Azimuth Calculations in TAP™
Q: How does the azimuth value for a path computed in TAP relate to my compass readings in the field?
A: All azimuth or bearing values computed in TAP are relative to True North. To compute the compass reading (relative to magnetic north) you must add the magnetic declination for your area.
Note that all values in this example are for demonstration purposes only. You should determine appropriate settings for your application.
The magnetic declension depends on the location of the path. You may be able to find the magnetic declension printed on many USGS maps, giving the declension value for the map area.
You can compute the declension for your area, for example at a National Oceanic and Atmospheric Administration web site:
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp
After entering the coordinates (or ZIP code) of your area, the declension value will be computed.
The instructions in the NOAA Frequently Asked Questions section describe the correlation between magnetic north (indicated by a compass bearing) and True North (computed in TAP):
5d. How do I correct my compass bearing to true bearing?
You can compute the true bearing from a magnetic bearing by adding the magnetic declination to the magnetic bearing. This works so long as you follow the convention of degrees west are negative (i.e. a magnetic declination of 10-degrees west is -10 and bearing of 45-degrees west is -45).
The NOAA FAQ includes a helpful graphic illustrating the relation between magnetic and True North:
 |
Copyright 2009 by SoftWright LLC