[Qgis-user] Request for exporting arcs from dxf to kml

[Andrea Giudiceandrea]

> *davidlurie at libero.it*davidlurie at libero.it 
> <mailto:qgis-user%40lists.osgeo.org?Subject=Re%3A%20%5BQgis-user%5D%20Request%20for%20exporting%20arcs%20from%20dxf%20to%20kml&In-Reply-To=%3C262521302.801307.1650790933783%40mail1.libero.it%3E>
> /Sun Apr 24 02:02:13 PDT 2022/
> ------------------------------------------------------------------------
> I always use default SR, Epsg 4326 - Wgs 84, the Google Earth one.

I Mauro,
probably there is a misunderstanding here.

Google Earth uses WGS84 datum as reference system for the coordinates. 
However Google Earth represents such coordinates not in a WGS84 
Cartesian plane (as it happens with a WGS84 map in QGIS), but in a globe 
/ planisphere like representation called General Perspective projection 
[1] [2].

This means that a "circle" drawn on a layer in WGS84 CRS, and 
represented in QGIS on a map/project with WGS84 CRS, will appear as a 
circle. The same happens if the layer is exported as KML and displayed 
in QGIS in map with the same WGS84 CRS or if exported as a DXF and 
displayed with a CAD software.

The "circle" will appears similar to an ellipse instead if the KML layer 
is displayed using Google Earth, because the Google Earth projection is 
not a WGS84 Cartesian plane, but a General Perspective projection.

The same happens for a polygon which resemble a "circle" in Google 
Earth: if exported as KML it will appear similar to an ellipse in a 
WGS84 map in QGIS.

So you need to use the same (or very similar) projection to display the 
geometries with the same (or very similar) shape.

The General Perspective projection is an Azimuthal projection [3] and is 
similar e.g. to an Orthographic projection [4] centred on a suitably 
chosen centre.
If your "circle" drawn in Google Earth is , e.g., centred at 15° E Lon. 
and 40° N Lat., than you could create a custom Orthographic projection 
with the following proj4 string and use such custom projection as the 
map/project projection in order to properly display it as a "circle" in 
QGIS:

+proj=ortho +lat_0=40 +lon_0=15 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs

or you could similarly create e.g. a custom Azimuthal Equidistant 
projection:

+proj=aeqd +lat_0=40 +lon_0=15 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs

See also [3], [4] and [5].

Regards.

Andrea


[1] https://en.wikipedia.org/wiki/General_Perspective_projection
[2] 
https://www.researchgate.net/publication/340162875_Google_Earth_and_Map_Projections_Google_Earth_i_kartografske_projekcije
[3] https://bnhr.xyz/2018/09/21/create-a-globe-in-QGIS.html
[4] 
https://docs.qgis.org/3.22/en/docs/user_manual/working_with_projections/working_with_projections.html
[5] 
https://docs.qgis.org/3.22/en/docs/gentle_gis_introduction/coordinate_reference_systems.html
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