Principles of Operation 15
© 2018 Nortek AS
1.6 Coordinate System
You may specify the preferred coordinate system in the Deployment Planning menu of the
instrument specific software. The velocity measurement is a vector in the direction of the transducer
beam, which we refer to as beam coordinates. Beam coordinates can be converted to a Cartesian
coordinate system (XYZ) by knowing the beam orientation. Furthermore, the flow can be presented
in Earth normal coordinates (ENU - East, North and Up) if the instrument is equipped with a
compass and tilt sensor.
The coordinate systems are defined as follows:
In Beam coordinates, a positive velocity is directed in the same direction as the beam points.
Beam 1 is defined as the arm with the black / red marking. If the marking is missing, beam 1 can
be identified as the arm opposite of the engraved serial number on the probe. Beam velocities are
not corrected for probe geometry.
In XYZ coordinates, a positive velocity in the X-direction goes in the direction of the X-axis arrow.
The X-axis points in the same direction as beam 1, and newer models have the direction engraved
in the probe endbell itself. Use the right-hand-rule to remember the notation conventions for
vectors. Use the first (index) finger to point in the direction of positive X-axis and the second
(middle) finger to point in the direction of positive Y. The positive Z-axis will then be in the direction
that the thumb points. Remember that XYZ coordinates are relative to the probe head and
independent of whether the instrument points up or down. For instruments with 4 receiving
transducers, an extra Z velocity is outputted (X, Y, Z1 and Z2).
In ENU coordinates, a positive east velocity goes toward east. This is also a right-handed
orthogonal system.
ENU and XYZ coordinates are the most practical when handling data. Beam coordinates are
primarily useful for higher-level turbulence calculations and for dealing with phase wrapping issues.
Figure: Coordinate systems. From left: Beam coordinates, XYZ coordinates and ENU coordinates
The figure below shows an assortment of Velocimeters probes and the definitions of beam and XYZ
coordinates.
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