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.fer3.com/arc/img/102321.mob%201.pdfyou 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!"

=============

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 15º 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.