The United States Geological Survey (USGS) has provided an indication of the deviation of magnetic north from true north on each of their topographic maps. This indicated declination has typically been used as the value the magnetic reading obtained in the field needs to be adjusted by to obtain true north. While the declination does change from year to year, the change is relatively small compared to the typical accuracy of most wind measurement systems. Even with the correct determination of magnetic declination, and the absence of magnetic aberrations in the local environment, there is still a chance that the magnetic declination may be applied in the incorrect direction due to simple operator error.
To avoid the problems that the
application of magnetic declination can introduce, various solar methods
have been used. Any of the solar methods require that the user know the
exact location of the site and have an accurate measure of the true time
(and date). The location is either used to calculate solar angles
or to determine the time of solar noon, (the time the sun crosses the
north/south axis). With the current technology in handheld Global
Positioning Systems (GPS), finding the location to an accuracy suitable
for the calculation of solar angles is a simple and inexpensive task.
In 2001, I presented a paper at the AMS 11th Symposium on Observations and Instrumentation providing a step-by-step process for aligning wind sensors. The tools needed to implement the method includes a transit or compass, GPS, and a program for the calculation of solar angles. Details on the method can be found in A Simple Step by Step Method for the Alignment of Wind Sensors to True North, 2001, with more reference details on my publications page. Highlights of the method include:
The calculation of solar angles is easily performed using a program called
COMPASS. The origin of the program is a BASIC program developed by Blackadar
in 1985 entitled ALMANAC. It provided the ability to calculate the azimuth
and elevation angles to the sun, moon and other celestial objects knowing
the latitude, longitude, year, month, and exact time of day. For
the revision to the USEPA quality assurance guidance in 1989, Tom Lockhart
extracted the subroutines from the Blackadar program that calculated
the sun's azimuth, elevation and solar noon values based on the required
inputs. The information from this revised program could then be
compared to the magnetic azimuth measurements of the sun to calculate the
local magnetic deviation at the point of measurement. A general description
of this methodology is provided in the EPA Quality Assurance Handbook
for Air Pollution Measurement Systems, Volume IV: Meteorological Measurements
(1995). In 1990, the BASIC program extracted by Lockhart was refined and I placed a simplified front end to input the needed variables to calculate a series of 45 solar azimuth and elevation angles at specified time intervals, and provide a more "user friendly" means of inputting the site data. A subsequent rewrite of the program resulted in COMPASS, which allows the site variables to be stored and later retrieved. A sample screen from the program is shown above. The program COMPASS provides the basic input information, along with the site latitude and longitude, which is used in the step-by-step true north determination methodology. The COMPASS program is freeware. Contact me and I will forward a copy to you.