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i have a land parcels dwg file in an unknown co-ordinate system and i want to georeference the file in ArcMAp and convert it to shapefile but the problem is that whenever i georef the cad drawings directly or or through spatial adjustment by converting it into shapefiles, the distances get distorted. for instance, if the length of line is 31 metres it changes to 0.000334 metres after projecting and georeferencing drawing. i have my control points in UTM easting and northing. can anyone tell me the detailed method to finish this task? The precise steps that i performed are:

  • importing dwg file in arcmap and assigning projected co-ordinate system to file.
  • georeferencing the dwg file through two control points by using an already georefenced tif file for destination points.

my cad drawing overlays on the tif file but main problem are the distances an area. they are inaccurate.

  • I think you should check the coordinate system on your shapefile by adding it to a blank map and measuring the distance between points for which you know the actual distance. – PolyGeo Feb 26 '15 at 10:25
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    It seems really suspicious that there are so heavy distortions when spatially adjusting the dwg to a tif. Have you checked that the tif really is correct in your reference system? And that you are not georeferencing the tif to your dwg and not the other way round? – Martin Feb 26 '15 at 12:07
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    If you are going to georeference the dwg through control points I wouldn't assign a projection to it at the beginning. Can you make things work by getting rid of the prj and starting all over again? – johns Feb 26 '15 at 14:55
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    I think you're georeferencing to data in a geographic coordinate ref system (latitude-longitude / degrees). Try georeferencing to a projected coordinate system that's appropriate for the area. At minimum, the local UTM zone. – mkennedy Feb 26 '15 at 17:49
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    If you have a 31m line coming in at .000334m (and it is meters, not degrees), that would suggest a scaling issue. Spatial adjustment will have the potential to create distortions and alterations in the data. If you don't want any change to the original data, you must use a projected CRS as mkennedy mentions, and at most you can do an origin placement and a rotation about that origin. Any further transformation or scaling (assuming your units are meters in both spaces), could and probably would result in distortions. You also can't georef to a tif with precision - tifs have pixels not points. – Chris W Feb 26 '15 at 21:58
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You have commented "when i georef the shapefile" which makes me think your procedure is astray. Perhaps you are defining a Projected Coordinate System on a shapefile in a Geographic Coordinate System (which has lat/long values) i.e. using Define Projection when you need the Project tool.

You should be georeferencing the CAD file not the shapefile.

There is a good help document entitled Georeferencing CAD datasets which starts:

The best practice for georeferencing a CAD dataset is to use the Georeferencing toolbar. The toolbar includes interactive tools to help you nudge the dataset into place as well as precision tools for registering control points to specific geographic coordinates.

If this does not just work then I recommend editing your question to include the precise steps that you perform.

The steps are:

  1. Check that the coordinate system of your control (TIF?) is a Projected Coordinate System and that its values make sense by adding your control to a Blank Map and looking at the values displayed at lower right as you move your mouse around.
  2. Georeference your DWG to the control.
  3. Convert your georeferenced DWG to shapefile if that is needed.

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