Plotting position lines

Plotting Position Lines
i.e. Position Lines obtained from
Celestial observations


What is a Position Line?
As we have already discovered, a position line is an
arc of a position circle whose radius is the Zenith
Distance, also:The observer will be somewhere on the position line
The Position Line always runs at right angles (900)
to the true bearing of the body
If we can obtain two position lines, then the point
where they intersect will be the observed
position of the observer


Plotting Position Lines?
When we take an altitude of the sun outside of
meridian passage we calculate the True Altitude
and from this, the True Zenith Distance ZX
Using the Marc St. Hilare method we can calculate the
Zenith Distance for our assumed or DR position
The difference between the TZX and the CZX will give
us the Intercept, named Away or Towards the
celestial body
Using the ABC method, we can also find the bearing of
the celestial body


Plotting Position Lines?
Now that we have collected the following information:The DR Position

The Intercept
The true bearing of the body
We can now plot the information on a chart using the
following procedure


Procedure
Plot the DR position
Lay off the bearing of the celestial body from the DR
position in the direction of the bearing
Using the Latitude scale, measure off the intercept
along the bearing from the DR position, either Away
or Towards the observed body to give us the
Intercept Terminal Point or ITP
Plot the Position Line through the ITP


Latitude
Latitude is marked vertically along both sides of
the chart
The Latitude scale is used to measure distance
on the chart


Latitude
It should be noted that due to the features of the
Mercator chart the lines of Latitude get further
apart the further north/south you go, depending
on which hemisphere you happen to be in
This means that, particularly on small scale

charts, distances should be measured adjacent
to the area being worked on



0.2’
0.4’
0.6’
0.8’

410 59’
420 01’
420 02’


Longitude
The Longitude is marked at the top and bottom of
the chart
Unlike Latitude it is constant over the whole of
the chart
The Longitude scale should never be used to
measure distance


1750 19’

1750 21

1750 22’
0.2’ 0.4’ 0.6’ 0.8’

Question 1 from Paper 1
Observer in DR Lat: 510 25’N
Azimuth
3120
2540
Find the observer’s position

Long: 0060 10’W
Intercept
3.4’ (T)
1.0’ (A)


DR