ICTworks

Understanding the difference between Magnetic (Compass) bearing and True bearing is key to aligning WiFi links.  

Most wireless network planning software, RadioMobile included, will give you the correct bearing (also known as azimuth) to point all the antennas in your network.  However, when it comes time to go to the field and install you’ll need to convert these bearings to magnetic.  Compasses measure magnetic, not true bearing and the compass will be your primary tool for figuring out where to point each antenna.

Here’s the easy way:

1. Go to www.magnetic-declination.com and plug in your city.  It will tell you your declination (in degrees).  If the declination is positive, you have east declination.  If the declination is negative you have west declination.
2. Remember CADET (I printed a label and stuck it on the back of my compass). C=Compass T=True and ADE=Add DEclination.  This means to go from Compass to True you Add DEclination.

If your Compass bearing is 100 degrees and your declination is 15 degrees (east), 100+15=115 True.
If your Compass bearing is 100 degrees and your declination is -15 degrees (west), 100+(-15)=85 True.

3. To go the other way, you read CADET from right to left instead.  To go from True to Compass, you do the opposite of add declination.  You subtract it:    

If your True bearing is 100 degrees and your declination is 15 degrees (east), 100-15=85 Magnetic
If your True bearing is 100 degrees and your declination is -15 degrees (west), 100-(-15)=115 Magnetic

Here’s the even easier way if you have a Garmin GPS with you:

1. If waypoint A is where you are and waypoint B is where you’re trying to point your antenna, go stand at A with your GPS.
2. Make sure B is stored in the GPS as a waypoint.  Press find, and arrow down to select point B (don’t push enter)
3. The GPS will show distance and bearing from your current position to the selected waypoint.  If you see a small “M” after the bearing, it’s showing magnetic bearing…use that for your compass alignment.  If you see a small “T” it’s showing true.  Go into global settings -> units and change it to Magnetic.