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3 Most Costly Errors in CAD to GIS Conversion

by Mike on July 11, 2014

In recent installments, I discussed the efficiency of using AutoCAD Map’s “MAPEXPORT” and “MAPIMPORT” commands to create ESRI-compatible Shapefiles to move data from CAD to GIS.  This installment provides the rest of the story, with three caveats one for shapefiles in general, and two for general use in both CAD and GIS.  This installment will get deep into the weeds of CAD and GIS, so if you are a construction manager or coordinator interested in saving time and money on a project but aren’t a CAD jockey, feel free to forward this to the engineers on the project who are.

1.      Arcs: Unfortunately, arcs do not exist in shapefiles; therefore, they don’t survive in the CAD to GIS conversion using MAPEXPORT or MAPIMPORT in AutoCAD Map.  Here are some basic work-arounds:

  • Copy the features from CAD to GIS separately and usually by layer, using ArcGIS.
  • Minimize the issue by exporting to GIS for viewing purposes only, without “round-tripping” back into CAD.
  • Be sure you have the original CAD data and cross-reference it to the GIS data using metadata or a “Source” field in the attributes.
The method you choose will depend on your data’s lifecycle and your business needs.
 
2.      Asymmetrical symbology: In any CAD or GIS application, there is occasionally an abhorrent practice employed wherein a symbol for a point-type of object is created asymmetrically – that is, the graphic sub-entities composing the symbol are not arranged around a central point.  This means that when objects are scaled their apparent position changes, and it also means that “round-tripping” data between CAD and GIS platforms results in an apparent positional shift – that is, the features will be centered on their insertion point, not the apparent center of the symbol.

 
 
Logically, symmetrically-constructed blocks will maintain their apparent coordinate positions when scaled or translated between platforms.  Always be sure to use symmetrical complex symbol creation for best results, and verify this characteristic before translation.  Beware of multiple symbol definitions, too, which can present seemingly identical symbology that has a completely different underlying composition.  That is, you may find that your CAD drawings use two or more differently-named “block definitions” for what are essentially the same objects (e.g., manholes represented by two different blocks – one named “MH” and the other named “MANHOLE”, with differing CAD properties such as layer, size, and construction.

3.      Non-topological features:  Sometimes CAD features are created using techniques that make sense for plotting construction documentation, but they don’t support logical relationships for the resulting features when translated into GIS.  For example, circles and polygons translate as linear features, not point features, resulting in lost functionality and value.
 
When the Indiana Geographic Information Council (IGIC) convened their workgroup on CAD-to-GIS procedures a couple of years ago, one of the recommendations they made was to ensure topological sense when creating features, so that point objects in the real world were represented by point features in CAD and GIS, lines (or more often, polylines) for linear ones, and mpolygons in CAD (or better yet, topologies) for area-type features in GIS (polygons).  As the former Chair of the Workgroup, I heartily endorse their findings.  This was also reinforced by the FAA and the SDSFIE (Spatial Database Structure for Facilities, Infrastructure and the Environment) in their CAD-to-GIS data models.
 
Along similar lines, topological rules should be enforced where logical – lines breaking at intersections, polygons not overlapping, etc., depending on the real-world “rules’ for those objects (or features).  Networks consisting of both point and line features should be “snapped” together, so that endpoints of lines and insertion points of symbols coincide.  Ensuring this topological integrity will save you hours of rework.
 
These are the biggies, in terms of endemic errors most frequently occurring.  Generating effective as-built data from digital construction documents shouldn’t be seen as an added cost.  However, with the proper approach, they save money for active, dynamic facilities.