YSI SonTek RiverSurveyor, System Manual

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SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
45
Compass Calibration Procedure
To calibrate the compass:
•
Perform compass calibration before deployment with the ADP in its mounted position.
•
Click
Calibrate Compass
in the
User Setup
dialog box to display the
Compass Calibration
dialog box (
Figure 3-22
).
•
Follow the instructions in the
Compass Calibration
box, which require you to rotate the
compass/ADP through two complete circles while varying the roll and pitch angles as much
as possible (within the limitations of the vessel).
•
Determine if the final “score” is acceptable. If not, you may need to change the survey site
or the ADP mounting material or location (e.g., away from the boat motor). Magnetic
strength fields greater than 25 should be avoided and may indicate a large ferrous object in
close proximity to the compass. If this occurs, try moving the transducer away from the ob-
ject and rerun the compass calibration. Typically, moving the transducer away from the
large ferrous object by less than a meter dramatically reduces the ambient magnetic field.
Compass Information Display in RiverSurveyor
RiverSurveyor displays compass/tilt information in both real-time (during data collection) and
post-processing. This information (heading, pitch, and roll) is shown in the
Sensor Data
tabular
display (
View|Data Display|Show Sensor Display
).
3.8.3. What is Magnetic Declination?
Note that calibration of the internal compass and setting the magnetic declination in your ADP
are critical for accurate current velocity and discharge measurements when using GPS (§
3.4
).
On the earth's surface, a properly calibrated compass points to the magnetic North Pole rather
than the geographic North Pole. The angular difference between these two directions is called
magnetic declination (also known as variation, magnetic variation, or compass variation). Mag-
netic declination varies both across the earth's surface and over time.
There are two ways that the magnetic declination can be set correctly for your survey site.
•
Automatic – This method uses the magnetic declination value output from the GPS system
connected to the ADP. Simply follow the procedure for connecting to the GPS, and then
click the
Set magnetic declination
button located in the
GPS Setup
window. The RiverSur-
veyor program will automatically input the magnetic declination value retrieved from the
GPS output string to the
Magnetic Declination
field in the
User Setup
dialog box.
Figure 3-22. Compass Calibration dialog box