Geoff Hutchinson raises a valid point. On a nautical chart the vertical scale
is in degrees and minutes of latitude and the horizontal scale is in degrees
and minutes of longitude so who would want to perform two calculations every
time you plotted a position! Under the metric convention the nautical mile
has been retained for the measurement of distances a sea.
One nautical mile = 1' of latitude = 1852 metres. Distance on the chart can be
measured directly with dividers on the latitude minute scale.
If you want kilometres, multiply by 1.852.
I have noticed in some recent seabird papers in refereed journals, positions of
birds seen in the open ocean are given in degrees, minutes and 3 places of
decimals of minutes of latitude and longitude! This is accuracy to a
nonsensical degree (minute) given that the bird may be 20 to 200 metres or more
from the
vessel and the third decimal of a minute of latitude represents 1.85 metres and
the second decimal 18.5 metres (the longitude distances are even smaller).
If the observer holding the GPS moved from one side of the vessel to the other
the third decimal could change substantially and if they moved from the
bow to the stern the second decimal could change. The third decimal might be
appropriate on land if you wanted to locate a bird's nest,burrow or the
particular shrub on which it was perched. My personal preference is to forget
the seconds and use decimals of a minute if extreme accuracy is necessary.
Neil Cheshire
==============================www.birding-aus.org
birding-aus.blogspot.com
To unsubscribe from this mailing list,
send the message:
unsubscribe
(in the body of the message, with no Subject line)
to:
=============================
|
