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Difference between map and trail distance?
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Curtis B.
(rutilate) - MLife

Locale: Pacific Northwest
Difference between map and trail distance? on 08/08/2013 19:31:16 MDT Print View

I've been on a number of trails where the posted mileage is different from the map distance listed in the trail books or calculated by Trails Illustrated or gmap4. Am I correct in my assumption that the map distance is calculated using the small map wheels and only accounts for flat distance, ignoring the vertical elevation (ie. calculating the base of the triangle from point a to b, rather than the hypotenuse)?

Or was someone in the woodshop simply overly optimistic when they carved the signs?

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Difference between map and trail distance? on 08/08/2013 19:43:28 MDT Print View

Trail distance seems to be computed differently in each jurisdiction where I've hiked. There used to be a joke about how to compare National Park Miles to Forest Service Miles.

I do not believe that you will find the elevation gain or loss to be that much of a factor. The maximum that I have ever seen was 10% error.

Also, trail signs are not necessarily wood, so they might not have come from a woodshop. For example, in many national parks in California, the trail signs are cut out of iron plate to prevent them from being used for campfires.

I've seen major errors in the mileage reported on some trail signs within Yosemite. For example, the first sign will say that it is 5 miles to XYZ lake. Then the next sign will say that it is 6 miles to the same lake. The last sign will say that it is still 3 miles to the lake. It turns out that they interchanged some of the signs. They would have all been correct if they had been installed in the correct spots.

Seasonal rangers will sometimes tape a "correction" onto the metal sign.

--B.G.--

Ian B.
(IDBLOOM) - MLife

Locale: PNW
I'd go with the signs on 08/08/2013 19:44:18 MDT Print View

The Green Trails map for Rainier is absolutely horrible. A 24k map doesn't have good enough resolution to account for all the twists and turns but even measuring straight line distance revealed that there were some pretty significant errors. A few legs were off by more than two miles. I knew what my total mileage was going to be for the day but it'd be bad for someone who didn't do their homework. I found the signs in Rainier NP to be pretty accurate.

One of the best tools I have for estimating distance is my watch. I normally truck along at a 2mph pace give or take a little. Not accurate enough to call in an airstrike but good enough for my purposes.

Edited out an obvious error.

Edited by IDBLOOM on 08/08/2013 19:47:54 MDT.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Difference between map and trail distance? on 08/08/2013 20:14:56 MDT Print View

About the steepest slope I see is 1000 feet elevation gain in 1 mile.

Distance along diagonal is sqrt(1000^2 + 5280^2) = 5370 feet = 1.02 miles. So there's a 2% difference where there's a steep slope - pretty insignificant.

The way to calculate distance is to take a GPS track, analyze it on your PC removing any bogus points like clouds where you stop or any side trips off the trail.

Signs on trails aren't very accurate. Most modern maps use GPS tracks, but they may not be that careful about removing bogus points because it's sort of obscure.

Curtis B.
(rutilate) - MLife

Locale: Pacific Northwest
Re: Re: Difference between map and trail distance? on 08/08/2013 20:18:55 MDT Print View

Jerry wrote: "Distance along diagonal is sqrt(1000^2 + 5280^2) = 5370 feet = 1.02 miles. So there's a 2% difference where there's a steep slope - pretty insignificant."

Ah, go ahead and spoil my delusion with hard facts! Very well done. My mind is at ease now. :-)

And thanks to all for the other comments. I've never seen metal trail signs. Here in the Whites I've started seeing newer signs made from the synthetic decking material.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: Re: Difference between map and trail distance? on 08/08/2013 20:30:45 MDT Print View

metal trail sign

A metal trail sign is pretty indestructible.

--B.G.--

Edited by --B.G.-- on 08/08/2013 20:31:52 MDT.

Curtis B.
(rutilate) - MLife

Locale: Pacific Northwest
Re: Re: Re: Re: Difference between map and trail distance? on 08/08/2013 20:34:29 MDT Print View

"A metal trail sign is pretty indestructible."
Yeah, that would tend to last a while!!

Jake D
(JakeDatc) - F

Locale: Bristol,RI
Re: Re: Re: Re: Re: Difference between map and trail distance? on 08/08/2013 21:04:10 MDT Print View

I'm pretty sure that the distances in the White mountains guide were done with a wheel. So i'd trust the signs and the maps more than the computer maps.

I use the 4000 footer map with the distances on the trail segments and those seem accurate?

Stuart R
(Scunnered) - F

Locale: Scotland
GPS overestimates distance on 08/09/2013 01:47:31 MDT Print View

The way to calculate distance is to take a GPS track, analyze it on your PC removing any bogus points like clouds where you stop or any side trips off the trail.

GPS tracks over-estimate distance, even when bogus points are removed. The slower you walk the greater the error. This is easy to prove:
Walk slowly (<2mph) along a straight-ish track for ~1/2 mile, stop and save the track. Walk briskly (~4mph) back to your starting point, stop and save the track. Finally, run or cycle the same route and save the track.
The slowest track will be the longest. The run/cycle track will be shortest, but not much shorter than the 4mph track.

Hiking Malto
(gg-man) - F
Re: GPS overestimates distance on 08/09/2013 06:20:35 MDT Print View

"The way to calculate distance is to take a GPS track, analyze it on your PC removing any bogus points like clouds where you stop or any side trips off the trail.

