problems georeferencing a dwg file

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05-27-2015 11:49 PM
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New Contributor

I am having trouble fitting a dwg file to the display and existing base map, georectified image and polygons within my workspace.  I have set the coordinate system to WGS_1984_UTM_Zone_56S but it seems to be way off - like of the base map.  I can get it to display in a new workspace and when adding a new base imagery it has appeared but of the world projection.

Any ideas on what I am doing wrong and how I can get it to fit my display on the zoomed in aerial imagery?

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MVP Frequent Contributor

Some things to be aware of:

  • Most AutoCAD files do not employ a Projected Coordinate system.  Instead, they are set up with a local non-Projected coordinate system, typically with the lower left corner being 0,0.  So as a result they will not align with any Projected Coordinate system.
  • AutoCAD does not normally provide for projections unless one gets added components like Autodesk Civil 3D (which is not that commonly used), so the resulting files, even if set up in the same coordinate system as desired, will be off from projected data as it is not projected (i.e. most CAD files are "flat-world").

As a result, the data almost always has to be adjusted.  Typically this is done by Georeferencing.

A quick tour of working with CAD data

ArcGIS Help (10.2, 10.2.1, and 10.2.2)

Chris Donohue, GISP

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MVP Regular Contributor

David,

Look at this post: CAD to GIS

The post describes how I georeference cad drawings.

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New Contributor

Thanks all for your help.  My problem seems to be getting it in to my existing work space.  Once I bring it the polyline feature class that I need as a layer from Arc catalogue, when I try and zoom to layer nothing happens.  I basically need to overlay it on to a small georeferenced aerial image and then from my limited understanding of ArcGIS I should be able to georeference the dwg file. Is this the easiest approach and any ideas why it will not zoom to layer?

Thanks again

David

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Occasional Contributor III

It should be able to zoom to the layer, but sometimes CAD drawings have single entities at 0,0 that make it appear that zoom to layer isn't working correctly because of this one small feature way out in the distance.

Another useful technique is to use the georeferencing toolbar's "Fit to Display". 

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Regular Contributor III

Try selecting some points lines or polygons in the CAD tables and then zoom to selected that should get where you can see it.

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Occasional Contributor II

I come across this from time to time and here are some things I always start checking.

  • Does the CAD file have a coordinate system internally defined? Like Chris said, many don't. If it doesn't, but the coordinates still match the data frame coordinate system, it will still display correctly, after getting the ESRI message about your "data missing a spatial reference and able to display it but not reproject it".  This is a tip off that it might not display correctly. You have to know what the coordinate system of the CAD file is intended to be, even if it is not defined in the DWG.
  • Is the base imagery being used have a coord system that really matches what you have the data frame set to? Cross check with imagery from another source. If it really is Zone 56 and you say it is Zone 54, obviously that won't work. I have seen mis-displays where the CAD file was correct and the base imagery was incorrect.
  • Go to your CAD staff, if you don't run CAD, and insert imagery of a known source and coord system. If it matches then you know your system.
  • Go to a local surveyor who would be familiar with coordinates in the area where the CAD file depicts data. He may be able to tell you a probable coord system for the CAD file.
  • I have created Projection (PRJ) files to store with the CAD file when the coord system does not match the data frame system.
  • If all else fails, and you know where on your data frame imagery the CAD file should be, and there are enough photo identifiable sources you can pick out from the CAD file (like road intersections maybe), then you can georeference the CAD file. If your CAD data is good and just an issue with figuring out what the coord system is, you should not have to do this.