[Qgis-user] importing dwg/dxf

[Richard Greenwood]

Any AutoCAD drawing *can* be on real world coordinates. They're often not
because no real world coordinates were readily available for the project
and there wasn't a compelling need for real world coordinates. AutoCAD
Map3D and subsequent versions with different names like "Civil 3D" have
support for coordinate system conversions, but that doesn't mean that a
DWG/DXF that was produced in vanilla AutoCAD can't be on a real world
coordinate system.

AutoCAD is always Cartesian projected coordinates (not spherical geography)
so you can open a DXF into a QGIS project and then use the QGIS move tool
to move it onto whatever reference data you have in the project. The
AutoCAD data might be in different units that you need but you can take a
guess and use the QGIS scale tool. A common problem in the USA is an
AutoCAD file that's in inches but your project is in feet. So scaling by 12
fixes that. And QGIS has a rotate tool that allows for aligning
different "norths". So with the three QGIS tools move, scale and rotate you
can do an affine transformation. Maybe not as accurate or as clean as you
might hope for, but it's a solution that I've used many many times.

On Sun, Nov 21, 2021 at 11:59 AM Nicolas Cadieux <
njacadieux.gitlab at gmail.com> wrote:

> Hi,
>
> Only AutoCAD Map3D was a certain comprehension of what a CRS is.  To my
> knowledge all other Autodesk Pilotdo not used any CRS.  It is my honest
> opinion that AutoCAD Map is the worst attempt to make a GIS.
>
> Nicolas Cadieux
> https://gitlab.com/njacadieux
>
> Le 21 nov. 2021 à 13:52, Bernd Vogelgesang <bernd.vogelgesang at gmx.de> a
> écrit :
>
> 
> On 21.11.21 15:35, Nicolas Cadieux wrote:
>
> Hi,
>
> .dwg or dxf have no CRS.  They can be in inches, feet, mm, cm, m...
> Usually meter in a local CRS like a local WGS84 UTM ZONE is used.  You can
> usually find this in the metadata if available.
>
> Nicolas
>
> I have no deep technical insight into dwg or dxf, but I am pretty sure
> that those  CAD-"products" are able to be produced with valid coordinates,
> fitting to a common CRS. Most people using CAD-systems simply seem to be
> either too stupid for that, or just do not care.
>
> One of the reasons, CAD-"data" is produced with a local reference system
> instead with a normal CRS is, according to an CAD-operator I once asked
> about this, that some CAD-systems just slow down to in-operability when
> using real-world coordinates because of the huge numbers, compared to the
> small coordinates in their own system.
>
> So, I would not even try to fix this, but instead ask those guys to stop
> scratching their balls and better send you proper real-world data and tell
> you which CRS they are in . The handling of this "data" is punishment
> enough afterwords.
>
> Hope my dislike for this "technology" was not too obvious ;)
>
> Cheers,
>
> Bernd
>
>
>
> On 2021-11-21 9:07 a.m., Greg Troxel wrote:
>
> Boaz Bar Ilan<boazprosie at gmail.com> <boazprosie at gmail.com>  writes:
>
> i always have problem importing dwg or dxf .  the layers  dont fit the
> coardinations and even when i set the layers crs it doesnt work.
>
> I am far from an expert, but recently tried to deal with a dwg.
>
> My impression is that they are almost always in local coordinates, and
> the path to success is something like using GeoScience plugin to define
> a local CRS based on control points where you know global coordinates
> and local, and then to use that CRS for the data.
>
> I recently imported some "PNEZD" data (csv with point it, northing,
> easting, vertical, and description, all in an unspecified local grid,
> from a total station data collector) and used geoscience to align it
> wtih RTK obsservations of a few points, and things fit quite well.
>
> How are you getting dwg?  Are you using the proprietary dwg library with
> gdal, or is there some open source path?
>
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-- 
Richard W. Greenwood
www.greenwoodmap.com
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