I once went through the exercise of finding North from a station at
home.
A gps fix would give me +- - 1metre northing and + - 1 metre
easting, another
fix 1000m away would be the same, a lousy base for an accurate azimuth.
Using an ephemeris for constants, a theodolite fix on a circumpolar star
taken at 6 hour intervals starts to get a value with the limits being
the
usual limits of observing stars but less the refraction errors.
I recorded the data with respect to a Referred Object, ( a street
light 4 km away)
Several times I repeated this exercise ( on other nights) and I was
disssapointed by the precision.
Now my theodolite is a Wild T1, and the method does not lend itself
to getting the maximum
precision from the instrument, but I was deeply impressed by the
difficulty of getting
an accurate measure of true North.
cheers, Neville Michie
On 22/11/2009, at 1:13 PM, J. Forster wrote:
Not so. I'm very familiar with laying in accurate North lines for gyro
testing. To get anything close to accurate (1 arc second or better)
takes
many hours of stellar observation with a Wild T-3 class instrument.
-John
===============
Neville Michie wrote:
When you think of time specifications from GPS, the GPS system is a
poor way to find north.
Even with a base line of 1000 metres you only have a fraction of a
degree.
The GPS system may be useful to get accurate time to simplify a star
observation, from a known (GPS)
position on this planet, but finding north is still a problem
because
of the accuracy of a small
number of observations from a star fix.
Gyrocompasses take some time to get a measurement
( one hour) but even their estimate of North cannot match the
precision that the GPS system can get us with time.
cheers, Neville Michie
If you are taking star shots a stellar compass can easily provide a
boresight pointing accuracy of a few arcsec.
Bruce
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