birding-aus

Decimal Vs Sexagesimal Notation

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Subject: Decimal Vs Sexagesimal Notation
From: "Neil Cheshire" <>
Date: Wed, 12 May 2010 00:11:54 +0930
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
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