YSI SonTek RiverSurveyor, System Manual

Page: 58 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
46
•
Manual – Enter the magnetic declination into the
Magnetic Declination
field in the
User
Setup
dialog box. You can determine the magnetic declination for your survey site from
several sources: magnetic declination maps provided by the U.S. Geological Survey, com-
mercial software, or Internet sites (see
Glossary listing for details).
3.9. Velocity Coordinate Systems
The following topics are covered in this section:
•
What are the different coordinate systems?
•
What are Beam Coordinates?
•
What are XYZ Coordinates?
•
What are ENU Coordinates?
3.9.1. What are the different coordinate systems?
There are three coordinates systems available for display of velocity vectors in RiverSurveyor.
•
Beam Coordinates
•
XYZ Coordinates
•
ENU (East-North-Up) Coordinates
3.9.2. What are Beam Coordinates?
Beam coordinates are relative to the orientation of the transducers on the head of the ADP. Beam
coordinates are typically used for diagnostics purposes only. The beams are referenced by a
number – Beam 1, Beam 2, and Beam 3 would exist on a 3-transducer RiverSurveyor system.
3.9.3. What are XYZ Coordinates?
This coordinate system is a Cartesian system relative to the orienta-
tion of the ADP. For rotations in this coordinate system:
•
Heading is defined as a rotation about the Z-axis.
•
Pitch is defined as a rotation about the Y-axis.
•
Roll is defined as a rotation about the X-axis.
On the ADP’s transducer head, you will find either an indented “dot”
(mini-ADPs) or an arrow with an X marked next to it (standard
ADPs). This dot/arrow (
Figure 3-23
) shows the direction of the X-
axis in the XYZ coordinate system. The Y-axis is 90° counter-
clockwise to the X-axis, when looking
at the transducers.
The XYZ coordinate system can be useful because it is always relative to the orientation of the
system. This means that for fixed deployments, the ADP can be rotated to a specific orientation
and measure the relative current velocities without the requirement for a compass.
Internally, the ADP measures currents in XYZ coordinates and uses compass data to “rotate” the
velocities to ENU coordinates. In the event data is recorded in XYZ coordinates, the
RiverSur-
veyor program can translate this into ENU coordinates if the system has a compass.
3.9.4. What are ENU Coordinates?
ENU is the abbreviation for East-North-Up, or “Earth” coordinates. ENU coordinates are relative
to the earth’s surface. The use of this coordinate system requires a reference to a heading angle.
For ADP systems, this heading angle typically comes from an internal compass (§
3.8
). As such,
Figure 3-23. Direction of ADP’s
X-axis in XYZ coordinates