that I manually converted the points in my Polygon to Spherical
>> Mercator values which should be based on meters so I would expect the
>> getArea result to be somewhat meaningful as a meters/km but it's still
>> way way too high...
>> 
>> Thanks a bunch for the input!
>> 
>> 
>> mpriour wrote:
>>> 
>>> Looking at the actual source code, I don't think this method works
>>> properly on irregular polygons. It also makes the calculations in map
>>> units. So if you are using feet you get square feet, decimal degrees you
>>> get square degrees, etc..
>>> 
>>> To perform the getArea calculation it does rectangular area measurements
>>> between the points of the line segments making up the polygon. This
>>> works in a highly predictive manner with any regular polygon, rectangle,
>>> or trapezoid. However, it is very unpredictable on free-form geometries.
>>> 
>>> 
>>> cmose wrote:
>>>> 
>>>> Hmnn after giving this a go I seem to be getting wildly inaccurate
>>>> results, at least I believe I am. For instance, I used a polygon over
>>>> an area I know to be roughly 400k square kilometers. Using the getArea
>>>> on the polygon (after converting points to sphericalmercator) I'm
>>>> getting a result of 748,912,714 sq. km...
>>>> 
>>>> Anyone have any pointers or ideas of where I might be going wrong here?
>>>> Thanks!
>>>> 
>>>> 
>>> 
>>> 
>> 
>> 
> 
> 

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