How Establish Magnetic North line on ground
I need to establish either a true north or a magnetic north line on the ground at the airport to calibrate my magnetometer. There is no compass rose here or at any other airport around this area.
I have several compass units but they don't seem what I can use...I want pretty great accuracy. If if had (and I do not) a theodolite I could 'shoot' Polaris, but can't find one...
What suggestions can you give me. I really need some way to get a very accurate line.
Airnav.com's airport information includes the true and magnetic orientation of the runways at many (most? all?) airports. So why not use that info and the runway C/L as reference data? You could even use little geometry and create a N/S line on the parallel taxiway.
I have an old "see through" (or more accurately "look across") compass with an eyehole to look through, a vertical wire on the other side to line up on, and a little mirror so I can read the compass and see the wire and the distance all at the same time. Mine I think was my grandfather's, but I have seen similar units on sale in the hiking catalogs (like REI or Campmoor) for not a lot of money. Here's one for $14. Larger would be better if you want accuracy.
That would give you an accurate magnetic direction from your eye to a vertical pole held by a helper several feet away. Then draw a straight line between the two points.
If you want true instead of magnetic, factor in the magnetic variation for the location which should be accurate to better than half a degree.
Remember Polaris is close to but not right on the true north celestial pole, so if you want real accuracy you'd have to take that into account.
Use your GPS to mark a position (perhaps at one end of a taxiway), go to a second position as far away as possible (like the other end of the taxiway) and note that position. Plot both positions and determine the azimuth between the Two points. That will give you the True Bearing between the two points. Consult a local Sectional chart and get the local magnetic Variation. Remember that True Virgins Make Dull Company Add Whiskey. Add Westerly variation (subtract Easterly) to the True Bearing to get the Magnetic Bearing between the two points.
In a coarse sense, at 60ft between the points, one foot right or left is one degree. So, at 600ft between points, one foot right or left is one tenth of a degree; et cetera.
Also, GPS error in a local area is fairly constant and slowly varies with time. So if your first reading is biased 10 yards in a particular direction, if you take the second position within minutes of the first, it will have essentially the same error in the same direction.
Professional Shipboard And Aerial Navigator
former Member of The Institute of Navigation
Last edited by TedK; 09-01-2013 at 07:06 AM.
Get Solar Noon from a weather site. http://www.timeanddate.com/worldcloc...omy.html?n=137 At that time of day, have a friend hold a pole vertical and mark the shadow on the ground. This line will point north and south. Too simple, huh? -glen