GPS mileage discrepancies – Updated Oct 2026

TL;DR: GPS watches, smartphones, and handheld receivers are excellent navigation tools. But the mileage they report is an estimate derived from recorded positions, not an infallible measurement. Depending on reception, terrain, recording intervals, and software, a GPS track can overestimate or underestimate distance. Map traces have limitations, too. Know your tools, and use some judgment.

For the technical background, see GPS.gov’s explanation of GPS accuracy and research on GPS distance overestimation.

Updated for Oct 2026

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Back in 2016, my friends and I did an off-trail option of a popular backpacking route.

The route went over an unnamed pass and was more technically challenging than the traditional all-trail route.

But this route was shorter, since we didn’t have to drop into a valley and climb again. We took a straight shot that hugged the Continental Divide for a bit, then dropped into a valley.

One friend brought along a mileage tracker.

At the end of the hike, he reported that we did the same mileage as if we’d taken the trail route.

That made no sense. How could that be?

Around that same time, I received a polite, friendly but firm email saying the mileage I reported from mapping software was off.

He and his friends GPS’d a route I suggested, and it was five miles longer than my mapping software reported. The email also suggested I update my reported mileage to match their GPS units.

Is that correct?
2026 UPDATE: Ten years later, the same discussion!

I revisited this article after a recent trip into canyon country.

Recent flooding made the day tougher than expected. We forded a flooded wash, scrambled up embankments, and generally worked harder than we’d anticipated.

As we sat around eating dinner, several people remarked on how difficult the day had been.

Several concluded the map was wrong.

Why?

Because their smartwatches reported more miles than the map indicated!

When I mentioned that GPS watches don’t always report accurate mileage, I received some rather skeptical looks.

But here’s the thing: A difficult day doesn’t necessarily mean a longer day.

Fording a flooded wash, scrambling up steep embankments, and navigating rough terrain take more time and energy than walking the same distance on a maintained trail. The extra effort doesn’t automatically add miles.

Could the map have underestimated our mileage? Certainly! We might also have walked farther because of detours.

But could the watches have overestimated it? Absolutely.

Without comparing the recorded tracks to the route we actually walked, I can’t say which figure was closer. That’s precisely the point.

Ten years after I originally wrote this article, GPS technology is considerably better. Yet we still tend to assume the number displayed on our wrists is correct.

Let’s revisit why.

In the racing world, people wearing GPS-enabled devices sometimes claim that professionally measured half-marathon courses of 13.1 miles are too short. Some people, less politely and less friendly but certainly more firmly, write to race directors and complain that the course is longer than advertised. Race organizers have even published explanations of why GPS watches disagree with measured courses. Why? Because their GPS-enabled devices report higher mileage than the race course.

What is going on here?

Is mapping software, professionally surveyed courses, or established maps less accurate than GPS-enabled devices? Are people hiking or running more than reported on known trails, routes, or races?

Not necessarily.

A GPS watch may be wrong. A mapped route may be wrong. Or the two may reflect different paths. What matters is understanding how each figure was calculated rather than assuming one screen settles the argument.

Note: I am not a GIS professional. This is a high-level discussion from a backpacker’s perspective, with links to more technical explanations for anyone who wants to dig deeper.
How professionals measure distance, and what has changed

Back then, a measuring wheel and traditional surveying equipment helped establish distances along roads, race courses, and trails. These tools still have their place. Surveyors also use specialized satellite receivers, correction services, and careful processing when precision matters.

The traditional distinction between recreational, mapping-grade, and survey-grade receivers remains a useful starting point, but the boundaries are less clear than they once were. Receiver capabilities, correction methods, software, experience and training, and how someone collects and processes the data matter more than a price tag or label alone.

And the fact that a professional surveyor uses satellite positioning does not mean my smartwatch produces survey-quality distance figures. 🙂
Recreational GPS devices: Better but still not perfect

Your smartwatch, handheld GPS, or smartphone can be an excellent tool for locating yourself, recording a track, and navigating unfamiliar country. I use digital mapping tools extensively myself.

Since I first wrote this article in 2016, many devices now support multiple satellite constellations, and some use multiple frequencies. These improvements can help, especially where signals bounce off canyon walls or other obstacles. Garmin has a useful explanation of multi-band GNSS technology.

But better positioning doesn’t mean perfect positioning. GPS.gov outlines the factors that affect accuracy, including blocked or reflected signals and receiver design.

In canyon country, those limitations are not merely theoretical. And the canyon walls don’t care how much you spent at REI.
So why doesn’t my GPS always give accurate distances traveled?

A short, bald backpacker with a phone will not get the same results as Jane GIS using specialized equipment and carefully processed data.

On a technological level? Let’s go down the rabbit hole a bit…

According to research scientist Peter Ranacher, a study published in the International Journal of Geographical Information Science states that

The statistics are just not in favor of GPS when it’s being used to collect distances.

This thread from Views from The Top offers a helpful discussion of GPS technology from a backpacker’s perspective.

Consumer GPS devices also rely on software algorithms, including heuristic software, to process the data they receive. Some algorithms may use fuzzy-logic techniques, though the precise methods vary by device. People much more intelligent than I can explain it better and in more detail.

In essence, the receiver and its software must decide how to interpret imperfect positional data, including when to record a point, smooth a track, or treat you as stationary. Remember, Jane GIS uses her experience and education, along with expensive equipment and software, to produce accurate results for your race course, property boundaries, or hiking maps.

