NTAD to ESRI Shapefile Converter
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DISCLAIMER:  This ArcView extension is not a supported product of ESRI.  
It is provided to you "as is".  Technical support for this extension is not available from ESRI.  No other versions of this extension will be developed or provided.

This extension has not been tested on all NTAD datasets.  It has never be tested on the National Highway Planning Network (NHPN) files, though it has been reported to work.  Also, it was developed and tested only under the Windows NT operating system.  It should work on other platforms, but this is not guaranteed.  This extension may not be able to convert the larger NTAD files on minimally configured PCs.  

ArcView, relative to native C programs, is slow at reading text files and manipulating text strings.  Even on a moderately powerful PC, conversion of an average-sized NTAD data file can take as much as an hour.  The NHPN could potentially take days.  Please plan accordingly.

There are known problems when converting some of the datasets.  In particular, the converter will fail to create some of the long, narrow polygons in the National Parks dataset.  These problems will not be corrected.

The Bureau of Transportation Statistics will be releasing a new NTAD data viewer/converter along with their updated CD in the Spring of 1997.  Once available, that program will be the preferred mechanism for converting NTAD data to shapefiles.
   

Installation
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Copy ntadconv.avx either to your personal extensions directory (make sure the environment variable $USEREXT points to that directory) or to $AVHOME/ext (may also be named ext16 or ext32).  See the ArcView Help system for more details on installing extensions and using environment variables.

After copying the extension, start ArcView, and under File select Extensions.  In the Extensions browser window, check off "Convert NTAD Data".  The extension is now loaded into the project and ready to use.


Operation
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With a view active, select "Convert NTAD Data" from the File pulldown menu.  Select the NTAD datafile you wish to convert.  Valid files for conversion have the extensions .pnt (point data), .lnk (link or line data), .nod (node or line endpoint data, treated here as points), and .are (area or polygon data).  Next, select the states you would like to convert.  Multiple states can be selected by holding down the Shift key.  Finally, name the output shapefile.

The status area at the bottom of the ArcView window will say "Opening data files ..."  It may remain this way for many minutes while Avenue loads the NTAD text files.  Next, for line and polygon data, the geography (.geo) file is scanned.  Then the NTAD file you selected is processed and the new shapefile is written.  During this phase you can click on the "Stop" button to interrupt the process.  The shapefile will contain only those shapes that it had found up to that point.

When finished, a prompt will appear asking you if you want to add the shapefile as a theme in the active view.

The shapefile will only have the attributes that are contained in the input text files.  Additional attributes are provided in separate dBase files, and relate back to the shapes based on the LINK_ID attribute.  Use ArcView's table joining and linking capabilities to connect to these other attributes.