I have a question regarding CAD and QGIS, but unfortunately I don't have a clue of CAD.

Here it goes: Real estates data has been mapped. There are several reference points surrounding the area. So there is an own coordinate system being used. The coordinates of the corners of each polygon are known. Each polygon holds different information regarding size, owner etc.

The task is to have this finished mapping converted to be used in QGIS. The person wants me to have it correctly transferred into UTM-PCS. The outcome is an editable map of the real estates and each polygon is given it's equivalent attribut table.

Any hints???

  • 1
    What do you know about the coordinate sytsem of the drawing? Have they set the center on one reference point, or did they use projected coordinates of the points? Is the y-axis North bound? You would have to use some kind of Affine transformation to gte the drawing to real projected coordinates. – AndreJ May 5 '14 at 7:13
  • ok, I'll let you know in some minutes. Do you think its possible to get the text into an attribute table? Does it somehow have to be combined? I think the biggest hindrance might be that in autocad lines combining the points are just sticks, but no real polylines. – GregorR May 5 '14 at 7:30
  • Alright. In the past the local government (Philippines), geodetic engineers, set up several tie points all over the town. They use these up to today. They are currently creating several smaller maps of the town (sharing the work) according to district boundaries. Unfortunately one one map there might be estates from one tie point, when another estate from another tie point further north. They use a local PCS, the y- axis is north bound. They are using Autocad 09, 10 and 13. Guess it's because of financial issues. – GregorR May 5 '14 at 8:10
  • For converting lines to polygons, see gis.stackexchange.com/questions/93369/…. It currently works only with QGIS Master. Can you supply WGS84 and local PCS Coordinates of a sample reference point? You can edit it into your question. – AndreJ May 5 '14 at 8:15
  • I'm not even sure if QGIS would generate polylines out of these lines in autocad. There's a referenced point (local pcs) and a line that leads towards the next referenced point. This line will be given the direction (degrees north and east e.g.) plus it's length. The tie points are not available in WGS84. Let me find out. – GregorR May 5 '14 at 8:26

Once you know the WGS84 coordinates of the tie point (which is supposed to be 0;0 in the CAD coordinate system) you can set up a custom transverse mercator CRS as I described here:

Using customized Coordinate System in ArcGIS Desktop?

With the local axis going north, you don't have to use the omerc projection.

If the tie point is only known in a projected CRS, you have to reproject it to WGS84. You can do this with QGIS, or GDAL cs2cs. But you have to know which CRS is used for the tie point coordinates.

If you have no information, but know where the point is, you can try to locate it with Google Satellite imagery or using a GPS unit.

  • I'm not sure if you understood me correctly, but they use their very own CS. coordinate (0;0) is with their first tie point! Thats the problem. I don't know how to transfer that to real coordinates.. – user29939 May 6 '14 at 9:36
  • You need to know the coordinates of that tie point in WGS84, UTM, Luzon 1911 or PRS92 to reproject the Autocad data to UTM. – AndreJ May 6 '14 at 11:51

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