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Gary wrote-


"But for calculating the slope the DR doesn't need to be particularly 
accurate as the slope is determined only by your latitude and the azimuth 
of the body, which you can measure to sufficient accuracy. If you scan the 
Motion Of the Body table from H.O. 249 at:

http://www.navlist.net/img/102321.mob%201.pdf

you will see that the rate of change in altitude is very insensitve to 
errors in latitude and azimuth. For example, looking at the MOB table for 
my data where my DR latitude was 14� 25' and the azimuth was 103� true, the 
tabulated values for the change in altitude for one minute of time changed 
only 0.1' for a 2 degree azimuth change and by only 0.2' for a 5 degree 
change in latitude. So for a five minute observation period, if my DR 
latitude had been wrong by five degrees the slope would be off by only one 
minute of altitude. Similarily, the measured azimuth would have had to have 
been off by four degrees to produce a one minute error in the slope. 
Scanning H.O. 249, volume 2 shows that to produce an error of four degrees 
in the azimuth my DR longitude would have had to have been off by ten 
degrees, 600 nm!"

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Gary's conclusion is right, though his example was not a typical one. 
Observing from within the tropics a body that's nearly due East, that body 
will be climbing more-or-less vertically , and so its rate-of-change of 
altitude will be near the maximum possible, of 15� per hour, and will 
therefore be very insensitive to changing the parameters.

But it's not the same the whole World over. From higher latitudes, when 
observing bodies that are climbing diagonally across the sky, the 
rate-of-change of altitude will be less, and it will have a greater 
dependence on latitude and azimuth.

Even so, it will often allow the rate of change to be calculated more 
precisely than it can be measured, depending on the precision of the 
observation and the accuracy with which the position is known.

George.

contact George Huxtable, at  george{at}hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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