[Qgis-user] ref pb

[chris hermansen]

Mark and list;

On Thu, Feb 9, 2023 at 8:24 AM Marc Millas via QGIS-User <
qgis-user at lists.osgeo.org> wrote:

> Hello,
>
> I got an autocad dxf file describing objects in Djibouti.
> no pb to load it in qgis as a vector layer.
> but... the srid seems to be 0 and QGIS ask for a valid ref
> as the data looks x,y, I did try epsg 32638, which is standard in Djibouti.
> loads fine... but the objects are around 200km west of where they should
> be.
>
> any idea ?
>
> Perhaps your dxf data is not georeferenced.  It may be in rectangular
coordinates (x, y) representing metres from some origin point that makes
sense when thinking of it as a drawing (like the centre of the drawing or
the southwest corner or a nearby landmark).

Therefore when you assign a projection to it, its origin point will be at
the origin of the projection.  In your case using EPSG 32638, that is UTM
zone 38N whose centre coordinates are 500000 4649776 (see
https://epsg.io/32638).

The origin point of your drawing will be in that coordinate system; (0,0)
will be 500000 metres west of 45° east longitude and on the equator; in
other words, 40.511256° east longitude and 0° latitude.

The latitude and longitude of Djibouti City is 11.572076, 43.145645 (see
https://www.latlong.net/place/djibouti-city-djibouti-24884.html).  You can
see that your origin point (0,0) in UTM zone 38M is 2.634389° west and
11.572076° south of Djibouti City.

Working back to UTM coordinates, in UTM zone 38N Djibouti City is
297791.1138793365 east and 1279891.9276488272 north (see
https://epsg.io/transform#s_srs=4326&t_srs=32638&x=43.1456450&y=11.5720760)

If your DXF data coordinates - say the centre of the drawing file - aren't
somewhere around (297791,1279892) then you need to transform your data as
you import it (or afterward).

Please read these articles

https://opengislab.com/blog/tag/DWG+DXF+QGIS
https://gis.stackexchange.com/questions/22691/georeferencing-dxf-using-qgis
https://www.mkrgeo-blog.com/good-solution-to-georeferencing-the-autocad-dwg-drawing-in-qgis-3x/

which will provide you with some ideas and more background info.

-- 
Chris Hermansen · clhermansen "at" gmail "dot" com

C'est ma façon de parler.
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