Re: How to Read / Navigate via GPS

To: "Birding-Aus" <>
Subject: Re: How to Read / Navigate via GPS
From: "Paul Taylor" <>
Date: Wed, 19 Apr 2000 22:53:19 +1000
Irene Denton wrote:
> For example, it has several screens including a compass and a depiction
> of a road but nothing about "how to use" these screens to, eg, get to
> where you want to go.

N.B.  I'm assuming that the Magellan GPS units operate in much the same
way as the Garmin GPS II+ that I am most familiar with.

The compass and "highway" views are equivalent; both modes indicate the
direction you are currently travelling in (heading), and the direction
to your selected "waypoint" (bearing).  (I personally prefer the compass

On the GPS II+ compass view, the "ring" of the compass indicates your
heading, and the arrow indicates the bearing; if the arrow is pointing
upwards, you are going in the right direction.  In the "highway" view,
the bend of the road indicates whether you need to steer left or right
to reach your waypoint.  Obviously you need to have defined a waypoint
to get the most of these views!

> It is also full of terminology such as heading, bearing etc. with no 
> explanation of what these are.

See above.

> There even appears to be a difference between latitude/longitude in
> DEG/MIN.MM format and DEG/MIN/SEC format.

The display formats are a matter of taste/convenience.  If the map
you are using only has degree markings e.g. Atlas surveys, the
DEG.DDDD format is probably the most convenient to use.  Likewise,
using the AGD(?)66 Northing/Easting coordinates may be more convenient
than Lat./Long. coordinates when working with AUSLIG maps (assuming
your GPS supports UTM/UPS coordinates.)  As an analogy, using cups and
teaspoons is usually more convenient in cooking than measuring in
millilitres (though the US vs. Australian/UK definitions of "tablespoons"
is a real headache - 15ml vs. 20ml is a 25% error tolerance!)

> For atlassing, is True North or Magnetic North the best GPS setting? Etc.
> Etc.

For atlassing, it shouldn't make any difference; this setting affects only
the heading/bearing values, not the position.

"True north" is a fixed direction; "magnetic north" varies over time, and
the difference varies depending on where you are. AUSLIG maps usually
indicate both true north and magnetic north, but you *must* correct the 
magnetic north value by the supplied correction factor (usually given in 
degrees per year.)  My preference is to use magnetic north since it is more
intuitive, but assumes the GPS correction factors are correct.  (Given the
average errors in hand-held compass readings, this is unlikely to be an

   Paul Taylor                                  Veni, vidi, tici -
                           I came, I saw, I ticked.

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