SUMMARY

Rotates a pair of data components through an angle.

SYNTAX

ROTATE {to GCP|TO v|THROUGH v|,{NORMAL|REVERSED}

INPUT

TO GCP: Rotate to the "great circle path". Both components must be horizontals. The station and event coordinates header fields must be defined.

TO v: Rotate to the angle v (degrees). Both components must be horizontals.

THROUGH v: Rotate through the angle v in degrees. One component may be vertical.

NORMAL: Output (horizontal) components with the second leading the first by 90 degrees (clockwise rotation looking down).

REVERSED: Output (horizontal) components with the second lagging the first by 90 degrees (counterclockwise rotation looking down).

TO GCP:	Rotate to the "great circle path". Both components must be horizontals. The station and event coordinates header fields must be defined.
TO v:	Rotate to the angle v (degrees). Both components must be horizontals.
THROUGH v:	Rotate through the angle v in degrees. One component may be vertical.
NORMAL:	Output (horizontal) components with the second leading the first by 90 degrees (clockwise rotation looking down).
REVERSED:	Output (horizontal) components with the second lagging the first by 90 degrees (counterclockwise rotation looking down).

DEFAULT VALUES

ROTATE TO GCP NORMAL

DESCRIPTION

Pairs of data components are rotated in this command. Each pair must have the same station and event header variables, NPTS, B, and DELTA

TO option: Both components must be horizontals: CMPAZ must be defined and CMPINC must be 90 degrees. After the rotation is completed, the first component of each pair will be directed along the angle given after the TO keyword. If the TO GCP option is used, the station and header fields STLA, STLO, EVLA, and EVLO must be defined so that the backazimuth (BAZ) can be calculated. After the rotation, the first component will be directed along the angle given by the station-event backazimuth plus or minus 180 degrees (to keep the final angle between 0 and 360 degrees). This component therefore points from the event toward the station (the radial direction), and the second component is called "transverse" or "tangential". (The (upward) vertical, radial, and transverse directions form a left-handed coordinate system.)

The NORMAL and REVERSED options apply only to horizontal rotations. If the NORMAL option is used, the second component leads the first by 90 degrees. If the REVERSED option is used, it lags the first by 90 degrees. ROTATE TO GCP REVERSED results in a transverse component in the opposite direction from ROTATE TO GCP NORMAL, a convention preferred by some researchers.

EXAMPLES

To rotate a pair of horizontals to a specified angle for the first component:
SAC> READ XYZ.N XYZ.E
SAC> ROTATE TO 123.43
To rotate two sets of horizontals so that the first component in each set along the great circle path and then write SAC files for the radial and transverse components:
SAC> READ ABC.N ABC.E DEF.N DEF.E
SAC> ROTATE TO GCP
SAC> W ABC.R ABC.T DEF.R DEF.T

HEADER CHANGES

CMPAZ, CMPINC, KCMPNM, DEP*

ERROR MESSAGES

1301: No data files read in.
2001: Command requires an even number of data files.
2004: Insufficient header information for rotation:
- STLA, STLO, EVLA, EVLO must be defined for GCP option.
2002: Following files are not an orthogonal pair:
2003: Following files are not both horizontals:
- TO option only works on horizontals.

LATEST REVISION

January 8, 1983 (Version 8.0)