SonTek
ADP Software Manual Version 6.42 (November 1, 2000)
21
SECTION 5. Obtaining Correct Magnetic Variation (MAGDECL)
In order to facilitate correct measurements of velocity direction SonTek supplies utility
MAGDECL.EXE which provides access to a magnetic variation database MAGDATA.GRD.
This program computes the magnetic declination of Earth's magnetic North as a function of Date,
Latitude and Longitude. Magnetic Declination is the horizontal angle between the Magnetic
North and TRUE North. For example, a declination of +10 deg means that Magnetic North is 10
deg to the East of TRUE North. SonTek's ADP, ADV, and ARGONAUT systems use a
magnetic compass to measure the direction of North, which results in velocities referenced to
magnetic North instead of TRUE North. Magnetic Declinations permit to correct these velocities
so that they are referred to TRUE North.
The program usage is as folows:
MAGDECL <Date> <Latitude> <Longitude>
where:
• <Date> is in the format yy/mm/dd.
• <Latitude> is in decimal degrees (from -90 to 90)
• <Longitude> is in decimal degrees (from -180 to 180)
Example:
MAGDECL 97/10/24 32.7 -117.2
Since magnetic variation is not only a function of the exact position but also the time it is advised
that the user obtains an up to date magnetic variation before setting up a deployment. Although
the Earth magnetic field slowly changes, for most latitudes (away from poles) the rate of change
is very small so a few days, or even months, only cause a fraction of a degree change.
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