Geometrics Inc. G-882 Cesium Marine Magnetometer Page
All depth values are assumed to be in
meters.
With the sensor at the surface:
What number is displayed for the depth? 112
With the sensor at a known depth:
What depth did you place the sensor at ? 9
What number is displayed for this depth ?
917
Enter a BIAS value of : -1.2522
Enter a SCALE value of : 0.0112
NOTE: Some older versions of the DepthCal program instruct you to measure
the “Zero Depth” value on the deck of the boat. Instead tie off the G−882 over
the side at the surface but still submerged to do this measurement. See the
discussion about temperature affects of the depth sensor in the paragraphs
above.
Here is what the DepthCal numbers mean:
We entered a depth change of 9 meters and got a change of 805 meters (917
at depth minus the 112 at surface). To get your logging software to correctly
display a change in depth for a given depth change we have to multiply the
incoming data by (9/906) which equals 0.0112. This is the scale factor
calculated by DepthCal above.
We read 112 meters out of your logging software when the magnetometer was
at the surface. Its not really 112 meters though. Using the scale factor
correction above we get an actual error of 1.2522 meters (0.0112 * 112). Thus
to make it read correctly, we have to add a bias of –1.2522 meters which is
the bias value calculated by
DepthCal.
Miscellaneous:
Note that once these values are entered, if your logging software remembers
them, you will not have to enter them again. Of course, if a different instrument
is connected at some later time, new values will be required corresponding to
the new instrument.
Slope – Intercept Method
Here is a method for calculating the Scale Factor and Bias for depth calibration.
At the first depth, let the depth in meters be d1, and the raw reading r1.
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