[at-l] Measuring Wheels (More)

Wed Feb 14 00:03:09 CST 2007

Accuracy is an inherent property of the device, not the user's perception. A
wheel's accuracy depends on how good the manufacturer's tolerances are on
getting the diameter of the wheel right. It also depends on not slipping
which on ice is a problem. Otherwise it measures whatever it rolls over as
accurately as it is calibrated. What it measures may not be what you
perceive as distance on non-flat surfaces. I think some of us perceive of
distance as the length of a piece of string stretched along the path but off
the surface enough that it does not see the small ups and downs of rocks but
does see the small cliff that goes vertically up or down 30 feet. I know of
no device that measures that other than the piece of string with a crew
arguing about where to place it.

A GPS's accuracy depends a lot on where the satellites are at the moment but
is typically 3-15 feet unless you use fancy GPSs with differential
correction, etc. That accuracy is of any particular position. Since it is
measuring absolute position the errors tend to cancel out. The shortage on
one position may show up as an overage on the next if all the errors were
along the path. But they aren't so you tend to get a path with more jags
than are really there. Ideally for measuring the distance you would sum up
all the individual distances between track points. The randomness of the
errors tends to cancel out the distance errors. Most GPSs I have seen have a
display of distance travelled but no explaination of exactly what they are
computing to arrive at it. In other words it is proprietary and unknown. On
rough terrain my cheap Garmin tends to be about 10% lower than what my wheel
reads. Some people will quote distance numbers computed from their GIS
program after they have uploaded the data. These typically correct for ups
and downs based on the contours which are derived photogramatically from
aerial photos and reported on a 10x10 meter grid and interpolated along the
path, so they certainly don't see small elevation changes over short
distances. One thing you need to know about GPSs is that they take track
points every so many seconds, typically 1 to 5. This means how dense your
points are depends on your speed of travel and if you don't slow down going
around a switchback you are likely to get one point 10 feet short of the
turn and one 10 feet after the turn resulting in 3 feet of distance instead
of 20.

-----Original Message-----
From: Bob C [mailto:ellen at clinic.net] 
Sent: Tuesday, February 13, 2007 11:55 PM
To: Walt Daniels; at-l at mailman.backcountry.net
Subject: Re: [at-l] Measuring Wheels (More)

I believe a measuring wheel more accurately measures the trail as a hiker
experiences it. I forget the name of the wheel I use. But it is a large
wheel that clicks a counter every foot. I think it measures around five feet
or more with each rotation. What's the formula for circumference? I forget.
Pi (3.1416) times the wheel's diameter?

I have never used a gps to measure a trail, though I suspect its inaccuracy
is offset by being much easier to use than a wheel. Also I don't think a
small diameter wheel would work for trail conditions. A large wheel will
roll over everything. A small wheel, it seems to me, would be constantly
fetching up on rocks and blow downs.

I suspect the small wheel versions are mostly used by engineers measuring
paved roads, and by assessors measuring properties for tax purposes.
Foresters I've seen have all used large diameter wheels as does the Maine
Appalachian Trail Club.

Weary

> ------------Original Message------------
> From: Walt Daniels <wdlists at optonline.net>
> To: at-l at mailman.backcountry.net
> Date: Tue, Feb-13-2007 5:41 PM
> Subject: Re: [at-l] Measuring Wheels (More)
>
> As previously posted we use a Rolotape 623 which measures 6 ft per 
> revolution. It has pegs aroung the rim so it counts in feet. I have 
> used smaller ones and they are difficult to control on steep rocky 
> stretches. You need a large enough counter that you don't have to 
> reset it during the day or count rollovers on a piece of paper, so you 
> need to go as high as 15 miles or so. We are currently measuring 
> trails in smaller parks so we rarely have a single trail longer than a 
> mile. I have never tried doing a bushwhack with a wheel but it not 
> unusual to have to deal with measuring through a large messy blowdown. 
> If it is bad enough we estimate or pace off the distance, walk the 
> wheel around and then spin it enough for the estimated distance. Ours 
> has open wire spokes and occasionally a stick gets kicked into it and 
> stops the rotation. It is easily removed and causes no problems.
> If this happened frequently it might be a pain. Since the distance you 
> measure depends on the diameter of the wheel to some extent (what 
> bumps it
> sees) it is good to standardize on a single size. NYNJTC has at least 
> 5 wheels and perhaps a few personal ones as well.
> 
> -----Original Message-----
> From: at-l-bounces at backcountry.net
> [mailto:at-l-bounces at backcountry.net] On Behalf Of Jim Lynch
> Sent: Tuesday, February 13, 2007 5:20 PM
> To: at-l at mailman.backcountry.net
> Subject: [at-l] Measuring Wheels (More)
> 
> Thanks for all the input.
>    I've found some more information.
>    Wheels are made by the following companies (Keson, Digiroller, 
> Redington, Meter-Man, Rolatape, Lufkin, Hanson, Berger, MeasureMark, 
> Calculated Industries).  There may be others, but these are what I've 
> found so far.
> There are several variables.  One is wheel size which varies from 6" 
> to 25"
> diameter.  While something larger than 6" would seem to be better for 
> trail measuring, I don't know how large is 'good enough'.  Wheels come 
> in either spoke or solid types.  The literature indicates that solid 
> wheels are better for working in brush (any experience with these 
> wheels?).  And the counters reset at 1000 (feet or meters), up to 
> 100,000 (any thoughts about what is the best for typical trail work?).  
> And the readouts can be either analog or digital.  I guess the analog 
> simply counts the rotations of the wheel.
> Not
> sure what the digital units do for you.
>    And of course I'd just as soon not spend an arm and a leg either. 
> :)
> 
> 
> 
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