Coordinate system (CRS) of drawing

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andrew
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Coordinate system (CRS) of drawing

Post by andrew » Fri May 28, 2021 10:56 am

From a QCAD user:
How can I find out or change the coordinate reference system (CRS) of a drawing file?

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andrew
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Re: Coordinate system (CRS) of drawing

Post by andrew » Fri May 28, 2021 11:07 am

It's a common misunderstanding that CAD systems are somehow aware of the particular coordinate reference system (CRS) a drawing uses. This is usually not the case. For a CAD system, coordinates are simply X/Y values. The meaning of these values is entirely up to the user's interpretation.

If your drawing is, let's say in EPSG:2056, this means that the coordinates in the drawing are to be interpreted as EPSG:2056 coordinates. One unit in the drawing equals one meter in the real world, the coordinate 2'600'000/1'200'000 refers to a place somewhere in Bern (Switzerland), etc.

The file itself or the CAD system are not aware of that fact. However, you might want to name your drawings in such a way that you as a user always know the coordinate system they're in (for example "myfile_epsg2056.dxf").

If you do not know the CRS used by a file, you'd pretty much have to make an (educated) guess to find out its CRS.

If you have a file in a particular CRS and want to convert it to another, you can use the dwgmapconvert command line tool that comes with QCAD Professional.

Alternatively, you can use QCAD menu Misc > Projection > Map Projection as follows:

- Select > Select All
- Misc > Projection > Map Projection
- Enter the source and target CRS identifiers in the options toolbar (see https://epsg.io/).
- The "Segment length" option can be used for the segmentation of arcs and curves.
- "Retain Texts" can be used to only move texts to the projected position without exploding texts into polylines.

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