[gdal-dev] proper way to get area of a lon/lat polygon

[Mikhail Kruk]

Yes, you are correct, that was one of my first attempts and was wrong
because I don't really understand these projection systems.
I now have a better one which sort of works and produces areas in meters by
setting first system to
sr.SetWellKnownGeogCS("EPSG:4326")
Now I am struggling with selecting the projection coordinate system.  Some
suggested that I use cylindrical projection EPSG:9835 which should preserve
the area anywhere on the planet but when I try
sr_proj.ImportFromEPSG(9835)
I get error:
PROJ: proj_create_from_database: crs not found


On Fri, Sep 18, 2020 at 9:50 PM Michael Patrick <geodesy99 at gmail.com> wrote:

> ... but basically I came to the conclusion that when people talk about an
>> area of something on Earth they really mean the area of a projection (am i
>> right about that?) and so i need to specify a projection before getting a
>> meaningful area.  So i ended up with this code:
>>
>> wkt = "POLYGON ((lon1 lat1, lon2 lat2, lon3 lat3))"
>> poly = ogr.CreateGeometryFromWkt(wkt)
>> sr = osr.SpatialReference()
>> sr.SetWellKnownGeogCS("NAD83")
>> sr.SetProjCS("SRS_PT_TRANSVERSE_MERCATOR")
>> poly.AssignSpatialReference(sr)
>> poly.GetArea() <https://lists.osgeo.org/mailman/listinfo/gdal-dev>
>>
>> I might be dense, but it seems you are assigning sr as a specific
> transverse mercator, i.e.  UTM zone 18N, or a custom one centered on the
> centroid of your polygon. i.e. you don't have sufficient arguments.
>
>
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