Dear all,
I want to display the area of a polygon, for example :
POLYGON((45.179 1.94,45.179 1.95,45.182 1.95,45.182 1.94,45.179 1.94))
I cannot get the area either in m^2 or deg^2.
Am I missing something ?
Below are some tests I went through.
1) Area measured with QGIS :
Ellispsoïdal
square degrees : 0.00003 deg²
square meters : 369063.9 m²
Cartesian
square degrees : 0.00003 deg²
square meters : 371759.7 m²
2) Area displayed with mapserver
2.1) based on attribute [shape]
LABEL
TEXT (" surf=" + tostring(area([shape]), "%.1f") )
...
END
The area value depends on zoom level : the more I zoom in, the higher the
area value.
It turns out that the area is expressed in pixel^2
2.2) based on hard coded polygon
LABEL
TEXT ("surf=" + tostring(area(fromText('POLYGON ((45.179 1.94,45.179
1.95,45.182 1.95,45.182 1.94,45.179 1.94))')), "%.10f") )
...
END
The area value is exactly what I would expect (deg^2) : 0.00003
3) Conversion from deg^2 to m^2
The relationship is :
* Surf_m ~ Surf_deg * (R * pi/180)^2
* R ~ 6378137 (the earth radius in meters)
which yields to :
Surf_m ~ 371760.8 m^2
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