Garmin query

John Brown

My buddy and me have identical Garmins. Recently we compared stats at end of
a walk. Mine showed approx 9 miles his approx 8 miles.
Any ideas why this should be so.
Settings were all identical except he had his set-up for world geodetic
system (WGS84), mine for OSGB grid references.
Thanks. John

Paul Saunders

John Brown wrote:
> My buddy and me have identical Garmins. Recently we compared stats at
> end of a walk. Mine showed approx 9 miles his approx 8 miles.
> Any ideas why this should be so.

If you lose lock your GPS stops recording, so if you each show a different
distance at the end of the walk, it's likely that one of you lost lock more
often than the other. That's the main reason for a distance discrepancy.

Two obvious reasons for this spring to mind.

1. Different battery settings. I once did a walk with my brother using
almost identical Garmins, and when we entered a forest his went haywire and
couldn't keep a lock while mine kept a reasonable lock. It turned out that
he had his set to "battery save" mode, while mine wasn't. Battery save mode
is more likely to lose lock in difficult conditions like forests.

2. Different GPS carrying positions. Where you carry your GPS can make a big
difference to your reception during a walk. The biggest obstacle that can
block GPS signals is your own body. If you carry your GPS low, for example
on a belt, the signal can be blocked more often than if you carry it high,
such as high on a rucksack strap or on your shoulder, or even in the top
pocket of your ruscksack. I've been on walks with people who've carried
their GPS low on their belts and recorded less distance than mine high up on
my rucksack strap (a hat with an aerial would be best!).

If you study the track afterwards (on a computer overlaid onto a map), it's
usually obvious if you lost lock during the walk, since there are sporadic
jumps in position. A closer examination of the track details can reveal
errors if there are sudden changes of speed that make no sense.

> Settings were all identical except he had his set-up for world
> geodetic system (WGS84), mine for OSGB grid references.

That wouldn't make any difference. All GPS units use WGS84 internally and
only use other settings for display of the data.

Other factors could be involved, such as different settings for track
recording, but they probably wouldn't make that much difference. I'd check
the two obvious ones first.

1. Don't use battery save mode (unless you really have to).
2. Do try to carry your GPS so that it sees as much of the sky as possible,
most of the time.

Note that different models use different antennae. eTrex models like to face
upwards (i.e. be carried horizontally), while older models like the GPS12
and newer models like the 60 and 76 series like to be carried vertically
(facing forward).

Paul
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