Calculating mileage for a hiking guide is no walk in the park. What sounds like a simple process actually took the collaboration of many individuals and agencies, so the mileage can be as accurate as possible.
In the book I talked about the shocking difficulty of accurately measuring trail mileage. Civilian GPS devices are far from accurate. Each reading can be 20'-30' from its actual position. One reading may be 20' to your left and the next reading could be 30' to your right. When the device adds these points it results in lots of additional mileage. Measuring with a wheel is one of the most accurate ways but the fact that every time the wheel climbs a rock and descends the other side it adds to its distance makes it less than accurate. Simply measuring using existing maps is not accurate as there are many mistakes on the various maps available, many of them due to changes in trails and also due to the fact that most of the trail maps are based on a larger scale than is needed for real detailed accuracy. Using satellite imagery can be helpful but a majority of the trails head through the woods and so can't be followed easily with satellite imagery.
What is needed is to do a proper survey of the park and its trails, but this would take years of work with many people and very expensive specialized equipment. With the national park funding as it is, this is not likely to ever happen. So, what we did in the end was we gathered data from various companies that track GPS data for outdoor users. While GPS is not accurate enough to use as a measurement tool when using a single device, we were able to gather years of data from a large pool of users. This enabled us to find the middle point between their tracks and compare it with our existing maps. Using this data we were able to identify places where the trail had been moved or mis-recorded and then adjust it accordingly in our maps. It took over a month of work with this data by GIS professionals to develop a new digital trails layer of the park that is the most accurate we've ever had. It may not be perfect, but it is as accurate as we currently know how to get it.
This new trails data was then used to calculate the distances to waypoints and destinations. You are likely to find that these distances will be different than some of the signs you'll see in the park and also different from some of the maps and even from your own GPS device, but we can say with some confidence that our numbers are the most accurate to date. We expect this information to eventually be used to update maps, signs and everything else over the coming years. As technology improves, we will continue to tweak our data and release new versions of the hiking guide with the very latest mileage and elevation figures. If this has happened you'll find a record of it under the "Updates" section of this website.