Trees weaken and scatter GPS signals, so under heavy canopy your speed reading gets jumpy and your trip distance usually comes up short. Wet leaves are the worst, which is why a forest road in summer after rain gives the shakiest readings, and the same road in winter under bare branches can be almost as good as open sky. You can’t calibrate this away with a setting, but you can mount the phone for the best sky view, get a strong fix before you enter the trees, and measure how much a regular route loses so you know what to expect.
This page is about foliage. If your route is also steep or twisty, GPS distance on hills and curves covers what slopes, switchbacks and valley walls add.
Why does GPS lose accuracy under trees? #
GPS signals are weak by the time they reach the ground, and foliage attacks them in three ways:
- Absorption. Leaves are mostly water, and water absorbs GPS radio signals. A thick, wet canopy can cut the signal enough that the phone loses some satellites completely.
- Reflection. Trunks and branches bounce signals around. A reflected signal arrives slightly late, so the phone miscalculates its distance to that satellite and puts you a few metres off.
- Fewer satellites. When part of the sky is blocked, the satellites the phone can still use may be bunched together, which makes each position less certain.
The result is a bigger accuracy radius. On an open road a phone often knows its position to within 5 metres. Under dense summer canopy that can grow to 15, 25 metres or more.
What do trees do to speed and distance? #
Speed #
GPS speed comes from the Doppler shift in the satellite signals, which holds up better than position. Still, with weak signals you’ll see the number wobble by a few km/h or mph, lag more than usual, and occasionally jump when a reflected signal comes and goes.
Distance #
Distance suffers more, and it can go either way depending on the app:
- Short, when the app throws away weak fixes. If it refuses to use a position with a 30-metre accuracy radius, the stretch covered while the signal was poor may not be counted.
- Long, when the app keeps every fix. Positions that wander side to side draw a zigzag track that’s longer than the road.
A good app leans toward short, because under-counting is honest about uncertainty while a zigzag invents distance you never travelled. Speedometer GPS takes that approach: fixes worse than ±20 m are shown on its GPS pill but not used for speed or distance, and it ignores small movements that look like drift.
How much error should you expect? #
It depends on how dense the canopy is, how wet it is, and how long you’re under it:
| Setting | Typical GPS behaviour |
|---|---|
| Open road or field | Accuracy within about 5 m; distance usually within 1 to 2% |
| Tree-lined road, open sky above | Small wobbles; distance close to open-road accuracy |
| Dense canopy, dry | Pill often FAIR; distance a few percent short |
| Dense canopy, wet summer leaves | Pill FAIR to POOR; noticeable speed jumps; distance can be well short |
| Bare deciduous forest in winter | Much better than the same forest in summer |
| Forest in a steep valley | Worst case: trees and terrain both block the sky |
Rather than trust a general number, measure your own route. That’s the practical form of “calibration” for trees.
How to measure your route’s tree error #
- Pick a route you use often that passes through a wooded stretch.
- Get a reference distance. Use your car’s odometer (after checking it against GPS on an open highway), a bike’s calibrated wheel sensor, or a route measured on a map.
- Record the route with GPS three times, ideally in different conditions: dry and wet, or summer and winter.
- Compare. If the reference says 20.0 km and GPS averages 19.4 km, that route loses about 3% under the trees.
That number tells you how much to add back on that particular route. Don’t apply it to every trip, because an open-road drive doesn’t need it and a denser forest will need more. For testing an app on open roads first, follow our guide to calibrating a GPS app for accurate distance.
How to get better GPS readings in the forest #
- Give the phone the best view of the sky. In a car, the top of the dashboard or high on the windshield. On a bike, the handlebars or stem. For hiking and running, a shoulder strap or the top of a pack beats a trouser pocket, where your body blocks half the sky.
- Get a strong fix before the trees start. Open the app in a clearing or on the open road and wait until the fix is good. In Speedometer GPS, wait for the GPS pill to show GOOD (within 10 m) or EXCELLENT (within 5 m).
- Keep precise location on. On iPhone, Precise Location must be enabled for the app. On Android, keep the location accuracy setting on.
- Keep the phone’s software up to date. Phone makers regularly update how GPS is processed.
- Check the fix quality, not just the number. When Speedometer GPS’s pill turns amber (FAIR) or red (POOR), treat the reading as approximate until it recovers. If you’re driving, leave that to a passenger or look at the saved trip afterwards.
- Use a wheel-based reference where trees are thick. A car’s odometer or a bike’s wheel sensor doesn’t care about leaves. Cyclists can set one up with our GPS vs wheel sensor guide.
Does a better phone help? #
Somewhat. Many recent phones receive signals on two GPS frequencies and from several satellite systems at once, which helps them reject reflected signals and keep enough satellites in view. They still can’t create signal that the leaves have absorbed. A dedicated GPS unit with a larger antenna does better again, which is why some hikers and mountain bikers carry one.
Which apps work best under trees? #
Look for an app that shows you the fix quality instead of hiding it. For drivers, cyclists and paddlers, Speedometer GPS displays the accuracy radius live, discards fixes worse than ±20 m, drops the speed to 0 with a SEARCHING label if the signal disappears for five seconds, and takes max speed from the smoothed reading so one bad fix on a wooded descent can’t set a false top speed. It’s free on iOS and Android and works without mobile data, which matters on forest roads with no coverage.
It isn’t the right tool for hiking distance. Its distance filter ignores steps under 5 metres between one-second fixes to keep drift out of vehicle trips, so below about 18 km/h (11 mph) it under-counts badly, and at walking pace most of your movement wouldn’t be counted. Hikers should use a dedicated hiking app or a GPS watch built for walking speeds. If you compare a wooded drive against your car, why GPS trip distance doesn’t match your odometer explains the other causes of a gap.
Frequently asked questions #
Why is my GPS distance short on wooded trails? #
Tree cover weakens satellite signals, so the phone’s position becomes less certain. Many apps skip fixes that aren’t accurate enough, which means some of the distance covered under the trees isn’t counted. Apps that keep every fix can read long instead, because the wandering positions draw a zigzag track.
Is GPS worse in summer than winter? #
Under deciduous trees, yes. Full summer foliage, especially when wet, absorbs far more signal than bare winter branches. Evergreen forest affects GPS all year.
Does rain affect GPS accuracy? #
Rain in the air has little effect on GPS signals. Wet leaves overhead are the problem, since water on and in the foliage absorbs the signal. The same road can read noticeably worse right after a storm than on a dry day.
Can I set a correction factor for tree cover? #
It’s better not to. The loss changes with canopy density, moisture and how long you’re under the trees, so one factor would over-correct some trips and under-correct others. Measure the routes you use often and adjust those individually.
How do I get accurate distance in a forest? #
Use a wheel-based reference if you can: your car’s odometer or a calibrated bike wheel sensor. If you’re relying on GPS, mount the device high with a clear view of the sky, get a strong fix before entering the trees, and choose an app that shows its accuracy so you know when to distrust it.