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A Data Converter for DLG Vector GIS Files

April 2, 2018 2 comments

My colleague Dr Bruce Ralston, a geography and GIS professor from the University of Tennessee, wrote a very useful program some years back to convert SDTS format (Spatial Data Transfer Standard) DLG (Digital Line Graph) vector files from the USGS into shapefiles.   With Dr Ralston’s permission, I have placed the program on the following site as a zip file:  https://esri.box.com/v/sdtsdlg2shp.  The reasons why this program and this format are useful touches on a key theme of this blog and our book:  Some data formats remain cumbersome (to put it mildly) to use.  SDTS is one of those formats.  Conversion programs like this one enable data in SDTS format to be read directly by a GIS program, such as ArcGIS.  Futhermore, the site that these programs were archived on is now blocked for non university users.  Changing sites and changing access is another theme of this blog!

To use, access the link above and download the file to your local (Windows) device.  Unzip the file, which will result in the following files:  setupdlg.exe, entity.zip, entity.dbf, and dlgmanual3.pdf.   The PDF is the well-written manual from Dr Ralston.  To start, access the setup program, setupdlg.exe.  This will install the program DLG2SHP, which is a Windows application for converting USGS Digital Line Graphs-3 (DLG3) in Optional Format or SDTS Format to Esri shapefiles.  For more about SDTS, read the Library of Congress information here and at the USGS, here.  This was a format widely promoted in the late 1990s and early 2000s, but was not widely adopted.  However, web sites such as The USGS National Map continue to serve very useful and detailed data in this format.  Fortunately, once the DLG files are downloadded, DLG2SHP makes converting these files easy.

The program automatically decompresses zipped DLG files, can batch process them, and allows the user to specify the output folder.  Hypsography shape features are automatically assigned elevations (lines and points).  There is no need for joining an elevation table to the shapes—DLG2SHP does this automatically.  In addition, the program performs complete SDTS Format Attribute Coding.  The SDTS format DLGs contain many tables. DLG2SHP converts these tables to dbf files with key fields for easy joining and linking to the geographic entities to which they refer. The shapes and attribute tables have descriptive names for ease in linking.

The program works with point, line, and polygon layers for a particular feature type.
The major feature types that DLG3 files cover are:   Hypsography, Hydrography, Vegetative Surface Cover, Non-vegetative Features, Boundaries, Survey Control and Markers, Manmade Features, the US Public Land Survey System, and Transportation.  In SDTS format, all transportation features are in a single DLG. In Optional format, they are broken into 3 groups: Roads and Trails, Railroads and Pipelines, Transmission Lines, and Miscellaneous Transportation Features.

For each type of feature in a DLG, the program will generate shapes for points,
lines, and areas. Each of these topological structures contains certain basic attributes.  For nodes, the basic attribute is the node ID. For Lines, the basic attributes are the line ID, the from node ID, the to node ID, the polygon on the left of the line, the polygon on the right of the line, and the length of the line.  For Areas, the basic attribute is the area id. For Optional Format DLGs, the X coordinate of the polygon centroid, and the Y coordinate of the polygon centroid also are included. For SDTS format DLGs, the centroids are stored as a separate point shape.  More attributes are stored in tables that can be joined or linked to each map layer.

One of our exercises makes use of these types of files for wildfire analysis.  The program looks like the graphic below when it is accessed.  For more details, see the SDTS2DLG manual.

 

sdtsdlg2shp

Running the SDTS2DLG program.

dlgroads

Digital Line Graph (DLG) road and hydro files for a single 1:24,000-scale area.  The DLG2SHP program enables the DLG files to be used inside a GIS, such as ArcGIS Pro, as shown here.  The lines marked by the arrows are cartographic neatlines that mark the edge of the area covered by the 1:24,000 cell.  They are not features, so they need to be selected by an attribute query and filtered out of any subsequent analysis.