Skyline/C2C & CalTopo Distance Accuracy?

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Skyline/C2C & CalTopo Distance Accuracy?

Postby mattytreks » Thu Jun 16, 2016 7:36 pm

I've been using CalTopo a lot lately to route-plan for an upcoming trip to the Sierras, and today I decided to map out Skyline/C2C out of curiosity.

But now I'm questioning the accuracy of it's distance data because it is telling me the Skyline portion of C2C is under 9 miles (isn't it 12?) and that the entire C2C is only 19 miles (isn't it 23.5?). Huh?

Have I been miscalculating Skyline/C2C all these years, or is something fishy going on with CalTopo? As you can see from the map I drew out below, I have not done any shortcutting whatsoever on the route, and I'm measuring the distance from: museum parking lot -- to summit -- to the top of the ramp at the upper tram station.

Feel free to take a look at the map I made and let me know if I've done something wrong:
http://caltopo.com/m/FAD1

Thoughts?

ImageC2C Overview by mattytreks
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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby Florian » Thu Jun 16, 2016 8:43 pm

My GPS usually says Skyline is around 10 miles. Also i've found drawn routes on a map generally come out shorter than actual. And you gain a mile and a halve vertically on Skyline too.

-Florian
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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby Hikin_Jim » Thu Jun 16, 2016 8:54 pm

Dude. The rock says 8 miles. Rocks are, like, solid.
Image

And of course that rock might be a mile in, maybe (if you come in from Ramon Road). Maybe. I'll buy Florian's overall 10 miles.

Nice map. Where'd you get it?

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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby mattytreks » Thu Jun 16, 2016 9:32 pm

Hikin_Jim wrote:Nice map. Where'd you get it?

You mean the one I linked to? I made it 8)
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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby Hikin_Jim » Thu Jun 16, 2016 10:56 pm

Yeah, but where did you get the underlying map? It has some interesting trails on it, trails that aren't on some other maps.

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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby mattytreks » Fri Jun 17, 2016 8:53 am

Hikin_Jim wrote:Yeah, but where did you get the underlying map? It has some interesting trails on it, trails that aren't on some other maps.

This was made using the "MapBuilder Topo" Base Layer.

In looking at some of the maps you've made, I believe you typically use the "US Forest Service" Base Layer.
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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby Hikin_Jim » Fri Jun 17, 2016 9:12 am

Yeah, I've been noticing that Map Builder layer lately. Particularly in the Sierra Nevada where some of the maps are in meters and others in feet (which makes my head all screwy), :? it's nice to use the Map Builder layer. I like the FS layer because it has the best resolution... but of course any of these maps can have the base layer changed at the click of a button.

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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby Ed » Fri Jun 17, 2016 11:36 am

Hard to see how a trail traced on a map can capture all the distance due to switchbacks. A GPS distance should be much better, though I suspect even a GPS may not capture switchbacks as fully as you would like, by truncating some of the turns. But the vertical distance probably has little affect. 10 miles horizontal and 8,000' vertical translates into a slope distance of 10.1 miles.
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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby GoPlayer » Fri Jun 17, 2016 5:55 pm

Ed wrote:Hard to see how a trail traced on a map can capture all the distance due to switchbacks. A GPS distance should be much better, though I suspect even a GPS may not capture switchbacks as fully as you would like, by truncating some of the turns. But the vertical distance probably has little affect. 10 miles horizontal and 8,000' vertical translates into a slope distance of 10.1 miles.


The GPS distance needs to be taken with a grain of salt also. There are measurement errors, interpolation errors, etc. A lot depends on how a device or app handles the data. I recently hiked ~20 miles around San Jacinto using my phone's GPS and 2 different apps concurrently. One app reported 21.66 miles the other 20.4 miles. I exported the tracks to GPX and opened them in GoogleEarth which reported 19.7 and 19.6 respectively. OziExplorer on the other hand reported 22.48 and 20.4 for the same GPX files. The GPX files contained 44693 and 3660 points respectively.
Of course reported elevation gain was all over also.

Hikin_Jim wrote:Yeah, I've been noticing that Map Builder layer lately. Particularly in the Sierra Nevada where some of the maps are in meters and others in feet (which makes my head all screwy), :? it's nice to use the Map Builder layer. I like the FS layer because it has the best resolution... but of course any of these maps can have the base layer changed at the click of a button.

HJ


I'd just like to use this post also to Thank You for those great maps HJ!
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Re: Skyline/C2C & CalTopo Distance Accuracy?

Postby Hikin_Jim » Fri Jun 17, 2016 7:56 pm

GoPlayer wrote:I'd just like to use this post also to Thank You for those great maps HJ!
Hey, glad to hear someone's actually using them.

[Bragging on]
The guys at CalTopo noticed the traffic coming to some of my maps and gave me a free account. How cool is that?

I also got mentioned on Andrew Skurka's blog. Kind of makes me feel like I done good.
[bragging off]

But most of all, I'm glad they're useful. People using my maps is the best confirmation that I did something worthwhile.

Map geekily yours,

HJ
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