YSI SonTek RiverSurveyor, System Manual

Page: 37 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
25
3.3.3. How does bottom-track work with RiverSurveyor?
As previously explained (§
3.3.2
), bottom-track is a great tool for measuring vessel velocity and
water depth. Unlike GPS (§
3.4
) and echo sounder (§
3.5
) inputs, bottom-track data does not re-
quire any additional serial ports (i.e., inputs) to your computer while still providing all the neces-
sary information for discharge measurements. As such, there is no benefit to disabling the bot-
tom-track feature in the
RiverSurveyor program.
In fact, all RiverSurveyor systems are shipped from the factory with bottom-track enabled. We
recommend that you do not disable bottom-track at any time. The increase in file size and power
consumption is negligible when bottom-track is enabled. When using the
RiverSurveyor pro-
gram, you should verify this setting each time you connect to your ADP (the current bottom-
track setting for your system can be found on the
Advanced
page of the
User Setup
dialog box
[
Figure 3-20
]).
When the ADP measures the velocity of the water beneath it, the ADP is actually measuring the
velocity of the water relative to the ADP. If the ADP is stationary, it is measuring the velocity of
the water relative to both itself and the Earth. But what happens if the ADP is not stationary,
such as on a moving vessel?
In this case, the water velocity relative to the ADP is the vector sum of the ADP’s (or vessel’s)
velocity over the ground and the water’s velocity over ground. As such, we must subtract the ve-
locity vector of the ADP/vessel to compute the speed of the water over ground (i.e., the true wa-
ter speed). By default, the
RiverSurveyor program does this automatically (
Velocity Coordinate
System
set to
Bottom Track
).
Within the RiverSurveyor program, bottom-track information (speed and direction) will be
shown in the tabular
Track
display. The individual depth to the bottom as detected by each of the
transducer beams, and the mean depth for all beams is also displayed. This is a useful tool be-
cause significant changes in the slope or bottom gradient will be indicated by variations in each
of the beam depths.
Data analyzed in RiverSurveyor can be referenced to either bottom-track or GPS (§
3.4
) for dis-
play of the vessel track (known as the track reference ; §
3.10.1
). The track reference will auto-
matically default to bottom-track if it is present, but the user can modify this setting. The depth
of water can be referenced using either bottom-track or an echo sounder (if available). This se-
lection is known as the
depth reference . By default, this setting will be set to bottom-track.
3.3.4. When and where should I use bottom-track with RiverSurveyor?
The simple answer to this question is always. While it is true that GPS will give you positional
information, and an echo sounder will give you depths, there are several reasons why you should
always plan on using bottom-track.
•
When operating in the proper conditions during a discharge measurement, using bottom-
track as a reference for velocities is significantly more accurate than using GPS as a refer-
ence. This is especially true for river widths less than 100 m. When conditions permit, bot-
tom-track should always be used in place of either GPS and/or echo sounder data.
•
Certain environmental conditions can limit the use of GPS for positional data (§
3.4.7
).
•
Bottom-track can be combined with either GPS or echo sounder data to give a higher degree
of confidence vessel position data and depth data. However, keep in mind that bottom-track