Raw velocity measurements are given in beam coordinates, which is a vector in the direction along each of the beams. Depending on the objective of the measurements, coordinate transformation may be beneficial. Beam coordinates can be converted to a Cartesian coordinate system (XYZ) or Earth normal coordinates (ENU: East, North and Up). The orientation of the beams/transducers, which is defined in the instrument specific transformation matrix, is used to convert from beam coordinates to XYZ. By additionally knowing the orientation in space, such as tilt and compass heading, ENU coordinates can be determined.
During transformation to ENU only the slanted beams are used in the transformation. The data represented in ENU originates only from these beams, and the centre beam (if collecting current data) is given individually and always in beam coordinates.
If the instrument is configured to measure data in XYZ or ENU, this coordinate transformation is automatically performed by the firmware and the output data will be in the selected coordinate system. The transformation can also be done in post-processing software. If the data is collected in beam coordinates, the transformation can also be carried out manually if desired. How the manual transformation from beam coordinates through XYZ to ENU is done is described below.
The starting point is a binary-to-ASCII conversion of the raw data file so that you have access to the three velocity files, .v1, .v2 and .v3, the transformation matrix in the .hdr file, and the .sen file. Heading, pitch and roll are presented in the .sen file (ref. the .hdr file), and these angles are output in degrees. The figure below indicates the three coordinate systems that the velocity data in .v1, .v2 and .v3 are measured in: Beam, XYZ or ENU. The coordinate system selected during the deployment planning is indicated in the .hdr file.
The three relevant coordinate systems. The figure shows an AWAC, but this applies to the other instruments as well.