Distance accuracy of GPS tracks, High Sierra Trail
Problem Statement
Assess the accuracy of distance measurement using GPS tracks by comparing measurements with reference data published by Sequoia Kings Canyon National Park.Summary
How accurate are distance measurements using hiker GPS tracking? The GPS track is a popular way to obtain the distance of a planned hike. GPS tracks are often available when trusted sources are not. However, GPS tracks are subject to errors from undersampling or satellite noise.
This is an investigation of distance measurements from GPS tracks as compared with reference distances published at the Sequoia Kings (SEKI) Canyon National Park website, The mileages from SEKI are not necessarilty error free but derive from the best external and authoritative source for this trail. Trail distances from the government land management agencies most likely come from surveys using the calibrated measuring wheel, still the best known instrument for trail distance measurement.
In this analysis we take a random sample of three GPS tracks for each trail segment listed at the High Sierra Trail website. We measure the distance of each GPS track using software The statistical analysis will check for an overall bias relative to the government data and for variability between GPS track measurements.
This research found that GPS tracks are remarkably consistent when sampled from the same trail segment. However, the average distance of multiple GPS tracks taken from the same trail segment usually differs from the government data. Sometimes, the difference between the average measured and reference distance can be over a mile.
Box plots of measured distance compared with reference distance
The root mean squared distance measurement error, when compared to reference distance was 1.3 miles. However, the track measurements were reliable. In other words, when repeated for the same trail segment, the GPS track measurements gave consistent results to within 0.3 miles (standard deviation). The paper conjectures on how there can be such a difference between these two types of errors. I now think it is the same as the reasons why many of us hikers have seen different distances reported by trail markers, maps, trail guides and other trusted sources. See this Washington Trails Association article for a good discussion with the problems with trail distance measurement.
Read the Details
- Distance accuracy of GPS tracks, High Sierra Trail .
- Technical paper, rendered with R Markdown, contains discussion, data analysis approach, results, conclusions and embedded R code.
- Distance accuracy data set.
- Distance measurements and reference distances used in the research, 51 observations on 7 variables, .csv format.