2DD: 2D Coordinate Difference Header

This record specifies the beginning of a 2D coordinate difference observation set.

The format for a 2D coordinate difference observation group is as follows:

2DD [optional sigma ID]
[Set of 2D coordinate records]
[Covariance matrix header record]
[Matrix elements records]

The "Set of 2D coordinate records" must contain only PL, NE, or XY coordinate records. It may also contain HI, HT, 4PAR, and 7PAR records.

GeoLab will automatically subtract the first of these records from the others to arrive at the actual coordinate difference measurement values. Note that the covariance matrix elements entered must correspond to these computed coordinate differences.

The "Covariance matrix header record" can be one of the following records: COV, WGT, CORR, or GENC.

The matrix elements records are set up as described in Input Matrix Formatting.

The format of the 2DD record is as follows:

Columns 002-005: 2DD
Columns 007-009: Sigma record identifier (see the SIGM record).