SIGM: Sigma-Record

This record specifies information for the various observation types. Any number of sigma-records can be used. If the sigma-record identifier (columns 7-9) is identical to that of a previously defined sigma-record, the new specification overwrites the old one (you can re-use the identifiers). Note that it is your responsibility to use a defined sigma-record for the appropriate type of observation (see table below).

If the standard deviation computed from sigma-record information is zero, a valid (non-zero) standard deviation (or covariance matrix) must be given in the usual way for the observation (this would be the case when you specify only an auxiliary parameter on the sigma-record). The standard deviation computed from the SIGM record is always added to the standard deviation (or diagonal elements of the covariance matrix) that is given in the usual way.

You can use the SIGM record to define information for all observation types. In the case of 2DC, 2DD, 3DC, and 3DD observation groups, the values are used to modify the diagonal elements only.

The following shows which values on the sigma-record are used for which observation types (the abbreviations are the same as those in parentheses in the SIGM record description below):

ANGL: STD, FR_C, TO_C, AUX
AZIM: STD, FR_C, TO_C, AUX
GAZI: STD, FR_C, TO_C, AUX
DIST: STD, PPM, FR_C, TO_C, AUX
EDIS: STD, PPM, FR_C, TO_C, AUX
DSET: STD, FR_C, TO_C
EHDF: STD, PPM, FR_C, TO_C
EHGT: STD, AT_C
OHDF: STD, PPM, FR_C, TO_C

OHGT: STD, AT_C
VANG: STD, FR_C, TO_C
GVAN: STD, FR_C, TO_C
ZANG: STD, FR_C, TO_C
GZAN: STD, FR_C, TO_C
2DC: STD, PPM, AT_C, AUX
2DD: STD, PPM, FR_C, TO_C, AUX
3DC: STD, PPM, AT_C, AUX
3DD: STD, PPM, FR_C, TO_C, AUX

The SIGM record has two possible formats, a long parameter name format, and a short parameter name format. If column 10 contains an asterisk (*) the long name format is expected, otherwise the short name format is expected.

The short parameter name format is as follows:

Columns 002-005: SIGM
Columns 007-009: Sigma record identifier (1 to 3 characters)
Columns 010-010: (blank)
Columns 011-020: Standard deviation of observation (STD)
Columns 022-031: PPM (PPM)
Columns 033-042: At-station centering error (AT_C)
Columns 044-053: From-station centering error (FR_C)
Columns 055-064: To-station centering error (TO_C)
Columns 066-077: Auxiliary parameter name (AUX)
Columns 079-080: Linear unit name

The long parameter name format is as follows:

Columns 002-005: SIGM
Columns 007-009: Sigma record identifier (1 to 3 characters)
Columns 010-010: * (asterisk)
Columns 011-020: Standard deviation of observation (STD)
Columns 022-031: PPM (PPM)
Columns 033-042: At-station centering error (AT_C)
Columns 044-053: From-station centering error (FR_C)
Columns 055-064: To-station centering error (TO_C)
Columns 066-096: Auxiliary parameter name (AUX)
Columns 098-099: Linear unit name