NAME
sdts2dlg - Convert a USGS SDTS DLG to a DLG in the ‘optional’ format.
SYNOPSIS
sdts2dlg [-L] | [sdts_dlg_file.ddf [output_file_name]]
DESCRIPTION
The U.S. Geological Survey (USGS) provides sites on the Internet with a
lot of Digital Line Graph (DLG) data. Depending on the resolution of
the data, it may be available in one of at least two different formats:
the ‘optional’ DLG format, or the Spatial Data Transfer System (SDTS)
format. The 24K DLG data (which are also called 7.5-minute DLG data)
are only available for free download in the SDTS format.
The drawmap program can read files in SDTS format; but the SDTS
information comes in the form of archives, each of which contains
numerous files. It may sometimes be more efficient, and perhaps
simpler, to store the data in the ‘optional’ format. Sdts2dlg is a
program that converts each SDTS archive into a single optional-format
DLG file. Currently sdts2dlg only works with 24K and 100K DLG data.
If you invoke the program with the "-L" argument, it will print some
license information and exit. In normal use, the first argument is an
SDTS file name.
Each SDTS DLG archive should contain one or more files with names of
the form ????LE@@.DDF, where the ’?’ symbol stands for any single
character, and the ’@’ symbol stands for any single digit. If you
provide a single such file as an argument, sdts2dlg will produce an
optional-format DLG file, based on the given SDTS file and the other
files in the SDTS archive. (When you unpack the SDTS archives, you can
change all of the resulting file names to all lower case and/or
compress all of the files with the gzip program. If you are going to
change to lower case, change all of the files. If you are going to
compress the files, compress all of them.)
The USGS takes each 1-degree-square block of latitude and longitude,
and divides it into an eight-by-eight grid of 7.5-minute-square
‘quads’. The rows of this grid are labeled ’a’ to ’h’ from bottom to
top, and the columns are labeled ‘1’ through ‘8’ from right to left.
Each quad is then referred to by a name of the form ‘AABBBCD’, where
‘AA’ is the latitude of the southeast corner of the 1-degree block,
‘BBB’ is the longitude of the southeast corner, and ‘C’ and ‘D’
represent the corresponding row and column labels. If you don’t
specify an output file name, the output file produced by sdts2dlg will
have the form ‘AABBBCD.dlg’. For 100K DLG files, each of which
generally covers 16 quads, this naming is also used, with the ‘C’ and
‘D’ code being the same as for the quad with the same southeast corner
as the 100K DLG data. If you specify an output file name, then your
name will be used instead.
LIMITATIONS
Sdts2dlg attempts to recreate the original DLG-3 file, as it was before
conversion to SDTS. Node entries include line lists, but no area
lists. Area entries include line lists, no node lists, and no area
coordinate lists. Line entries include line coordinate lists.
Attributes are included as major-minor pairs.
Try as it might, though, sdts2dlg is going to produce files that differ
from the originals. In the Node entries, for example, the line lists
have no preferred ordering, and the output from sdts2dlg is unlikely to
reproduce the ordering of the original DLG file. In the Area entries,
the line lists do have a specified ordering, but there is no rule for
choosing which line to start each list with. Thus, the lists are
unlikely to be exact duplicates of the original file. Neither of these
differences should be a problem. The lists are still valid, even if
they are not identical to the originals.
For the universe polygon (which completely surrounds the data coverage
area) the Universal Transverse Mercator (UTM) coordinates of its
representative point are likely to be different from the originals.
The SDTS files do not appear to encode this particular representative
point, so sdts2dlg inserts the coordinates of the southwest corner of
the map area.
Some floating-point numbers will have a different format from their
original USGS versions. The USGS files normally put the first
significant digit after the decimal point, while sdts2dlg puts it in
front of the decimal point. The actual numeric values are the same,
but the format differs.
There are two fields in the header that don’t always show up in
existing optional-format files, although the specification allows them:
the horizontal datum and the vertical datum. These are too useful to
leave out, so sdts2dlg includes them.
Finally, there may be data discrepancies due to errors in the program.
Errors are a distinct possibility when it comes to attributes. There
are many different kinds of possible attributes, and I have not yet
found adequate test data for some of them.
SEE ALSO
drawmap(1), sdts2dem(1)
Jul 24, 2001