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AutoCAD Integration

Learning Objectives

  1. Understand basic data structure of CAD created data compared to ESRI format feature layers
  2. Georeference a CAD dataset in ArcMap
  3. Perform data conversion from CAD format into a geodatabase

Problem

The problem involves developing a file geodatabase for the NC State University Campus.  An AutoCAD 2005 Masterplan drawing file showing both on- and off-campus streets, buildings, sidewalks, streams, athletic fields, etc. must be used to develop this database.  It has been drawn to scale, but needs to be georeferenced to a coordinate system.  A 1999 panchromatic orthophotograph has been provided to assist in georeferencing the CAD file to NC State Plane coordinates with NAD83 datum in US feet.  All buildings in the geodatabase must exist as a polygon feature class.  Finally, only campus features are to be included within the file geodatabase. 

Analysis Procedures

In order to develop the file geodatabase, the AutoCAD file was added within ArcMap and georeferenced to the orthophoto.  This will be accomplished using the Georeferencing toolbar. The 1999 orthophoto and 2005 AutoCAD drawing were added to ArcMap.  (The entire CAD dataset was brought into ArcMap; however, individual feature classes could be added.)  Because the orthophoto contains only world file coordinates and does not possess defined coordinate system parameters, the data frame within ArcMap was set to the NC State Plane coordinate system.  This also sets the display properties for working in ArcMap. With the orthophoto in view, the Georeferencing toolbar was opened in ArcMap, and the Fit to Display command was used to bring the CAD drawing within view of the orthophoto.  Layers showing campus streets, campus sidewalks, streams, lakes and ponds, existing and future campus buildings, and athletic fields were symbolized to appear over the black and white orthophoto using the Layer Properties dialog box.  This was accomplished by selecting unique values with “Layers” as the value field.  All other layers were removed from the display.  Carefully selecting two control points, (stationary features that exist in both datasets) a trial-and-error process was used to update the display and eventually create a world file (.wld file) that georeferenced the CAD file to the orthophoto in State Plane coordinates. 

Next, a file geodatabase was created under the ArcCatalog tab in ArcMap.  In order to convert on-campus CAD layers into an ArcGIS file geodatabase, the Conversion, To Geodatabase, Feature Class to Feature Class tool was used from within ArcToolbox.  The CAD polyline features dwg file was selected as input with an SQL query to select a campus layer of streets, sidewalks, lakes, streams, or athletic fields with each run. Though the same CAD entity can exist in both a polyline and polygon CAD feature class, the polyline feature class will be used for geoprocessing, as it will usually have most of the needed data.  Each output feature class was assigned a coordinate system in the Environment Settings (since the CAD drawing had no reference system, and the default system is that of the input layer) and saved inside the new file geodatabase.  To ensure all campus buildings are included in the geodatabase as polygon feature classes, each existing and future campus buildings layer will be selected from the DWG polyline layer. All existing and future campus building entities within the dwg Polyline CAD files were then selected by attributes.  In ArcToolbox, the Data Management, Features, Feature To Polygon tool was used to convert the selections to an appropriately named feature class inside the geodatabase.  

  Optionally, because each CAD feature class shares the same coordinate system, a CAD feature dataset could have been created.  The flowchart below illustrates the conversion and analysis procedures.

   

 

Results

The map below shows the result of georeferencing a computer-aided design drawing over an aerial image that has been projected within ArcMap. The two datasets share the same scale and coordinate system.

Click image to enlarge.

 

Application and Reflection

Georeferencing is important for creating meaningful overlays and making accurate measurements.  Creating the world file for an image not otherwise georeferenced, can likewise be beneficial when referencing it inside your CAD drawing.  Often, an aerial image needs to be georeferenced inside a CAD drawing as a backdrop or point of reference.  When the CAD operator retrieves the image file, it is important to remember the accompanying world file for proper alignment.

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