Track shorter than expected

[+DeepButi]

Last weekend I was one of the 3000 (crazy) people that went for a "classic" on my country: a 85Km (aprox 53miles) mountain hike in less than 24h :blink: (yes, no need to tell me: it's more than crazy! ... but it's so rewarding that it was my 10th attempt with just 2 failures).

 

For the first time, I took my GPS with me. Not to use it, just I wanted to chek at home the whole path and speed and this kind of useless data

 

To my surprise the track was "only" 77Km long (using Google Earth). mmmm ... I start wondering and after some checking with the organisation data (they know what they are doing, it wass the 33rd edition) I discovered a couple of problems. They seem 100% logic but I never expected for such a big diference!

 

1. GE doesn't use the altitude to compute distances ... the world is flat!

The medium slope was about 8% (both + and -) with maximums of +37% and -30% ... using a constant slope of 10% this would add 1.2Km ... not bad

 

2. But ... a lot more important, we compared my track and the org's one. Mine had some 7000 points while theirs had more than 22000!! And here come to me the biggest surprise. "Of course" you would say, every small straight line between two consecutive points is a little bit shorter than the "real" path that's never straight, every small real curve gets shortened and the less points the less acuracy.

Ok, it's clear but I never expected such a big diference! Aprox a 8% shorter my track than the official one. Wow!

 

Lesson: don't trust your GPS for long tracks

 

Just sharing

[+Redwoods Mtn Biker]

What GPS? And what are your tracklog settings?

[+Atlas Cached]

What GPS? And what are your tracklog settings?

 

Exactly.

 

Do trust that your GPSr will record a track exactly as you have it set to do so.

 

Sounds like maybe the OP needed to adjust the track log settings for more or most often (if was a Garmin).

[+DeepButi]

Garmin Dakota 10

 

Settings to Automatic (options: time/distance) and Normal (five frequency options).

 

Of course setting it to maximum frequency will increase the number of track points and accuracy. I was simply surprised of such a big difference. With a sample every aprox 12m I didn't expect as much.

 

PS. My math brain must have been turned off, the "flat" world effect is a lot less than I calculated, not even 500m, so in fact the "error" due to low sampling is aprox 11%!!!???

[John E Cache]

My wild guess: Unless there is a a bread crumb right on the apex of a switchback, the tip of the switchback will be chopped off. The more crumbs the more likely you will have a crumb on the apex.

[Grasscatcher]

@ Deepbuti,

 

Can you get a gpx file of the "official" version of the trail?

 

If so, then you could compare it with yours, both visually and data differences.

 

Is there any areas of major visual differences? Where & why?

...or is their just a gradual difference in distance? (definitely you should use "more often" or "most often" with your "auto" setting).

 

With the comparison of the two files, you should be able to determine where, why, and how the different lengths happened.

[+EScout]

@ Deepbuti,

 

Can you get a gpx file of the "official" version of the trail?

 

If so, then you could compare it with yours, both visually and data differences.

 

Is there any areas of major visual differences? Where & why?

...or is their just a gradual difference in distance? (definitely you should use "more often" or "most often" with your "auto" setting).

 

With the comparison of the two files, you should be able to determine where, why, and how the different lengths happened.

 

the comments by Grasscatcher are right on.

It is just what I would do.

I would not assume the that "official" track is correct. It might have additional ground covered.

 

I addition to Google Earth for comparing the GPX log files, I would use some utilities such as the online: GPSVisualizer.

And utilities/apps such as "Analyze Track" and "G7toWin"

[Grasscatcher]

Personally, I would use Expert GPS from Topografix to compare the two files, both visually and data. It would probably take all of about 15 minutes.

[John E Cache]

I think some flat landers are missing my point. When you you hike up a mountain two things mess up a GPS, altitude change and switch backs. When I started the trail just below "Rock of Ages", my GPS said 700 feet to the cache and it took me an hour to get there. Also, the GPS tries to smooth zig-zags in a track caused by position errors and ends up smoothing out switch backs.

[snowfleurys]

If it is important to have a crumb at the point of each switchback, you need to occupy that point for at least the minimum interval time you have the GPSr set to (and if you stay there too long, there will be multiple crumbs within a cone of position error). That is just the nature of how GPS/GPSrs opperate.

[seldom_sn]

I think some flat landers are missing my point. When you you hike up a mountain two things mess up a GPS, altitude change and switch backs. When I started the trail just below "Rock of Ages", my GPS said 700 feet to the cache and it took me an hour to get there. Also, the GPS tries to smooth zig-zags in a track caused by position errors and ends up smoothing out switch backs.

Multi-path, and visibility always mess up my tracks when hiking near the bottom of a cliff, but they usually lengthen the track, not shorten it. But if you are in the mountains and you lose signal, then reacquire it, you could get a straight line running from end to end of a curvy route.

Edited September 26, 2012 by seldom_sn

[Grasscatcher]

I think some flat landers are missing my point. When you you hike up a mountain two things mess up a GPS, altitude change and switch backs. When I started the trail just below "Rock of Ages", my GPS said 700 feet to the cache and it took me an hour to get there. Also, the GPS tries to smooth zig-zags in a track caused by position errors and ends up smoothing out switch backs.

First of all, my house is at 8725 ft elevation, so why you callin' me a "Flatlander" ????

Second, you're walking almost too slow for the GPS to accurately detect movement if it takes you an hour to travel 700 ft ! HA !

The remainder is equally factually incorrect.

[seldom_sn]

When I started the trail just below "Rock of Ages", my GPS said 700 feet to the cache and it took me an hour to get there.

 

Was the 700 feet an accurate distance (crow flies) when measured in BaseCamp? How long was the track between them, and did it correspond with the actual path you traveled? I just tried to make an accurate track of the Fisher Towers trail near Moab. All the multipath caused my trip computer to show 3.5 miles for a 2.1 mile trail, and the track was 100 yards off trail at several points. GPSr was Etrex30 with WAAS and GLONASS turned off.

Edited September 27, 2012 by seldom_sn