Hello Alain MORY
As to the magnetic compass to point NS line, the most annoying problem is
that we cannot find out how much error include, even if we would correct the
deviation. We have the problem with the compass itself and the local
magnetic influence, such as iron plates and bars in the building, electric
wires in the surrounding and iron-contented soil under the foot.
I think that the astonomical method is the most desirable, as we would say
the true NS line is defined astronomically.
1. We can use the plumb-line shadow when the sun places on the true South.
We know the time from the Annual Astronomical Tables or the Ephemeris in
each country.
2. We can use the sun's shadow being cast to the concentric circles on the
ground, as Mr. Fernando Cabral said fully in his email. This would be the
classical method.
3. If you have a transit instrument, or a theodolite, for land-surveying,
you can find out the considerably exact NS line with one-time measuring of
the sun'position. We can also use the bright stars' position. There are two
methods independently, measuring the sun's altitude and measuring the sun'
time-angle. You need the sun-glass for the instrument.
I will show you the formular calculatng the azimuth from measuring the sun's
altitude.
cos(azimuth)={sin(latitude)xsin(sun's altitude)-sin(sun's declination)}
/cos
(latitude)/cos(sun's altitude)
Latitude: we know fromGPS
Sun's altitude: we know from measuring. But we must correct the astronomical
refraction according to the temperature and the
air pressure.
Sun's declination: we know from the Astronomical Tables or the Ephemeris.
You can calculate the azimuth, and then rotating the transit or the
theodolite, you will find out the true NS line.
I hope this mail is a useful information for you.
Sumi Yoichi
Japan
[EMAIL PROTECTED]
