[MapServer-users] Polygon area : cannot get the correct unit(e.g. square meters)

[Philippe Ghesquiere]

Dear all,

I didn't find a way to compute polygon areas in square meters.
As a workaround, I decided to add a new column in my GPKG database, with
the pre-calculated area :
ogr2ogr -append Output.gpkg Input.gpkg MyPolygonLayer -sql "select *,
ST_Area(geom, 1) as SURFACE_M2 from MyPolygonLayer" -nln MyPolygonLayer

I can now use the new attribute in my mapserver file :
LABEL
   TEXT " surf=[SURFACE_M2]  m2"
   ...
END

Philippe

On Wed, Jun 18, 2025 at 10:54 AM Philippe Ghesquiere <
philippe.ghesquiere at airbus.com> wrote:

> 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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