First GPS Hike

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First GPS Hike

Postby Hikin_Jim » Sat Jan 04, 2014 10:49 pm

I got a GPS for Christmas and went on my first hike with a GPS on Jan. 1.

Thank you to everyone who gave me input.

OBSERVATIONS
1. There does seem to be a loss of detail (and therefore an under-reporting of mileage) in the process of saving a track. At the end of my hike, my "odometer" read 6.28 miles. When I uploaded to my computer, my mileage read 5.94 miles. That's a loss of about 1/3 miles, which to me is fairly significant, particularly since it was only a 6 mile hike. It looks like I'm losing about 0.05 miles per mile recorded just due to the way the track is saved alone.

2. If you look at the plot of my route on a satellite photo (see above link), you can see that the GPS basically draws a straight line from when ever it marks a point. Trails are seldom straight lines. I think it's safe to assume that a GPS will always under report.

3. Give #1 and #2, above, it would seem that GPS unit reported mileages are going to be significantly lower than actual mileages. How much? I'm not sure, but perhaps 0.1 mile lost for every actual mile. The shape of the trail and general topography as well as things like how many GPS/Glonass satellites were in "view" would affect this.

4. If one were to go to the above link and toggle the view to topo map, one would see that the route of things like the Gabrieleno National Recreation Trail shown on the map is quite a bit different than what my GPS plot reveals. While topo maps in the US are generally pretty good, there are a lot of errors.

5. If one looks closely at the satellite photo, one can see some of the road or trail that I was travelling on. The GPS doesn't follow exactly. Sometimes it is in error off to one side; sometimes the other. Clearly, GPS, at least when used in the form of a hand held unit like mine, is an approximation.

GENERAL REMARKS
The Garmin eTrex 20 interface is pretty quick and easy to learn albeit primitive compared to my Samsung Smart Phone. The GPS is interesting, and I look forward to using it more.

Overall, a fun hike. It was even more fun because I had such a good hiking partner. :)
Image

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Postby OtherHand » Sat Jan 04, 2014 11:28 pm

Welcome to the 21st century. You might find the Internet of interest too!

The points shown on your upload to the Caltopo website are indeed sparse with straight lines in between. But if you look at your total points in something like the Garmin software you should see them much closer, maybe 10 to 20 feet apart. And the amount of points recorded is a setting you can change.

I have almost always noted a discrepancy between the odometer and the actual recorded track. I'm not sure exactly why that is, but the odometer for me usually reads just a bit more. Am am surprised at the amount of comment this gets, as if people are paid by the mile! I assume most national forest trails were measured using a wheel (and not pedometers!!), and if you ever have used one you know their questionable accuracy due to going over or around rocks. I think a GPS with close point recording will outperform a wheel on rough trails.
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Postby pdforeme » Sat Jan 04, 2014 11:38 pm

with my Garmin etrex i have found it NOT to work accurately unless i turn it on well before i start hiking. That is, i can start a hike (wrongly turning it on as i start) and until it both acquires the satellites...AND....seemingly get settled....i get grossly inaccurate data. But if i give it time, its all fine, save for the frequency i set it for tracking my points. if on a day hike w rechargeable batteries there's no real reason to economize on data points...
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Postby Hikin_Jim » Sun Jan 05, 2014 10:33 am

OtherHand wrote:Welcome to the 21st century. You might find the Internet of interest too!
The Internet? Nah. It will never catch on. I give it six months. Tops. :lol:


I have almost always noted a discrepancy between the odometer and the actual recorded track. I'm not sure exactly why that is, but the odometer for me usually reads just a bit more.
I don't want to read too much into one trip, but yeah I was a little disappointed with a five percent data loss just from saving the track.

On other fronts, I don't blame the GPS for CalTopo's limitations. The GPS lost 0.3 miles out of 6.25. CalTopo lost 0.5.

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Postby Hikin_Jim » Sun Jan 05, 2014 3:47 pm

OtherHand wrote:I assume most national forest trails were measured using a wheel (and not pedometers!!), and if you ever have used one you know their questionable accuracy due to going over or around rocks. I think a GPS with close point recording will outperform a wheel on rough trails.
That's a good point. Clearly the GPS track I have is far more accurate than the map, and presumably the mileages it calculates are as well. It seems a little "sloppy" to me to have a 5% discrepancy between the odometer and the track, but the mileage calculated by a GPS is probably the most accurate available. And besides that, what's the difference in perceived exertion or time required between a 5.94 mile hike and 6.28 mile hike? Not a lot. From a planning standpoint, I'd approach both about the same.

