YSI SonTek RiverSurveyor, System Manual

Page: 59 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
47
it is of paramount importance that the compass is working correctly, and that it is installed in the
correct orientation. Generally speaking, all discharge and current profiling measurements are
normally recorded and displayed in ENU coordinates.
3.10. Track, Velocity, and Depth References
The following topics are covered in this section:
•
What does Track Reference mean?
•
How will selecting a different Track Reference change my results?
•
What does Velocity Reference mean?
•
How will selecting a different Velocity Reference change my results?
•
How do Track Reference and Velocity Reference relate to one another?
•
What does Depth Reference mean?
•
How will selecting a different Depth Reference change my results?
3.10.1. What does Track Reference mean?
Track reference refers to the source of the input that will be used to calculate the system’s posi-
tion, speed, and direction of travel at any time. There are two track references available:
•
Bottom-track (available on all RiverSurveyor systems; §
3.3
)
•
GPS (available only if connected to your system; §
3.4
)
3.10.2. How will selecting a different Track Reference change my results?
As explained in the previous sections of this manual, there are certain advantages and disadvan-
tages to using either bottom-track or GPS as the track reference.
•
Bottom-track (§
3.3
) is a “relative position” system (e.g., my current position is 5 m North of
my position 5 seconds ago). Advantages: lower uncertainty at tracks <1000 m. Disadvan-
tages:
moving riverbeds ; excessive depths; compass misalignments. If conditions permit,
bottom-track should always be your first choice.
•
GPS (§
3.4
) is an “absolute position” system (e.g., my current position is X
2
Latitude and Y
2
Longitude, and my last position was X
1
Latitude and X
1
Longitude). Advantages: lower un-
certainty at tracks >1000 m. Disadvantages: multipath error; receiver inaccuracy.
Remember, as a significant distance is traversed (typically more than 1000 m), the uncertainty
associated with positional information from bottom-track increases, while the uncertainty from
GPS remains the same. So when covering shorter distances (e.g., <1000 m) it may be preferable
to use bottom-track because of its lower uncertainty. For longer distances (e.g., >1000 m), GPS
can minimize the positional uncertainty. For distances near these limits, either positional source
should be adequate, and a combination of the two can often be helpful.
One of the benefits of using RiverSurveyor is that the track reference can be changed at any time
during post-processing, or when the system is not collecting data, by clicking the appropriate
track reference icon on the
Processing
toolbar:
•
–
Use Bottom-Track for Track Display
•
–
Use GPS for Track Display