Determining true north and sensor orientation

Orientation of the wind direction sensor is done after the data logger has been programmed, and the location of true north has been determined. True north is usually found by reading a magnetic compass and applying the correction for magnetic declination; where magnetic declination is the number of degrees between true north and magnetic north. The preferred method to obtain the magnetic declination for a specific site is to use a computer service offered by NOAA at www.ngdc.noaa.gov/geomag . The magnetic declination can also be obtained from a map or local airport. A general map showing magnetic declination for the contiguous United States is shown in Magnetic declination for the contiguous United States (2015).

Declination angles east of true north are considered negative and are subtracted from 360 degrees to get true north, as shown Declination angles east of true north are subtracted from 0 to get true north (0° and 360° are the same point on a compass). For example, the declination for Logan, Utah is 11.78° East (11 August 2015). True north is 360° – 11.78°, or 348.22° as read on a compass. Declination angles west of true north are considered positive and are added to 0 degrees to get true north, as shown in Declination angles west of true north are added to 0 to get true north.

Orientation is most easily done with two people, one to aim and adjust the sensor, and one to observe the wind direction displayed by the data logger.

1. Establish a reference point on the horizon for true north.

2. Sighting down the instrument center line, aim the nose cone or counterweight at true north. Display the input location or variable for wind direction by using a laptop or keyboard display.

3. Loosen the U-bolt on the CM220 or the set screws on the Nu-Rail that secure the base of the sensor to the crossarm. While holding the vane position, slowly rotate the sensor base until the data logger indicates 0 degrees. Tighten the set screws.