Knowing the direction of true north is essential for interpreting direction data, and is also useful during the tower layout and installation. In surprisingly many monitoring programmes, direction vanes and anemometers are not oriented in the correct, or documented, direction. This can cause significant errors in wind flow and wake modelling and result in a poor turbine layout.
Often, directional errors arise because of confusion between magnetic and true north. Magnetic north is what a magnetic compass reads; true north is the direction along the local line of longitude to the North Pole. Sometimes the correction from magnetic to true north is applied wrongly, and sometimes it is applied twice, once in the field, once by the data analyst. When the tower installers use a magnetic compass, the risk of error can be reduced by instructing them to orient the wind speed sensors with respect to magnetic north, and by correcting the readings to true north when the data are analysed. A better option is to equip the installation team with a Global Positioning System (GPS) receiver configured to indicate true north. If a correction from magnetic north is required, the local magnetic declination (in degrees) must be established. This correction can be found on topographic or isogonics maps of the area (an example is provided in Figure below.)
How the correction is applied depends on whether the site is east or west of the longitude of the magnetic north pole. To the east, the declination is expressed in degrees west of true north, and the bearing towards true north therefore equals the declination. To the west, the true north bearing is 360° minus the declination. For example, if the local magnetic declination is 10°W, the bearing for true north is °10. If the declination is 10°E, the bearing is 350° (360° – 10°). For a met tower configuration, the same correction method is used to determine information such as boom orientations. For example, if a boom has an orientation of 150° from magnetic north, and the local magnetic declination is 15°W, then the bearing for true north is 15° and the corrected boom orientation is 165°.