YSI SonTek RiverSurveyor, System Manual

Page: 46 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
34
Compass Alignment and Magnetic Declination
When using GPS as a velocity reference to compute discharge, you must correct for any differ-
ence in the alignment angle between the internal ADP compass (referenced to magnetic north)
and the GPS compass (referenced to true north). As an example, a misalignment of 5° between
the GPS and the ADP internal compass results in an 8% error in velocity. This angle can be cor-
rectly determined and used during a discharge measurement by calibrating the internal ADP
compass and by using the correct
magnetic declination for the site being surveyed. You must
perform the following actions to compensate for this angle. Compass calibration and magnetic
declination are critical for accurate current velocity and discharge measurement.
Calibrate the internal ADP compass
Calibration of the internal ADP compass can be done via the
User Setup
dialog box (
Figure 3-20
)
after connecting to the ADP. Click
Calibrate Compass
and follow the instructions to calibrate the
internal ADP compass. At the completion of the calibration procedure, the program will provide
a quality score and record the calibration information in a compass log file (which is stored in
the default data file folder). A more detailed description of compass calibration is given in §
3.8
.
Set the magnetic declination
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. Note that the magnetic dec-
lination must always be set correctly when referencing measurements using GPS.
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 receiver
unit. Simply follow the procedure for connecting to the GPS, and then click the show prop-
erties link for your GPS system (
External
or
Internal GPS Properties
dialog box). Click
Get
Magnetic Declination from GPS.
The RiverSurveyor 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 3-20
).
•
Manual – Enter the magnetic declination into the
Magnetic Declination
field in the
User
Setup
dialog box (
Figure 3-20
). You can determine the magnetic declination for your survey
site from several sources: magnetic declination maps provided by the U.S. Geological Sur-
vey, commercial software, or Internet sites (see
Glossary listing for details).
3.4.7. What are the limitations of GPS?
Both GPS and DGPS have a few limitations. Sources of error include:
•
GPS and DGPS Accuracy
•
Multipath Error
•
Invalid GPS Strings
•
GPS Filters
GPS and DGPS Accuracy
Regular GPS is accurate to within
≈
100 m, which would require extremely long averaging times
(
≈
2.5 h) to achieve a 1 cm/s accuracy. Such measurements are useful only in the open ocean.