<div dir="ltr"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>... 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:</div><div><pre><code><span>wkt </span><span>=</span><span> </span><span>"POLYGON ((lon1 lat1, lon2 lat2, lon3 lat3))"</span><span>
poly </span><span>=</span><span> ogr</span><span>.</span><span>CreateGeometryFromWkt</span><span>(</span><span>wkt</span><span>)</span><span>
sr </span><span>=</span><span> osr</span><span>.</span><span>SpatialReference</span><span>()</span><span>
sr</span><span>.</span><span>SetWellKnownGeogCS</span><span>(</span><span>"NAD83"</span><span>)</span><span>
sr</span><span>.</span><span>SetProjCS</span><span>(</span><span>"SRS_PT_TRANSVERSE_MERCATOR"</span><span>)</span><span>
poly</span><span>.</span><span>AssignSpatialReference</span><span>(</span><span>sr</span><span>)</span><span>
poly</span><span>.</span><span>GetArea</span><span>()</span></code><a href="https://lists.osgeo.org/mailman/listinfo/gdal-dev" rel="noreferrer" target="_blank"></a></pre></div></div></blockquote><div>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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