Distance accuracy of the Geko 201, or other GPS-receivers?

View Full Version : Distance accuracy of the Geko 201, or other GPS-receivers?

Daniel

When I'm measuring the distance of a trail that is partly
going through the woods, and according to a map should be
10km, my Geko 201 says 8.85km. What experience do you
have of distance accuracy of the Geko 201 in particular,
and GPS-receivers in general? Do you think the map or the
Geko is wrong?

Thanks!

/Daniel

Dot

Daniel wrote:
> When I'm measuring the distance of a trail that is partly
> going through the woods, and according to a map should be
> 10km, my Geko 201 says 8.85km. What experience do you have
> of distance accuracy of the Geko 201 in particular, and
> GPS-receivers in general? Do you think the map or the Geko
> is wrong?

Short version: Probably both - depending upon a bunch
of things.

Maps - how accurate is the map, how did you overlay your
course on it, how did you measure it on the map, what
map, is the trail accurately displayed on map with no
reroutes, etc?

GPS - how good was your satellite reception (not just
numbers of satellites, but distribution across sky), what
was the reported error in the location, etc?

Long version: Where I am (Alaska), discussion of accuracy
of USGS topo maps is usually greeted with chuckles and/or
frustrated sighs (even for natural features, not to
mention cultural features like roads). A large percentage
of the state hasn't had topo work done since the 1950s.
Updates have been largely confined to the populated areas,
where I do
live. Some are still only available in the 1:63,360
(rather than
1:24k/25k). This is also true in other parts of the US, I
believe. Like everything else up here, USGS treats Alaska
differently (even the way they pack the DEM's in files).

One of my colleagues does gps/gis work and has spiffy gps
(survey grade, I think, but it might be resource grade), not
the consumer-grade handhelds that we use. We've overlaid gps
tracks from some of his stuff and my handheld (different
days) on topos and aerial photos (corrected for radial
distortion, etc). While much of the map, photo, gps agree,
there are a couple areas where the gps's agree with each
other, but they do not agree with the map and the photos are
also different. The disagreement is so consistent across
various devices and days of recording that we question the
accuracy of some parts of the map. OTOH, we know the other
things have error also, so we won't say they are the
"truth". We take each as an approximation.

I've been carrying my gps (Etrex Vista, sometimes 12XL) on
top my camelbak, and I've been getting decent tracking from
that position. Across routes that are "about" 7-8 miles, I'm
probably within .5 mile (usually less error) between gps and
map. Where I have a route on the map ahead of time, I've
been selecting a route to get in the 7-8 mile range (either
loop or out/back to something), but may make some deviations
along the way. (If no predetermined route, I just go run for
2.8-2 hrs or so.)

My map distances are based on tracing the route on All Topo
Maps: Alaska (iGage, NFI) which are electronic presentations
of the USGS maps (not vectorized like some products). I'm
more likely to believe the gps, esp. if the track is clean -
no skips and no out-of-whack readings - compared with
tracing on USGS map.

As the runs get longer, I depend more on the gps for the
distance / elevation change than I do on map (unless the
batteries die), since my tracing on the map is more likely
to introduce errors, esp. as I zoom out to display larger
areas. I could get more accurate map readings, by zooming
in, but then I have to go across so many screens, that it
gets to be more pain than it's worth. With the gps, I just
have to look at 2 windows (elevation is in different window)
and have everything I need and then some. That said, I
carried gps by hand one day and got a really messed up
track, but the total distance agreed with trail guide and my
estimates from topo map (whether coincidental random
numbers, I don't know). In places that I've biked, I'll
compare bike computer (not necessarily calibrated in fine
detail), gps, and map, and they're usually pretty close -
close enough for me.

FWIW, I'm happy if I'm within 5-10%, and I think I'm getting
close to 5% difference in readings between gps and map. I
really don't expect to get any closer than that on trails,
and generally don't know the true value anyway.

I train by time and effort, so the exact distance isn't that
relevant. Most trail races aren't measured accurately so I
just need to know that I can run at least that distance -
and elevation gain (and I don't trust either map or gps on
this issue on my small, steep rolling hills). That said, the
xc ski trails that I use have a map with a table of
distances between intersections (to 1-m precision), and I've
been told they were actually measured (not sure how, but
something on the ground, by a local, experienced hiker/trail
builder) and later checked with GPS. On those trails, I just
use those distances since the gps error in some cases is
larger than the distance between adjacent trails.

