[GIS] DXF entities not showing on import into QGIS

dxfqgis

I regularly get sent DWG files containing Ordnance Survey base mapping (variously with & without additional layers). I usually re-scale & reposition these in Draftsight, or just run a conversion in Teigha Converter. So far so good.

I have received a DWG with OS mapping & a site boundary. If I convert that to DXF(I usually have success with 2010 ASCII), all I get in QGIS is the site boundary polyline & a load of points.

If I open in a CAD program (Draftsight,QCAD etc) , the DXF that doesn't work in QGIS, I can see the whole drawing.

There is only 1 polyline entity in the import box, so its not that they're all down at 0,0 or something.

I can reproduce this on Linux & Windows, so not related to that.

Any idea where to look?

Best Answer

It turns out that the base mapping layers were in a block (unsupported entity presumably). I exploded the block & business as usual.

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[GIS] DXF (points and lines) in QGIS – Doesn’t import z coordinates

First forget about QGIS when playing with 3D, QGIS is not really there yet. If extracting vertices is enough or the lines are isolines (contours - constant elevation) then it is possible to use approach with free tools as explained here:

Importing 3D DXF file into QGIS with z information as attribute value?

The problem is when the lines in DXF have actually vertices in different elevations. In that case GRASS skip these lines when running v.to.rast. I think the only way in that case is to densify vertices on these particular lines to some needed minimum distance and extract them as points - v.to.points (dmax = your wanted minimum resolution value).

So in this case the process in open source GRASS (comes together with QGIS OSGeo4W installation):

v.in.dxf - to import contours including z dimension

v.to.points (dmax=) - to have all as points - while making sure lines become enough (dmax) dense points

v.to.rast (use=z) - convert dense points to raster

r.surf.contour - to linearly interpolate the rest of the surface (DEM) between points (lines)

In this case it is also possible to use other r.surf.* or interpolate from points v.surf.* or use v.out.ascii to export points to ascii x|y|z format and use any other software for surface interpolation

[GIS] Wrong scale when importing dxf into QGIS

Google satellite (EPSG:3857) is not a good idea. The units are metres, but only at the equator. The more to the North you go, the greater the difference gets.

Alternatively, you can use a custom CRS in transverse mercator projection, with latitude and longitude of origin (in degrees) at the point where your dxf file has its origin:

+proj=tmerc +lat_0=... +lon_0=... +x_0=0 +y_0=0 +k=1 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

Best Answer

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[GIS] DXF (points and lines) in QGIS – Doesn’t import z coordinates

[GIS] Wrong scale when importing dxf into QGIS

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