Downloading of multiple files from the same dataset can also be done using explicit FTP (file transfer protocol) commands, as follows
For users on the EESI network, this dataset reference code is linked to the actual dataset directory via entries in a top-level link directory which is pointed to by the environment variable DSREF. This permits referencing files in the dataset directory as
Copying these multi-file datasets into the user's working directory
using ftp or the unix cp command can be risky, especially, for Arc/Info,
if the user's working directory already contains an "info" subdirectory
(the "info" directory for the new dataset may contain files having the
same names as those in the "info" directory in the user's working
directory). For this reason it is safer to use the Arc/Info or LAS copy
Note -- Both Arc/Info and LAS may try to write temporary files into the user's current working directory. For this reason, the user should chdir to a working directory to which (s)he has write access.
For Arc/Info, the commands
may be used directly from the Unix prompt, where
chdir user-working-directory arc copy $DSREF/yyyy-nnnn/data-directory-name output
outputis the name the user wishes to give the dataset. Note that as of Arc/Info version 7.0, a multi-layer grid file ("stack") can be copied as a single unit using the
copystackcommand. To check whether a directory contains grid, polygon, or other data, the Arc/Info
describecommand may be used.
For LAS, an image file and its associated .ddr and .his files may be copied using the commands
and at the
chdir user-working-directory las
copy $DSREF/yyyy-nnnn/image-name output-image
Datasets with a .Z file extension are in Unix compressed format. They may be uncompressed using the Unix uncompress command or with gunzip, which automatically determines which compresssion method was used.
Software for reading the USGS ASCII DEM format is provided by many GIS and image analysis software packages, some of which also support the SDTS format. LAS program DEM2LAS handles both the ASCII and SDTS formats, and can directly process the gzipped files. A standalone program which can read a DEM in either ASCII or SDTS format, and convert it to a 16-bit binary array is available in both C-language source code and MSDOS executable form.
For MacIntosh users, Brian Wagner, San Diego, CA, has developed and made available DEM Reader software, which directly displays ASCII-format DEMs on the screen. He also is developing support for the SDTS-format DEMs.
Reblocking DEM files
Some DEM files are in a fixed-record-length format which some
programs, such as the Arc/Info DEMLAttICE, seem unable to process
correctly. On Unix systems, a DEM file may be uncompressed, its
blocked/unblocked stutus checked, and unblocking performed if necessary
using the commands
gunzip filename.gz od -c filename | more dd if=filename of=newfilename ibs=4096 cbs=1024 conv=unblockThe dd command is not needed if the output from od contains a linefeed character ("\n") at or before address 0002000, since this indicates the file is already unblocked.