Difference between map and trail distance?

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.–

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.

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.

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.

A metal trail sign is pretty indestructible.

–B.G.–

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

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?

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.

"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.

del

"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

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

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.–

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.

"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.–

"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.

> 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

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?" :-)

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.

"How do the sign makers know how fast I hike? distance is a known number.. you can calculate your time and speed from that."

When I did my first trek in the Mount Everest Region of Nepal, I noticed some of the trekkers would ask the Sherpa guide: "How far is it to the next village?" They expected to hear something about two miles or five miles.

Instead, the Sherpa would say "It is one day's travel to that village." –or– "It is one half day travel to that village." That perplexed the trekkers. They could not think in terms of a distance being a half day. I figured that was just the way it was there. Part of the reason is that there are no highways, there are no cars, and there are no car odometers, so a Sherpa has very little way of knowing just how far a mile is, and the Sherpa has never seen a rolling wheel measuring a trail distance.

As it turned out, that Sherpa was wise. He knew how much time it was going to take the trekkers just from past experience. He also knew that the fast hikers were often the first ones to fail from high altitude problems.

–B.G.–

I've found trail signs to be pretty unreliable in terms of mileage. The worst in my experience are Forest Service signs; NPS seems to do a better job, but I still don't trust them. I treat them as directional indicators only – they are pretty reliable in that regard – and estimate time/distance from the map as always.

> I still think it is easier to put a known mileage than it is to put a guesstimated time.
Yes, it is easier, but it can be misleading, possibly fatally.

There's a section on the Western Arthurs in SW Tasmania called 'the Maze of Beggary'. We KNOW it is about 2 km long. So how long will it take you? Oh, about 8 – 10 hours. You may need rope. The crux, if you can call it that, is a flying jump down across a gap. Dunno how you reverse that move.

Cheers

That is where the guidebook comes in handy?

how did we get from signs vs gps in the white mountains as per the OP to off trail, 4th class?

I know, for me, that i hike 2-2.5mph on all but the steepest stuff in the white mountains.. slower on steep downhills. that information is useless to someone else, especially the heavyweight weekend warriors and day hikers you get up there. On the Long trail in VT we ran into a group that did our 1 day mileage in 3 days.

as someone said, maybe it is different outside the US but around here it's by miles.