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 from Paper 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 [...]...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