MIDLG -- USGS Digital Line Graph (DLG) Tape Read

Reads vector information from a USGS (United States Geological Survey) DLG (Digital Line Graph) Optional Distribution Format magnetic tape (1:24000-Scale and 1:100000-Scale) into a PCIDSK database vector segment.

(NOTE: MIDLG does NOT read Standard DLG Distribution Format or 1:2000000-Scale Optional DLG Distribution Format magnetic tapes).

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PARAMETERS

MIDLG is controlled by the following global parameters: 

Name     Prompt                                  Count     Type
FILE     Database File Name                      1-64      Char
DBSN     Database Segment Name                   1-8       Char
DBSD     Database Segment Descriptor             0-64      Char
STARTFIL Start File on Tape                      1         Int

The following parameter receives output:

LASC     Last Segment Created                    1         Int

FILE

Specify the name of the PCIDSK file on which vector segment (type VEC:116) should be created to hold the read vectors. The PCIDSK file can have any type of image data or no image data. 

 EASI>FILE="filename"

DBSN

Specify an 8-character segment name to assign to the created vector segment: 

 EASI>DBSN="...string..."

DBSD

Specifies a 64-character descriptor for the new vector segment: 

 EASI>DBSD="...string..."      | Default, blank line

STARTFIL

Specify the DLG file from which to read vector data. There may be 3 files for every DLG (Header file, DLG, Trailer file). STARTFIL will refer to the DLG file to read. For example, if STARTFIL=2, MIDLG will skip the first DLG set (3 files) and a header file, before it starts reading from the second DLG set. 

 EASI>STARTFIL=1              | read first DLG file on tape
 EASI>STARTFIL=n              | read nth DLG file on tape

LASC

This parameter will contain the segment number of the newly created vector segment.

DETAILS

MIDLG is used to read vector (line) information from USGS (United States Geological Survey) Optional DLG (Digital Line Graph) Distribution Format magnetic tape files for 1:24000-Scale and 1:100000-Scale. Standard DLG Distribution Format and 1:2000000-Scale (Albers equal-area conic projection metres) Optional DLG Distribution Format magnetic tapes are not supported.

Although the topology of the DLG is not retained, MIDLG strives to retain as much information as possible. Line records are retained and node/area information is skipped. However, this information is partially retained, since lines will bound areas and node information is included in lines.

When MIDLG is run, a new vector segment (type VEC:116) is created on the PCIDSK file (FILE) to hold the information. Parameter LASC will be updated with the number of the newly created vector segment number when MIDLG completes. The PCIDSK file must exist, but does not have to have any image data (see EXAMPLE). The vector segment will be given a name (DBSN) and description (DBSD) specified by the user.

STARTFIL must be specified for a given MIDLG run. It refers to the DLG file to process, excluding header files and trailer files, if present. If the STARTFIL is not on the tape, MIDLG rewinds the tape and exits with an error message (E145, "Nonexistent file").

If the line has a Major and Minor attribute code associated with it, these are combined and saved in the Attribute field in the output vector record. The Minor code will be the last four digits, while the Major code will be the remaining high-order digits. For example, if the line has a Major code of 050 and a Minor code of 0200 (i.e. a shoreline), these are combined to produce 500200 which is stored in the attribute field for the line in the vector segment. If there is more than one set of Major and Minor attribute codes in the input DLG line record, only the first set is retained. The map scale, the name of the source map, and the overlay category are saved in the history descriptor for the vector segment. The segment history descriptor may be checked by specifying DTYP="FULL" and running ASL after the MIDLG run to ensure that the correct DLG specified by STARTFIL was read (see EXAMPLE).

There is no reporting function for DLG format magnetic tapes in the tape header read task MSH, so you will need the tape listing sent with the tape by USGS to keep track of what is on the tape.

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EXAMPLE

MIDLG requires that an image file exists before the program is run, but it doesn't matter whether the file has imagery or not. In the following example, an image file with no imagery is created to append the created vector segment to. The database size (DBSZ) is given the minimum values (Note: DBSZ would be important if you were defining bit map segments for the file at some later date, but does not affect vector segments for this example.) The number of channels (DBNC) is set to zero to indicate that there is no imagery in the database. ASL is run after the MIDLG run to check that the correct DLG was read (e.g., the segment history could read `1: 24000/LAKE MATHEWS, CA/BOUNDARIES (18 and 19)'). 

 EASI>FILE="DLG.PIX"
 EASI>DBSZ=8,1
 EASI>DBNC=0
 EASI>R CIM
 EASI>DBSN="BOUNDS"
 EASI>DBSD="Lake Mathews, CA - Boundaries"
 EASI>STARTFIL=1
 EASI>RUN MIDLG
 EASI>DTYP="FULL"
 EASI>R ASL
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