Newbie GPS questions

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Waffle Stomper
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Newbie GPS questions

Post by Waffle Stomper » July 5th, 2014, 5:41 pm

So I've dusted of my old Garman 60CSx and started to play with tracking. And now I have a couple questions to start.
http://gpsfly.org/g/3455
Question 1. It was an out and back hike (and very slow hike - even slower than a recliner hike :D). Is it my GPS or is it typical that it doesn't match the trails on the map?

Question 2. It was an out and back why the wild variations between the tracks?

Thanks.

Waffle Stomper.
"When we try to pick out anything by itself, we find it hitched to everything else in the Universe." - John Muir

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Waffle Stomper
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Re: Newbie GPS questions

Post by Waffle Stomper » July 5th, 2014, 5:49 pm

Oh, oh another lesson, learning how to link gpsfly correctly.
"When we try to pick out anything by itself, we find it hitched to everything else in the Universe." - John Muir

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adamschneider
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Re: Newbie GPS questions

Post by adamschneider » July 5th, 2014, 10:59 pm

Waffle Stomper wrote:Question 1. It was an out and back hike (and very slow hike - even slower than a recliner hike :D). Is it my GPS or is it typical that it doesn't match the trails on the map?
It's typical. The main reason is that the trails on the map are rarely 100% accurate. The 2013 USFS digital maps are often pretty close, but not always.
Waffle Stomper wrote:Question 2. It was an out and back why the wild variations between the tracks?
Because GPS isn't 100% accurate either. Your "wild" variations are well within the normal margin of error, especially when in mountainous terrain, and especially in a forest. Trees really mess with your signal.

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Waffle Stomper
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Re: Newbie GPS questions

Post by Waffle Stomper » July 5th, 2014, 11:25 pm

Thank you, Adam.
"When we try to pick out anything by itself, we find it hitched to everything else in the Universe." - John Muir

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kepPNW
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Re: Newbie GPS questions

Post by kepPNW » July 6th, 2014, 8:50 am

Waffle Stomper wrote:Oh, oh another lesson, learning how to link gpsfly correctly.
Been waiting for someone to ask that! Seems to be a lot of ones just like yours, because it's not very intuitive. On the page with your track, click the "<Embed>" button to get HTML that looks like this:

<iframe width="645" height="440" src="http://gpsfly.org/gps_map.php?gps_id=3455&w=645&h=440" scrolling="no" frameborder="no"></iframe>

Select the SRC attribute link (highlighted in red):

<iframe width="645" height="440" src="http://gpsfly.org/gps_map.php?gps_id=3455&w=645&h=440" scrolling="no" frameborder="no"></iframe>

Then put that between the gpsfly tags in your post. (Press the Quote button on this post to see exact syntax.)

http://gpsfly.org/gps_map.php?gps_id=3455&w=645&h=440
Karl
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kepPNW
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Re: Newbie GPS questions

Post by kepPNW » July 6th, 2014, 8:56 am

adamschneider wrote:
Waffle Stomper wrote:Question 2. It was an out and back why the wild variations between the tracks?
Because GPS isn't 100% accurate either. Your "wild" variations are well within the normal margin of error, especially when in mountainous terrain, and especially in a forest. Trees really mess with your signal.
These were some really bizarre deviations, though. I almost suspect the GPS was carried differently in and out? My track of the same hike didn't do get all ziggy in that same spot (near the big switchback). And it does appear a break was taken there - maybe an equipment adjustment too?
Karl
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Lumpy
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Re: Newbie GPS questions

Post by Lumpy » July 6th, 2014, 11:24 am

No consumer grade GPS works perfectly. There have been many improvements over the years, but you will never, ever see surveyor quality measurements from consumer grade stuff. I know that isn't the question you asked.

When you are moving, the GPS unit has to be constantly crunching information to keep up, and you can expect it to have many minor problems during this work: Location, speed, course, elevation, etc. Sometimes, the unit has to make calculations based on less information than it had a few seconds ago, so you get weird locations, and than a few seconds later, you have great information to calculate with, so your displayed location is much better. Since the unit will never ask you to backtrack so it can replace the work it already did with better work, you get a track that looks funny.

Also, as was already stated, "map" type information and gps tracks for many features rarely match up. Calculation errors, map tile placement in a program or website, on and on.
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vibramhead
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Re: Newbie GPS questions

Post by vibramhead » July 6th, 2014, 2:44 pm

I have a Garmin 62s, and I also often use my Nexus 5 phone as a GPS. The Nexus receives both GPS and GLONASS signals, and I think it generally tracks more accurately than the Garmin. But the difference isn't huge, and the Nexus's battery life is much less. I think your tracks look pretty typical. I find that the position shown by my GPS is generally highly accurate when I'm up high, in open terrain, or on the water, but its accuracy deteriorates under heavy tree cover and in canyons. It's worst in an urban setting with high buildings.

One trick that will give you somewhat better accuracy is to turn on the Garmin about an hour before starting your hike, such as while you're driving to the trailhead. That will allow it to acquire the complete "almanac" of current satellite positions in view, which improves accuracy. Obviously, you need to zero out your track log and odometer before starting your hike.

If you're going to use the GPS much, I highly recommend getting Eneloops rechargable batteries, at least for day hikes. For longer backpacking trips, e.g., week-long, I use lithium batteries, because they last longer and I don't need to carry many replacements.
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GPS tracks on Wikiloc.

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Re: Newbie GPS questions

Post by Lurch » July 8th, 2014, 10:24 am

Also make sure to clear your track before you start, and save the track at the end of your hike, or you'll end up with really annoying 'hops' between home/trailhead where the GPS just assumes you have a magical teleportation device.

As for the tracks being slightly off, that's what you get using satellites and radio waves traveling at the speed of light to triangulate your distance :P Even with a poor GPS fix and a +/- 5 meter margin of error, it takes that signal 0.0000000166 seconds to go that far.. I'd say that's damn close to magic for us to have a handheld device that can receive multiple signals that accurately and calculate their convergence

I would suspect that part of the discrepancies that you're seeing is in the algorithms that they use to fudge and filter out the errors. If you're traveling at a consistent rate and bearing, it's going to assume that you're still traveling on that bearing/speed. If the point that it gets is off of its estimate, it will fudge it a little in favor of where it thinks you should be. That's why if you're hunting for a very specific location, and stop exactly when it says you're dead on, you can stand and watch your coordinates drift away before slowly coming back. It causes drift in long switchbacks where it has a chance to figure out your new bearing/speed, and will drift in opposite directions when you're traveling back along the same route.

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Re: Newbie GPS questions

Post by Lumpy » July 8th, 2014, 10:37 am

Lurch wrote:As for the tracks being slightly off, that's what you get using satellites and radio waves traveling at the speed of light...
Very weak radio signals, at that. Add in possible sources of interference, multipathing, and anything overhead that can reduce or block the signal (tree canopy, weather, your own noggin, etc.), and I think these little handheld, lightweight, battery operated devices are just amazing!
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