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My question is somewhat related to this similar one, which links to a pretty complex solution - but what I want to understand is the result of this:

Using a Mysql Geometry field to store a small polygon I duly ran 

select AREA(myPolygon) where id =1

over it, and got an value like 2.345. So can anyone tell me, just what does that number represent seeing as the stored values were long/lat sets describing the polygon?

FYI, the areas I am working on are relatively small (car parks and the like) and the area does not have to be exact - I will not be concerned about the curvature of the earth.

2.345 of what? Thanks, this is bugging me.

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1 Answers1

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The short answer is that the units for your area calculation are basically meaningless ([deg lat diff] * [deg lon diff]). Even though the curvature of the earth wouldn't come into play for the area calculation (since your areas are "small"), it does come into play for the calculation of distance between the lat/lon polygon coordinates.

Since a degree of longitude is different based on the distance from the equator (http://en.wikipedia.org/wiki/Longitude#Degree_length), there really is no direct conversion of your area into m^2 or km^2. It is dependent on the distance north/south of the equator.

If you always have rectangular polygons, you could just store the opposite corner coordinates and calculate area using something like this: PHP Library: Calculate a bounding box for a given lat/lng location

The most "correct" thing to do would be to store your polygons using X-Y (meters) coordinates (perhaps UTM using the WGS-84 ellipsoid), which can be calculated from lat/lon using various libraries like the following for Java: Java, convert lat/lon to UTM. You could then continue to use the MySQL AREA() function.

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Chris Morgan
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  • Thank you for explaining that to me - happily you also just gave me an idea. In the UK we have a national corresponding Eastings / Northings system (distance in metres from btm LH corner of the UK) - and I am aware of converters. I will likely just create a 2nd field for E/N. – Cups May 12 '11 at 21:12