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Maps, Signs, and reality

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Re: Maps, Signs, and reality

Postby Wandering Daisy » Wed Sep 26, 2012 7:44 am

Maps are only approximations of terrain. The more you over lay on a map the more chance there is for errors. Google Maps have many errors in their overlay of roads.

Topography is now "drawn" from 30-meter pixel DEM's (Digital Elevation Models). A computer program interpolates between points. There are pretty sophisticated interpolation programs out there that "look" beyond the actual location being evaluated to see what the larger topographical trends are. Nevertheless, you can actually generate different topographical lines depending on which interpolation program is used. The older USGS maps (say 1970's and earlier) are often more accurate because they were actually created by cartographer. That is why some of the newer USGS map contours look "smoother" . On the old maps a cliff could have been actually hand drawn.

All maps have to be geographically "located". Depending on which map projection is used, actual points on the ground do not usually line up perfectly. So if you take a USFS map (for roads) and overlay on a USGS map (for topography) and overlay on satallite images; none of the layers will exactly match up. Some mis-alignments can by significant - say a quarter mile.

And trails are often re-routed and these changes do not show up on a lot of maps. Also, a map cannot show every little bend and switchback in a trail. When I talk of "miles" in my guidebook, I always clearify it as "Map Miles", and comment that "real miles" will be more - often as much as 20-30% more.

Not all GPS maps are equal! Each GPS uses specific maps as their bases. Most of the newer GPS units use the current USGS 7.5 min DEM based maps. However, the trails and roads that are shown in a GPS can come from many different sources of data. Also, YOU have to set your datum and there are two common datums and if the base maps the GPS maker uses does not correct for different datums, the GPS maps can be off.

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Re: Maps, Signs, and reality

Postby sparky » Wed Sep 26, 2012 7:10 pm

I have noticed the milage on the road signs on the 395 are off
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Re: Maps, Signs, and reality

Postby jrad » Mon Sep 05, 2016 11:24 am

I just hiked from Franklin Pass down to Mineral King. The discrepancies between Harrison and NPS on-trail signage were shocking:

4,6 miles - SEKI sign mileage - Farewell Gap trail to Franklin Pass
3.7 miles - Harrison Map mileage for same section

4.0 miles - SEKI sign mileage down from there (Franklin Pass jct. Farewell Gap Trail) to MK Trail Head
3.0 miles - Harrison Map mileage for same section

I emailed Harrison and he said ALL his data is from published official NPS and USFS data - GPS recordings published by the government. I asked if he used a wheel and if he knew why these discrepancies (the one I found)? He uses GPS only and had no idea why the signage on the trail should vary so radically from GPS provided by the very institution that makes the signs.

I tend to accept Harrison data, given the fact that I have again and again found self-discrepancies on-trail of NPS and USFS signages since my first trips 42 years ago. Thus I would finish a section purported to be so-and-so many miles one way only to find the mileage posted for the same section in the reverse direction to be slightly or significantly different.

I am afraid government incompetence might explain the lack of alignment. BUT WAIT ........... Maybe Harrison is making an error here, assuming the government uses a wheel.

That is, GPS data does not account at all for elevation change, leading to pretty significant errors sometimes! [I am assuming GPS does not provide elevation data or at least such data is not accounted for. Actually I think GPS DOES record elevation. Not sure, though.]

EXAMPLE: If one hikes 1 mile on flat ground, GPS says "1 mile traveled". Period. But if one climbs 1 mile vertically between those GPS readings, the actual distance covered is really 1.4 miles while GPS says "one mile traveled"!

This still does not account for trail signage being so different. But it may explain some.
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Re: Maps, Signs, and reality

Postby rlown » Mon Sep 05, 2016 11:34 am

Guess signage, maps and gps readings are "close enough" for me. You get there when you get there, esp if you decide to take small detours. Not a fan of overthinking it..

I just want the trails on the map to be semi-accurate. Off-trail, I don't really care. It's all a game at that point..
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Re: Maps, Signs, and reality

Postby SSSdave » Mon Sep 05, 2016 2:51 pm

Dredging up an old thread. There have been threads on this on numbers of enthusiast boards over the years.

The accuracy of GPS routes along trails and recorded mileages greatly depends on several factors, especially the level of the device and algorithms. Read this topic:


If mileage is derived from a trail drawn on a topographic map, although accuracy of the map line mileage can be calculated with pretty good accuracy, the true accuracy depends on how well the trail was placed on the map. Wilderness Press guide books were usually consistent the way they calculated mileage from topos and I tend to match their numbers closely. Although some drawn trails on maps are shown quite accurately, many are not and some quite grossly incorrect. The worst I've seen is at Mineral King above Groundhog Meadow. Nothing new and was the situation decades ago too. On some GPS sites members are correcting trails with results showing just what I mention above. Some trails match well while others are grossly incorrect.

Here is one old thread on the Shepherd Pass trail where I and Bob Burd destroyed the notion of accuracy of trails on maps:
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Re: Maps, Signs, and reality

Postby dave54 » Mon Sep 05, 2016 4:51 pm

I have seen K-tags off by 6 miles. I am pretty certain it was a simple mistake, scratching 33N on the sign when it was 32N. Everything else was correct. But a K-tag is a surveying marker, and should be right on to the sub-meter level.

Also some jerk though it would be funny to turn a road junction sign 90 degrees. The loose dirt around the base indicated it was recently done. I took the time to fix it.
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