[Qgis-user] ref pb

[Marc Millas]

👍
logical and clear
thanks

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com



On Thu, Feb 9, 2023 at 6:16 PM chris hermansen <clhermansen at gmail.com>
wrote:

> 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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