Downloading Data for DLGVU

Data for the dlgvu map viewer is available for downloading from the United States Geological Survey (USGS).

This page needs a rewrite. The downloading instructions below are obsolete. The new interface is available through the USGS DLG data page; select large or small scale SDTS format from the links at the bottom.

A Detailed Example

The process of downloading data from the USGS is best explained by example. There are several possible approaches; this example downloads some Key West quadrangle data using a map-based interface. The example will be most instructive and least confusing if you follow it exactly, even if you don't really want the Key West data.

Begin by creating a new, empty directory to receive and unpack downloaded data.

Open a second browser window and follow these steps. After each step, wait for a new page to load.

Go to [old download page now gone].
Click on 1:24K DLG segment in the header bar. Watch out for the similarly-named DEM segment.
Click on Graphics, near the bottom of the bulleted list.
Click on Conterminous 48 states.
Click on the extreme southern tip of the state of Florida.
Click on the small triangular island southwest of the mainland.
Click in the KEY WEST,FL square towards the left.
Click on KEY WEST,FL

If all that worked as expected, you now are at an Index page displaying several directories. The map comprises data from all of those directories, but for purposes of this exercise you can skip most of them. For both the hydrography and transportation directories, and for any others if you wish, follow these steps:

Click on the directory name.
Click on 1/, the directory that is always supposed to contain the latest data.
Download to disk every file whose name contains sdts.tar. There may be up to three files in the transportation/1 directory but typically just one in other directories.
Click the browser's BACK button twice to return to the index of map layers.

After downloading from all the directories of interest, you have a collection of gzipped tar files. In a shell window, go to this directory. Don't unpack the tar files in the obvious way, because they contain duplicate file names. Instead, download this dlgx script and run it this way:

dlgx *.sdts.tar.*

The dlgx script unpacks the tar files one by one, renames files to avoid collisions, and removes files not used by dlgvu. The end result is a directory full of .DDF files. Run

dlgvu .

to display the map encoded by the DDF files in the current directory. After verifying that the map is functional, delete the downloaded *.sdts.tar.* files.

If you have zip and unzip in your search path, you can combine the DDF files into a single archive for use with dlgvu:

zip keywest.zip *.DDF dlgvu keywest.zip

After this, the *.DDF files can be deleted, and only the ZIP file is needed.

Alternate Approaches

Many things about that example could have been done differently. There are different ways to select the same data. From the 1:24K DLG start page, the "FTP via State" and "FTP via Alphabetical List" links locate a USGS quadrangle by name. It is also possible to run ftp from the shell rather than using a web browser.

After finishing the Key West example, you probably want to download something that is more interesting. Bear in mind that the Key West files are uncharacteristically small and that most maps involve larger amounts of data.

Different map areas can be chosen graphically or by name by following the appropriate links. Remember that coverage is spotty, especially at the 1:24K scale; you may wish to check the status viewer first.

Different map scales are available by selecting 1:100K DLG or 1:2M DLG on the initial download page. Only the DLG choices are useful here; DEM files are another thing entirely and are not read by dlgvu.

dlgvu has the potential for reading DLG files from other sources, although it's unclear how well non-USGS files will work. Maps in SDTS form from the Map Layers Warehouse of the USGS National Atlas have been successfully tested.

Common Problems

Because of incomplete coverage, it may not be possible to find data for every area of interest. Some areas lack hypsography (contour line) data at all scales. Many areas are not covered at 1:24K scale, or are covered only by some map layers.

Irregularities are occasionally seen in the structure of the USGS file server. For example, the files in a 1/ directory may not appear to be the latest, or SDTS files may be missing when other formats are present.

Some 1:100K maps contain multiple boundary layers -- that is, multiple *.sdts.tar.* files within the boundary/1 directory. Unpacking these tar files results in filename collisions for which we have not found a satisfactory workaround.

By Murphy's law, an area of interest often falls close to a map edge. dlgvu can paste together multiple maps, so the solution is to download maps from both sides of the boundary (creating two directories or ZIP archives) and then display them together.

Back to the dlgvu page