Dot

--
"Success is different things to different people" -Bernd
Heinrich in Racing the Antelope

Miss Anne Throp

Buttercup, If there's already a trail, do you really need a
map OR gps?

Becky

Keep in mind thaat GPS measures in a straight line. If
your trail has twists and turns and you only took two
GPS readiings (start and end of run) then GPS will
come up short.

"Daniel" <nej@nej.nej> wrote in message
news:CPxFc.925$vH5.595@amstwist00...
> When I'm measuring the distance of a trail that is partly
> going through the woods, and according to a map should be
> 10km, my Geko 201 says 8.85km. What experience do you have
> of distance accuracy of the Geko 201 in particular, and
> GPS-receivers in general? Do you think the map or the Geko
> is wrong?
>
> Thanks!
>
> /Daniel

Mel Rimmer

In article <10eg392279mfd58@corp.supernews.com>, Becky
<bbkelher@remove. spamaculink.net> writes
>Keep in mind thaat GPS measures in a straight line. If
>your trail has twists and turns and you only took two
>GPS readiings (start and end of run) then GPS will come
>up short.

Is that how they work? So they'd be completely useless on a
circuit, wouldn't they? I assumed they got a position
reading every few minutes or whatever and worked out your
mileage from that. It would still underestimate on a very
twisty route, but it would give you a ball-park figure, even
for a circuit.
--
Mel Rimmer

Np426z

"Dot" <dot.h@#att.net> wrote in message
news:nWCFc.55995$OB3.29245@bgtnsc05-news.ops.worldnet.att.net...

> Short version:

This I liked.

> Long version:

This I didn't.

Dot, I know we never 'speak' on rec.running, but I feel we
have a superficial awareness of the other's prescence.
You're probably not minded to pay attention to my advice,
but I'll offer it for your consideration nevertheless.

You are undoubtedly a bright, outgoing and intelligent lady
with whom - were circumstances of our meeting to be
slightly different - I would enjoy many a happy few seconds
in conversation. But - and it's a very big 'but' - you do
go on a bit. Knowing when to *stop* telling people stuff is
almost as important as knowing *what* to tell people.
Without question, you know the *what*. Sadly for all
concerned, you rarely seem to know when the trickle of
information that people seek becomes a torrent which they
didn't. Seek, that is.

A few kinds words, a fact or two, then - if you must - a
link to somewhere else on the web would more than suffice.
Flaunting your extensive knowledge of trivia is best left to
trolls such as my good self. Trolls are the recipients of
extensive training in the presentation and dissemination of
dry, dusty, life-sucking facts, thus are best placed both to
inform and amuse the rec.runners without them even realising
they are being both educated and entertained. You are
treading on our turf. Please stop.

Would you be so kind as to convey my best wishes to your
Antelope? The poor beast must get bloody cold in Alaska.

Dot

Becky wrote:
> Keep in mind thaat GPS measures in a straight line. If
> your trail has twists and turns and you only took two
> GPS readiings (start and end of run) then GPS will come
> up short.

GPSr's usually track the path of travel every few seconds
(can't remember interval off top of head and probably varies
by device) or when you turn (even minor turns), creating a
breadcrumb trail, and sum those distances as you go. A
runner may also be changing sides of trail (except narrow
single track) where he/she runs also adding some turns.
While some distance is lost on windy trails, I think a GPS
is going to pick up those turns better than a map (at least
the maps I use), depending, of course, on detail and
accuracy of map. *IF* (notice the big IF) I have a clean
track log (no gaps, no obvious side fictitious
"excursions"), I tend to believe my gps is closer to reality
for distance estimates than my guesstimates on map. If the
track log has many breaks or hiccups (out of whack points),
then I go with my map guesstimates or a dart board.

GPSr can also be used to mark waypoints - such as the start
and end or some interesting feature along the way. If you
measured distances only between those points, then, yes,
you'd be way short on distance with the gps.

