Terascan Commands: lac and gac
lac - creates AVHRR datasets from LAC tapes or datafiles.
gac - creates AVHRR datasets from GAC data or datafiles.
This command is used to read Level 1B data tapes or files, and
convert them to a Terascan Data File for use in Terascan programs.
This is the first step in using AVHRR data. The lac and gac
commands are very similar, the only difference is that lac is used
on 1.1km Local Area Coverage Data, and gac is used to read 4.4km
Global Area Coverage Data.
Important things to think about
- Extract data using a master file?
Data can be extracted in two ways
Extracting data using a master file has a few quirks, and doesn't always
work as well as I would like. Typically, I use option number 2, and
start at the beginning of the data file and read in 1000-1500 lines. If
I miss the area of interest, I repeat the process.
- the smallest rectangular area required to cover a master dataset
- a rectangular area of specific size beginning at a specified time
and scan sample.
- Calibrate data at this time?
I usually calibrate the data when using lac or gac. The only reason that
you would want to answer no at this point is if the data file has
missing data scan lines. If it does, they can be fixed using
avfix, but that program can only be run on
- In order to read data tapes, the tape drive must be attached to
tephra. For example, in our current lab configuration, the 9-track tape drive
is hooked up to volcano, but Terascan cannot access remote tape drives. The
name of the 8mm tape drive on tephra is /dev/rmt/1n
- It works best if you allow the program to create the name of the output
file (see below). The program will examine the data header, and assign a
unique name to the output file. (eg. n14.96179.0303 where n14=noaa-14;
96=year; 179=day of year; and 0303=start time of data in GMT.)
GAC data file acquired from the
Satellite Active Archive
In this example, the data file is in the current directory and named
output file(s) : char(255) ? [.]
tape_device : char(255) ? [/dev/rmt/1n] ./A0009414.L0021911
tbm_header : char( 3) ? [yes]
use_master : char( 3) ? [yes] n
start_time : char( 15) ? [00:00:00]
num_lines : int ?  1000
start_sample : int ? 
num_samples : int ? 
channels : int ( 5) ? [1 2 3 4 5]
calibrate : char( 3) ? [no] y
temp_units : char( 10) ? [celsius]
Processing tape file 1
avcal.coef: No date passed for AVHRR calibration table
avcal.coef: Using pre-launch coef's for visible channels
./n14.96179.0303 starts at time 03:03:27.64, sample 1.
Actual size is 763 lines by 409 samples.
This page maintained by Dave Schneider (firstname.lastname@example.org)