GPS tracks over-estimate distance, even when bogus points are removed. The slower you walk the greater the error. This is easy to prove:
Walk slowly (<2mph) along a straight-ish track for ~1/2 mile, stop and save the track. Walk briskly (~4mph) back to your starting point, stop and save the track. Finally, run or cycle the same route and save the track.
The slowest track will be the longest. The run/cycle track will be shortest, but not much shorter than the 4mph track."

I assume this is due to a slower speed having more data points than a faster speed??? With each data point comes an error, the error is compounded more. I have also seen in bad reception areas the ability to make serious miles and elevation gain during breaks.

Rick M
(rmjapan) - F

Locale: London, UK
Re: Difference between map and trail distance? on 08/09/2013 07:39:00 MDT Print View

I prefer trail signs posted in time rather than distance when I can find them. Seems to be a more common mountain "standard" outside North America though.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: GPS overestimates distance on 08/09/2013 07:53:25 MDT Print View

"GPS tracks over-estimate distance, even when bogus points are removed. The slower you walk the greater the error..."

If you walk in a straight line, remove every point except the two end points

Make sure you don't remove any switchbacks

There isn't much difference between gentle curves and just the distance between end-points

Nick Gatel
(ngatel) - MLife

Locale: Southern California
Re: Re: Re: GPS overestimates distance on 08/09/2013 10:40:31 MDT Print View

Rock

I hate technology in the wilderness. This is close enough +/- a mile.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: GPS overestimates distance on 08/09/2013 12:17:31 MDT Print View

The correct answer, of course, is: It depends.

Every model of GPS receiver is a little different in how it calculates distance. Hopefully the error isn't much. For example, some older receivers came out during the period of Selective Availability, so there was a constant random error floating around, and the receiver would suppress data that showed a very slow ground speed, like less than 1 mph. But, when should it quit doing that and record normally? We've gotten away from Selective Availability now, but similar other random processes are still busy. Some receivers will discard data when you drive through a road tunnel. Typically they don't get any signal inside the tunnel anyway. But, if it had signal on one end and then signal on the other end, should it compute the distance as being continuous through the tunnel? Or, should it just omit the tunnel distance? (There is no perfect answer to this.) If you translate that into a hiker's progress, as you go underneath heavy tree canopy, what should it do for distance computations? Each receiver samples a little differently, so the distance results become different. To prove this, walk up or down the Mount Whitney 97 Switchbacks and then study the track log and distance. Sometimes I get a different distance whether I am going up or down.

--B.G.--

Stuart R
(Scunnered) - F

Locale: Scotland
Re: Re: GPS overestimates distance on 08/09/2013 15:00:14 MDT Print View

I assume this is due to a slower speed having more data points than a faster speed??? With each data point comes an error, the error is compounded more.

It depends on the recording algorithm you have selected for the track points, but in general yes, more points within a straight distance will give more error.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: Re: GPS overestimates distance on 08/09/2013 15:20:57 MDT Print View

"It depends on the recording algorithm you have selected for the track points, but in general yes, more points within a straight distance will give more error."

That is all kind of silly, because the user seldom has any control over recording algorithms.

If you are traveling in a perfectly straight line for a long distance, then more points within that straight distance will give more error, but it is a tiny error that you can't find within all of the other errors going on. That is also kind of silly, because backpackers are seldom going in a perfectly straight line for a long distance.

If you are traveling on a switchback trail (with zigzags), you will get the most accurate distance recorded by going slowly and letting the maximum number of data points accumulate, but that will vary if your position accuracy is unreasonable. For example, if you are on a switchback trail within a deep box canyon, then your horizontal accuracy is likely to be impaired. There are excellent ways of doing this, like if you are a surveyor, and they involve some expensive equipment. But for a typical backpacker, you just have to go with what you've got and learn how to make the best of it. Few people do this.

--B.G.--

Jake D
(JakeDatc) - F

Locale: Bristol,RI
Re: Re: Difference between map and trail distance? on 08/09/2013 16:11:00 MDT Print View

"I prefer trail signs posted in time rather than distance when I can find them. Seems to be a more common mountain "standard" outside North America though."

not quite sure how this works... How do the sign makers know how fast I hike? distance is a known number.. you can calculate your time and speed from that.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Re: Difference between map and trail distance? on 08/09/2013 16:17:15 MDT Print View

> How do the sign makers know how fast I hike?
They don't. They give a time for an 'average' walker. You calibrate yourself to that - pretty easy.

> distance is a known number.. you can calculate your time and speed from that.
Ho Ho Ho.
Wheelchair track? Grade 3 scramble? +2000 m that day? Thick scrub? Rough scree?

Cheers

Jim Colten
(jcolten) - M

Locale: MN
Re: GPS overestimates distance on 08/09/2013 16:21:42 MDT Print View

Nick said: "I hate technology in the wilderness."

As do I ... often don't even take a watch. But then I'm sometimes reminded to challenge myself with questions like "Since when is paint not technology?" :-)

Jake D
(JakeDatc) - F

Locale: Bristol,RI
Re: Re: Re: Re: Difference between map and trail distance? on 08/09/2013 16:34:39 MDT Print View

Roger, the trails that the OP is talking about have been measured with a rolling wheel so they are accurate. The elevations are on the topo map.

the white mountains "book time" is 30min per mile plus 30min for every 1000' elevation gain. I beat that quite regularly and am usually faster up hill than i am down hill so their math doesn't work for me.

I still think it is easier to put a known mileage than it is to put a guesstimated time.