[Acoustics] Methods for Determining Magnetic Variation

[David S. Mueller]

This Hydroacoustic Update (attached) includes information regarding the
determination of the local magnetic variation (declination) at a site for
use in ADCP data collection and processing. Properly conducted on-site
magnetic variation tests are the most accurate method for determining the
magnetic variation at a site, if there is sufficient space to perform the
test, good GPS positions are available, and no moving bed is present. The
magnetic variation can also be estimated from geomagnetic models. Three
common sources of local magnetic variation estimates based on geomagnetic
models are the two freeware programs Geomagix* and Geomag, and the RMC NMEA
data type available as output from most GPS-receivers. Estimates of the
local magnetic variation from geomagnetic models may be adjusted, but if the
adjustment is greater than +/- 1 degree, the cause of the adjustment must be
investigated and documented. If you have any questions, please contact David
Mueller (dmueller at usgs.gov or 502-493-1935) or email the USGS Hydroacoustics
Work Group, hawg at simon.er.usgs.gov.

This Hydroacoustic Update can also be accessed at:
http://hydroacoustics.usgs.gov/applications/HydroacousticsUpdate-Magvar.html

* The use of trade, product, or firm names is for descriptive purposes only
and does not imply endorsement by the U.S. Government.

David S. Mueller
U.S. Geological Survey
Office of Surface Water
9818 Bluegrass Parkway
Louisville, KY 40299

dmueller at usgs.gov

Voice: 502-493-1935
Fax: 502-493-1909
Mobile:502-558-5907

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