Bushy

And after you wade through all the maths that WGS84 or other different
standards give, the earth isn't even flat except on the sea on a very calm
day. There are mountains and caves and buildings that get in the road. Even
how high you hold your GPS could affect it, but how accurate do you want to
be? These figures are approximate!

Latitude

1 degree
= 60 nautical miles
= 111.12 kilometres
= 111120 metres
= 69.04694 miles
= 364567.8 feet

1 minute
= 1 nautical mile
= 1.852 kilometre
= 1852 metre
= 1.150782 mile
= 6076.131 feet

1 second
= 0.016667 nautical mile (1/60)
= 0.03086667 kilometre
= 30.86667 metres
= 0.01917971 mile
= 101.2688 feet

To work out Longitude, you have to get out your cosine tables or your
calculator. Just multiply the above by the cosine of the degree and it will
give you the distance.

The different models they use all apply a fiddle factor to either or both
latitude and longitude to adjust for a nominal change from a perfect sphere.
You could probably do a search on it and wade through 5000 matches and
eventually find it somewhere. I don't need that accuracy if I'm doing it
with a pencil and paper trying to navigate a plane, and the GPS does it for
me as long as I set it to the one I want to use. Normally this would be
because the map you have uses that setting.

Hope this helps,
Peter