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How To: Generate polygons from CAD lines with text, blocks, points or attributes inside

Summary

Instructions provided describe how to generate polygons from CAD lines with text, blocks, points or attributes inside. It is common in CAD to represent polygon features as a network of individual line boundaries rather than closed polygons. There is no standard way to create polygon attributes in CAD, and it is common to include a point, symbol, or text entity inside the inferred boundary as attribution.

The FEATURE TO POLYGON tool generates polygons from a linear network of lines, and can optionally include the attributes of point features found to be inside the newly created polygons.

Procedure

Run the FEATURE TO POLYGON tool with a CAD POLYLINE feature class as input to build polygons from CAD data. Be discriminating about the CAD data used to get the desired result.

Use any valid line and point feature class, including CAD polylines and points to create the input line features and optional input label features.

To generate meaningful polygons from CAD lines and points, consider the following before executing FEATURE TO POLYGON with the CAD data:

  1. Identify the source CAD data. It is rare that all the linear features or point features in a CAD drawing are used as boundary features and label points to build the polygons. Therefore, identify just the lines and points in a CAD file that are used to depict the desired polygon boundaries and optional descriptive attributes.

    CAD files typically organize data by layers, but it is equally valid to distinguish features by graphic properties in combination, such as line style, line weight, or color. Once it is determined how data is organized in the drawing, use the distinguishing information to filter the input data.
  2. Limit the candidates from the CAD file used to build polygons. Applying a filtering query to the input features allows the isolation of just the input features useful in creating polygons.

    The task of selection implies a multi-step process, and automating this task using a script or model is strongly suggested. Some samples models of CAD translation are available on ArcScript. See the link in the Related Information section below.

    The SELECT tool includes a query expression parameter that allows the creation of a feature class from a subset of CAD features and writes them to disk.

    The MAKE FEATURE LAYER tool also includes a query expression parameter to create a subset of a feature class, but retains this subset in memory rather than writing it to disk.

    Using either filtering method is acceptable.
  3. Build polygons using the FEATURE TO POLYGON tool. To actually create the polygons invoke the FEATURE TO POLYGON tool found in the Data Management Tools > Features toolset of the System Toolbox. Use the selected lines as the input line feature class parameter and optionally the selected point features as the label point parameter.
  4. Other issues for consideration are that some CAD polyline data can be generated in such a way as to record the vertices out of order. This does not affect their function in CAD, but does adversely affect their use as GIS features. Typically, these types of non-standard entities are generated from 3D party, and custom CAD applications, such as DXF conversion or applications, that generate geometry. AutoCAD and Microstation do not generate such features, although it may be possible to edit entities in such a way as to generate these types of errors. It may be necessary to use the REPAIR GEOMETRY tool on the selected line features, originating from CAD before passing them along to the FEATURE TO POLYGON tool.

    The FEATURE TO POLYGON tool has a tolerance value that can automate the snapping of vertices when data includes small topological errors, but may allow less control of the results.

    Consider the implications of using an automated snapping tolerance on CAD lines that may contain overshoot and undershoot errors. Data containing errors may be moved so that the resulting intersection point may be at the average of the coordinates within the small tolerance zone. Depending on the desired result, this may or may not be desired. If the lines should not be moved, even a small amount, or more control is needed, use the topological editing tools in ArcMap to identify and repair errors when building polygons, rather than using the FEATURE TO POLYGON tool.

Related Information