The user DougPaul on the Views from the Top forum explained some of these trade-offs in an older discussion:

There is no 100% reliable way for a [Recreational Grade] GPS to differentiate between slow movement in a small area and being stationary. As such, the manufacturer’s have to guess at some heuristics (ad hoc algorithms) and thresholds which result in different characteristics–the Magellans in question appear to have a higher threshold for separating the two states than Garmin. This may result is a prettier, but less accurate track which skips zones of slow movement (ie data is thrown away). In practice, the fuzz balls may not be pretty, but they do tell the knowledgeable user what happened.

There used to be a similar issue with what to do if the GPS lost satellite signals. Some (Magellans, IIRC) would extrapolate (ie guess) the track while the Garmins would declare a loss of signal. However, the extrapolated track was often wildly inaccurate… This also led some users to erroneously think that these GPSes were more sensitive than the Garmins.

That older discussion describes specific devices, not every modern receiver.

But the basic problem remains: your GPS may appear to track movement while you sit still, and software must decide what to do with missing or noisy positions. Some devices smooth tracks or estimate movement between recorded points. Challenging terrain and long recording sessions create more opportunities for errors to accumulate, though the resulting mileage isn’t always too high. Even something as simple as where you wear or carry the GPS-enabled device can affect data accuracy.

If your device places you a few feet to one side and then a few feet to the other, those little errors can add distance. It may even make it look like you moved while you were sitting still. Research by Peter Ranacher and colleagues explains why GPS positional errors can inflate calculated distance.

But GPS can also underestimate mileage. A device that loses reception, records too few points, or smooths a winding trail into a straighter line can miss distance. Recording settings and software algorithms also affect the result.

A GPS receiver doesn’t simply measure every step you take. It measures positions, and software turns those positions into an estimated distance.

The more complicated the terrain and reception, the more likely the results are to be skewed.

What about running races?

Runners sometimes finish a certified half marathon or marathon with a watch showing more than the official course distance. That doesn’t automatically mean the course was short or measured incorrectly. Certified road courses follow a carefully measured shortest possible route, while runners may take a different line. GPS errors add another variable. USA Track & Field discusses why GPS watches and certified course distances may disagree.

An article from Runner’s World also sums it up rather well:

Because most devices collect positions at a high frequency—though this can vary based on the brand as well as the current position of GPS satellites—Ranacher and his coauthors found it more likely that distance will be overestimated when using GPS technology.

For their study, published in the International Journal of Geographical Information Science, a participant walked on a premeasured track with a GPS device that collected positions every second. For several of the trials, the distance was overestimated by as much as 10 percent, Ranacher said.

(emphasis mine)

If GPS measurements can differ from a measured road race or simple track, imagine the additional challenges of hiking through trees, mountains, canyons, and lots of twists and turns. The result may be higher or lower than the actual distance.

In all cases, the watch’s number and the mapped route may not represent precisely the same path.
So what good is the GPS tool I bought?

Your GPS is a great tool for figuring out approximately where you are in the mountains, recording a route for later use, and reviewing elevation and pace. I use these tools myself. For many outings, the reported mileage will be close enough for practical purposes.

Just don’t assume the distance on the screen is automatically correct, especially on a long, complicated route.
Sorry. I know I walked further than what the maps, guides, or race director said!!!!

Besides putting faith in electronic gadgets, we sometimes remember a difficult day as longer than it was.

That wind blew just a bit harder. The climbs are just a bit steeper. And the mileage walked a tad longer.

At the end of the day, we are tired, so the miles do seem longer.

And people seem to forget that ELEVATION GAIN and ELEVATION LOSS will significantly impact hiking time. A steep climb or descent takes longer and taxes the body more. Needless to say, the rough terrain will make a hike longer and more arduous, too.

For me, I tend to think in terms of hours hiked versus miles hiked.

Experience and the ability to gauge trail vs. non-trail, elevation gain, terrain, and attempted mileage give me a reasonable sense of what I can accomplish in a given time.
So how do I get reasonably accurate figures for distances hiked?

Many recreational hiking maps already list distances, of course.

Official park, government, and trail-organization resources may also publish distances.

However, in obscure areas or when hiking off-trail, many mapping resources let you trace a route and show mileage.

But a trace is not entirely accurate, either. After floods or other landscape changes, the route you actually walk may differ from the one you originally drew.

A trace may not capture every nook and cranny of the trail. And few people follow a hiking trail 100% as laid down on the map.

Here is a rule of thumb I learned years ago to get a reasonably accurate account of distances hiked or planned: Trace your route in mapping software, then add a 10% fudge factor. This is merely a fudge factor, not a universal GPS correction or a claim that every mapped route is ten percent short.

We used to trace routes with a compass lanyard, but mapping software is easier!

In the example above, the map trace was 19.x miles. Add 10%? You get 21.X miles. Round up slightly to 22 miles. Voila!

Is this method accurate? Not 100%, of course. For planning purposes, I find this approach more useful than treating a consumer GPS mileage figure as gospel, given the quirks discussed above. And some of the people who explain these quirks are GIS professionals and research scientists. Or people who are “wicked smaht”, as I would say growing up.

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So there you go. Recreational GPS units are fantastic technology.

They have uses for backpackers, and as digital map data and tools have become more accessible, they’ve made planning and sharing routes easier than ever.

But like any tool, use them wisely and within their limits: a hard day doesn’t necessarily mean a longer-distance day, and a precise-looking number isn’t necessarily accurate.