The Flora of the PCT: Accuracy of Reported Trail Mileages Mileages and elevation gains are always a bit problematic for trails, and shouldn't be taken as being accurate to the last decimal point. For the PCT, in particular, the trail is always changing. The trail shown on the USGS Topo maps is quite commonly different from the trail shown in the

Pacific Crest Trailbook by Schaffer et al. In general, the actual trail is usually exactly that shown in the Schaffer book, since the Topo maps are much older. However, segments change yearly, and we know of several segments in southern California that now differ from the version in the 1995 edition of the Schaffer book.Even if one has the same version of the trail, the reported mileage is hard to determine accurately. Some of the problems are:

- Measuring the mileage of a fixed trail on a map is hard to do exactly. Mileages given by computer programs such as the National Geographic
Topo!program are done by spacing points regularly along the trail, and computing the distance between those points. This works great for roads and trails without tight switchbacks, but is problematic for trails with switchbacks. Unless the regularly-spaced points happen to be exactly at the hairpin turn in a switchback, the computer version of the trail will in effect cut off those switchbacks, resulting in a shorter estimate of the actual mileage.It is impossible to give a rule of thumb as to how large this effect is, since it depends on how many tight switchbacks there are, and the computer-spacing of the points to approximate the trail.

- Mileages reported by GPS units for hiking trails are always problematic, since this is a difficult task for the following reasons:
First, there is the error of the GPS points themselves, which can be 20 feet (or more in areas with exposed rocks that produce multipath error that can't be corrected for) in any direction. A GPS-derived version of the trail has these little 20 foot zigzags going forward, back and sideways.

Second, GPS units also approximate the trail by a series of discrete points. The software tries to be smart about where to take the points, but it has trouble at times knowing when the trail has made a switchback, or whether a jag in the trail is actually due to GPS point error.

Third, since GPS unit software tries to take into account the error of the GPS points, and not allow zigzags less than the typical GPS error, that software can cut off tight switchbacks as well.

Tom's experience is that his current GPS unit almost always overestimates the mileage by about 10%. Note that the same GPS unit may measure a highway distance traveled by car essentially perfectly, since the car is traveling much faster than a hiker does, and generally travels over highways without tight switchbacks. A 20 foot zigzag for measurements spaced one second apart makes little difference to the total mileages if a car has traveled 90 feet, equal to 60 mph; that zigzag changes the measured distance by only 2 feet. But it makes a huge difference if the hiker has traveled just 3 feet.

Hence a given mileage on any trail is probably never more accurate than 5 to 10%.

Examples: if a trail segment is 1.0 mile, that is probably accurate to 0.1 mile. But if a trail segment is 10.0 miles, that is probably accurate to only 1.0 mile. The error remains at about 10% as one adds together segments. Hence "Mile 100.0" on the PCT measured from Mexico, is probably only accurate to 5 or 10 miles.

Because it is fun to know how far a given segment is from Mexico, we have given the mileage from Mexico for each segment. We've arrived at that number by using the segment lengths we've adopted for each section, and then rounding the number appropriately.

Thus segment A1 covers miles 0.0 to 2.3 from Mexico, which is probably accurate to 0.1 mile. But even if a segment at mile 100 is said to cover mile 99 to 102, with no tenths of a mile quoted, the accuracy is much higher than one mile, and that segment might actually cover mile 97 to 100.

The mileages we quote from

Topo!are those derived either from our own GPS track of a segment, or using Half Mile's GPS track. We also quote the mileage from thePCT Data Bookfrom the PCT Association if it is different from theTopo!estimate. We use our best estimate, along with some rounding, to determine the mileages we give for each segment. In general, we err on quoting a larger distance, since hikers are never unhappy if a trail turns out to be slightly shorter than advertised, but are often very unhappy if the trail turns out to be a mile or more longer than advertised.

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Copyright © 2014 by Tom Chester, RT Hawke, and Shaun Hawke.
Permission is freely granted to reproduce any or all of this page as long as credit is given to us at this source:
http://tchester.org/pct/mileage_accuracy.html
Comments and feedback: Tom Chester
Updated 4 December 2014.
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