The Convergence angle is: -1° 24' +/- 1'
So the Geodetic azimuthm is here: map azimuth
- Convergence angle = 132° 38+/- 2'
So looking at this result, one needs to stand at least 5 yards
from a fence. The best way is to do a few measurements while
stepping one step back and forward and measure in reverse
direction. If these measurement differ much, there must be
something around that disturbs the magnetic field.
More information on error determination of magnetic grid
readings (such as using a reference measurements) check this page.
From this picture one can determine (called photometry method), based on the
width of the Sun (A) and the height above the horizon (B), an
approximate time when it was taken (19:45 BST). Using the
distance between the middle of the sun and Rockabill (C) one can
then calculate the position of Rockabill. Doing that gives an
Astronomic azimuthr of: 132° 24' +/- 10'.
Another picture has been taken on Dec. 21st, 2004 by Anthony Murhpy.
If the centre of the Sun in this picture is indeed on the
horizon, the Sun's azimuth is ~130° 50', furthermore the
distance between Sun's centre and lighthouse Rockabill looks to
be 3.25 times the diameter of the Sun, so around 3.25*0.542°
= ~1° 46', thus the Astronomic azimuth of the lighthouse on
Rockabill looks to be: ~132° 26'. Thus is again very close
to the values calculated above.
In the below picture a trial has been made to show how the
winter solstice Sun of 4000 years ago would have looked
like in relation to Rockabill (seen from Baltray). I have
used the moon as a proxy;-)
Moon just above lighthouse at Rockabill
on July 17th, 2000 at 22:30 BST
seen from Bettystown beach,
proxying the winter solstice Sun of 4000 years ago seen
It is also done from the beach of Bettystown but now
with the Moon as celestial object. The Moon was directly above
Rockabill at an azimuthr of 124° 45' +/- 1'.
In the above picture the Moon is precisely at the same position
above Rockabill as the winter solstice Sun would have been 4000
years ago seen from Baltray.
The Map grid en Reference object are some 8'
from each other. Longitude/latitude and Reference
object reading are some 4' off. The Magnetic grid
has the most variation.
I assume that the Reference object reading is at the end
the most accurate, but Longitude/latitude and Map
grid can do in most instances.