While discrepancies bug me, let's keep things in perspective. Take a look at the below photo. Waypoint 21 is the trail junction to turn off to Valley Forge Camp. You can see the actual junction in the satellite photo. The discrepancy? A foot or two at the most. Waypoint 23 is the creek crossing near camp. Looks pretty dead on. Waypoint 24 is the main sign for the camp, the one with the rules & regs posted on it. I can't quite see the sign in the Sat photo, but I know it's at the head of the parking lot (now closed). The waypoint is either dead on or very, very close.
Image

OtherHand wrote: if you look at your total points in something like the Garmin software you should see them much closer, maybe 10 to 20 feet apart.
Yeah, I have to play with it using other software. I've heard of GPS Visualizer and Garmin Connect. What other sites or software are good?

OtherHand wrote: the amount of points recorded is a setting you can change.
On my eTrex 20, I saw a "recommended default" and then another setting for every 30 seconds. I used the recommended setting on the trip down and the 30 second setting on the way back up. I'll have to dig into the XML to see if I can see any meaningful difference.

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Postby jfr » Sun Jan 05, 2014 8:45 pm

Hikin_Jim wrote:
OtherHand wrote: the amount of points recorded is a setting you can change.
On my eTrex 20, I saw a "recommended default" and then another setting for every 30 seconds. I used the recommended setting on the trip down and the 30 second setting on the way back up. I'll have to dig into the XML to see if I can see any meaningful difference.

HJ


I think that your settings are a bit too loose, Jim. In 30 seconds at 2 miles per hour you travel 88 feet or so. That's pretty far between points, and leads to the wildly erratic track when zoomed in. It also leads to a poor distance measurement, especially on tight switchbacks. In a perfect world your gps track would be a continuous line forming a perfect curve (not points at all). Sadly, the world isn't perfect. :evil:

My Delorme PN-40 gps is set at 10 feet between points. This setting leads to some nice looking tracks, that give you more confidence in the total distance measurement. I'd recommend that you change your recording interval to 10 feet (or 5 seconds) and see what happens. Meanwhile, all this experimenting is a great excuse to go on more hikes. :)


Image
What the smoothed tracks look like in Google Earth


Image
What the actual track points look like in the Delorme Topo software. (Delorme's imagery is a bit fuzzier than Google's)


Ones and zeros are cheap these days, so getting more data is a great idea provided that your SD card has the space for a bigger track file. And the great thing about data, from an ultralight backpacking perspective, is that ones and zeros are nearly identical in weight! :D
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Postby OtherHand » Sun Jan 05, 2014 9:12 pm

Jim, since you have a Garmin you can run either Mapsource or Basecamp for free. With those you can look at the properties of tracks on a point by point basis.

For chuckles I looked at my last track into Smith Water. It consisted of 1,395 points and totaled 7.1 miles. That works out to an average of 26.8 feet per point. I'm just using the standard recording setting on my Garmin, nothing special. From the images I saw on Caltopo, it looks like they process your file and reduce the number of points to saw storage space.
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Postby climbant » Sun Jan 05, 2014 11:11 pm

pdforeme wrote:with my Garmin etrex i have found it NOT to work accurately unless i turn it on well before i start hiking. That is, i can start a hike (wrongly turning it on as i start) and until it both acquires the satellites...AND....seemingly get settled....i get grossly inaccurate data. But if i give it time, its all fine, save for the frequency i set it for tracking my points. if on a day hike w rechargeable batteries there's no real reason to economize on data points...


I have found the same. When I get to where I'm going first thing I do is turn on my GPS. Then I get my stuff together, put my shoes on, etc and my gps is good to go. Zero out the data and tracks and start hiking.

Jim glad to see you got a gps, enjoy it.
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Postby Hikin_Jim » Tue Jan 07, 2014 12:29 am

In "Hikin' Jim Bumbles Around with a GPS" News:

1. I have now set my Garmin eTrex 20 to a 5 second recording interval. Hopefully that will give me some better tracks as compared to a 30 second interval. I'll do another hike this Saturday (possibly Holy Jim Falls) and see what things look like.

2. I tried GPS Babel to convert some KML (Google Earth) files to GPX, but didn't get very far. GPS Babel said it couldn't open the first one and then couldn't write output on the second one. :roll: Not the best user interface I've seen, but then it is freeware.

3. I haven't downloaded Basecamp or Mapsource yet. I'll check those out when I have time.

THANK YOU everyone for bearing with me. :oops:

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Postby Hikin_Jim » Tue Jan 07, 2014 1:27 am

Tried to get Garmin Mapsource from download.cnet.com. I've used Cnet.com dozens and dozens of times. Never had a problem. I got a boatload of malware this time. I'm now scrubbing my computer. I'll report the problem to Cnet.com when I have time. I hope they haven't become corrupt. I'll post more later.

For now: Beware of cnet.com.

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