Dot

--
"Success is different things to different people" -Bernd
Heinrich in Racing the Antelope

Bumper

Join

http://sports.groups.yahoo.com/group/GarminF/

while much of the discussion concerns the Garmin Forerunner,
several of the group are bona fide gps gearheads capable of
answers most all questions thereof.

np426z <np426z@btinternet.com> wrote:

> "Dot" <dot.h@#att.net> wrote in message news:nWCFc.55995$OB3.29245@bgtnsc05-
> news.ops.worldnet.att.net...
>
> > Short version:
>
> This I liked.
>
> > Long version:
>
> This I didn't.
>
> Dot, I know we never 'speak' on rec.running, but I feel we
> have a superficial awareness of the other's prescence.
> You're probably not minded to pay attention to my advice,
> but I'll offer it for your consideration nevertheless.
>
> You are undoubtedly a bright, outgoing and intelligent
> lady with whom - were circumstances of our meeting to be
> slightly different - I would enjoy many a happy few
> seconds in conversation. But - and it's a very big 'but' -
> you do go on a bit. Knowing when to *stop* telling people
> stuff is almost as important as knowing *what* to tell
> people. Without question, you know the *what*. Sadly for
> all concerned, you rarely seem to know when the trickle of
> information that people seek becomes a torrent which they
> didn't. Seek, that is.
>
> A few kinds words, a fact or two, then - if you must -
> a link to somewhere else on the web would more than
> suffice. Flaunting your extensive knowledge of trivia
> is best left to trolls such as my good self. Trolls are
> the recipients of extensive training in the
> presentation and dissemination of dry, dusty, life-
> sucking facts, thus are best placed both to inform and
> amuse the rec.runners without them even realising they
> are being both educated and entertained. You are
> treading on our turf. Please stop.
>
> Would you be so kind as to convey my best wishes to your
> Antelope? The poor beast must get bloody cold in Alaska.

Donovan Rebbech

On 2004-07-04, Mel Rimmer <mel.rimmer@timelord.org.uk> wrote:
> In article <10eg392279mfd58@corp.supernews.com>, Becky
> <bbkelher@remove. spamaculink.net> writes
>>Keep in mind thaat GPS measures in a straight line. If
>>your trail has twists and turns and you only took two
>>GPS readiings (start and end of run) then GPS will come
>>up short.
>
> Is that how they work?

No.

> So they'd be completely useless on a circuit, wouldn't
> they? I assumed they got a position reading every few
> minutes or whatever and worked out your mileage from that.

Every few seconds, in fact (though this depends on choice of
settings).

> It would still underestimate on a very twisty route, but
> it would give you a ball-park figure, even for a circuit.

Yep.

Cheers,
--
Donovan Rebbechi http://pegasus.rutgers.edu/~elflord/

Ellis

"np426z" <np426z@btinternet.com> wrote in message
news:cc74aj$7fi$1@hercules.btinternet.com...
> "Dot" <dot.h@#att.net> wrote in message news:nWCFc.55995$OB3.29245@bgtnsc05-
> news.ops.worldnet.att.net...
>
> > Short version:
>
> This I liked.
>
> > Long version:
>
> This I didn't.
>
> Dot, I know we never 'speak' on rec.running, but I feel we
> have a superficial awareness of the other's prescence.
> You're probably not
minded
> to pay attention to my advice, but I'll offer it for your
> consideration nevertheless.
>
> You are undoubtedly a bright, outgoing and intelligent
> lady with whom -
were
> circumstances of our meeting to be slightly different - I
> would enjoy many
a
> happy few seconds in conversation. But - and it's a very
> big 'but' - you
do
> go on a bit. Knowing when to *stop* telling people stuff
> is almost as important as knowing *what* to tell people.
> Without question, you know
the
> *what*. Sadly for all concerned, you rarely seem to know
> when the
trickle
> of information that people seek becomes a torrent which
> they didn't.
Seek,
> that is.
>
> A few kinds words, a fact or two, then - if you must -
> a link to somewhere else on the web would more than
> suffice. Flaunting your extensive knowledge of trivia
> is best left to trolls such as my good self. Trolls are
> the recipients of extensive training in the
> presentation and dissemination of dry, dusty, life-
> sucking facts, thus are best placed both to inform and
> amuse the rec.runners without them even realising they
> are being both educated and entertained. You are
> treading on our turf. Please stop.
>
> Would you be so kind as to convey my best wishes to your
> Antelope? The poor beast must get bloody cold in Alaska.

Bollocks.

Dot - please continue. I for one enjoy some of your
longer musings and anyone can stop reading at any time if
they want to.

Ellis
--
Apply yourself. Take a few risks. Have fun with it.

\"El Paisano\

Np426z

"Ellis" <ellis_paul@msn.comREMOVE> wrote in message
news:Yf6dnf6ZgvLWC3PdRVn-gQ@giganews.com...

> Bollocks.

'Bollocks', when used in response to a post, is considered
by many to be a rude word. I have reported you to your ISP.
Expect a repremand from one of their nice service people.

> Apply yourself. Take a few risks. Have fun with it.

I have. I do. I shall.