Quote Originally Posted by KilroyWasHere View Post
are miles on the AT measured by a measuring wheel these days or by GPS? More specifically, is all the walking on switchbacks part of that measurement or are switchbacks measured as the crow flies?
Without knowing which specific trail or dataset you're referring to, I don't think anyone can answer your first question. But I can talk a little bit about how, for example, your phone app tracks your hike, which is similar to how a trail would be mapped via GPS (spoiler alert: whether wheel or GPS, "as the crow flies" from end to end has never been how it's done).

Your phone app starts by getting a GPS fix, the more accurate the better. From GPS observations, the device makes an initial series of calculations to determine your latitude, longitude, and elevation, and the accuracy of those calculations.

Here are some sample data from around Max Patch:

maxpatch.png

In that data you'll see a timestamp, the primary calculations based on GPS observation (grey), and also secondary calculations (green) based on the primary calculation results.

There is no bearing or speed calculation in the first row, because the secondary calculations in green are based on comparing two adjacent rows of data. If I hike for two hours, and store a trackpoint e.g. every 5 seconds, I will record 1440 trackpoints, and can calculate 1439 sets of values between adjacent points (point1 -> point2, point2 -> point3, point3 -> point4 . . . . point1439 -> point1440).

The Haversine formula can be used to solve for horizontal distance between two trackpoints. Vertical distance is the difference between two <ele> values. The hypotenuse of the resulting right triangle is the distance hiked. (Add up the values of those 1439 hypotenuses, and you have total distance hiked.) Grade, speed, and compass bearing between each pair of points can be calculated as well.

If you have concerns about the accuracy of the initial observations (for example, if your device is in the forest, inside your pack brain), the same calculations can be performed between non-adjacent points (point1 -> point3, point1 -> point4, etc.) can be performed to check and smooth the resulting plot. (I can't really hike at 15mph.)
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The screenshots below show three different datasets plotted. The white dashed line is Google's plot of the AT (which, as you can see, is not always accurate). The red line is the ca. 2008 dataset from TNLandforms. The blue line is what my phone recorded on a hike in 2016.

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