Phone GPS Accuracy Explained: How Many Metres Off?

Under open sky, a modern phone usually places you within a few metres. Between tall buildings, indoors or underground, the error can grow to tens or even hundreds of metres. This guide explains how a phone works out its position, what the ±m accuracy radius really means, and how to get a fix you can rely on for work photos and attendance.

Where a phone’s location comes from

“GPS” is shorthand. In practice your phone blends several sources.

GNSS, not just GPS

GNSS (Global Navigation Satellite System) is the umbrella term for satellite constellations: the US GPS, Russia’s GLONASS, Europe’s Galileo, China’s BeiDou and regional systems such as Japan’s QZSS. Most current phones listen to several at once. The more satellites a phone can see, spread across the sky, the steadier the position. Some phones also support dual-frequency reception (commonly L1 and L5), which helps reject reflected signals in cities.

A-GPS: a faster first fix

To compute a position, the phone needs to know where the satellites are. Downloading that orbital data from the satellites themselves is slow. Assisted GPS (A-GPS) fetches it over the mobile network or internet instead, cutting the time to first fix from tens of seconds to a few. Without data, GPS still works; it just starts more slowly.

Wi-Fi, cell towers and sensors

The operating system also uses nearby Wi-Fi networks, mobile cell towers and motion sensors. The result is a fused location. It gives you a position indoors, but a cell-tower estimate can be a long way off.

What the ±m accuracy radius means

When an app shows “±8 m”, that is an estimated accuracy radius: a circle around the displayed point within which your true position probably lies. On Android, this figure is documented as roughly a 68% confidence level, so your real position will sometimes fall outside the circle.

Radius shown	Typical situation	Fit for work photos
Under 10 m	Outdoors, clear sky, satellites locked	Good
10–30 m	Dense streets, near a window	Fine for most purposes
30–100 m	Indoors, mostly Wi-Fi based	Use with care
Over 100 m	Basement, cell towers only, just after startup	Not reliable as location evidence

These bands are practical guidance, not a standard. The official GPS.gov accuracy page notes that GPS-enabled smartphones are typically accurate to a few metres under open sky, and less so near buildings, bridges and trees.

Why your location drifts

• Urban canyons: on streets lined with towers, such as central Ho Chi Minh City or Hanoi’s Cau Giay district, buildings block much of the sky and bounce signals around. The phone may place you across the road.
• Indoors and underground: concrete and metal-coated glass weaken or block satellite signals, so the phone falls back to Wi-Fi or cell towers.
• Cold start: if the phone has not used location for a while, or has travelled far, the first fix can be well off.
• Battery saver modes: some phones reduce location updates or disable GPS when power is low.
• Metal cases and car mounts can shield the antenna.
• Mock location apps: this is not natural error but deliberate spoofing, and it needs to be detected separately.

Tips for an accurate fix when taking photos

1. Enable high-accuracy location on Android, or Precise Location for the app on iPhone.
2. Open the camera app a few seconds before shooting so it can lock onto satellites.
3. Check the ±m figure before you press the shutter. If it is above 30 m, step outside or move to a window.
4. Do not shoot straight out of a basement or lift; wait for the position to refresh.
5. Remove metal cases if you see drift regularly.
6. Add a note indoors with the room, floor or work item, because coordinates cannot tell floors apart.

GPS accuracy and geofenced attendance

With photo-based clock-in, a company usually sets a geofence: the centre of a site and an allowed radius. Set the radius too tight and staff standing in the right place get flagged as “outside” purely because of GPS error. As a rule of thumb, the allowed radius should be comfortably larger than the typical error at that location. See how to choose one in geofence attendance radius, and how to spot cheating in stop attendance fraud.

How SnapID Mark handles location accuracy

SnapID Mark prints the coordinates together with their accuracy on the photo, so viewers know how far to trust the position instead of seeing a bare string of numbers. For photo clock-in, a weak GPS fix is flagged, and photos taken outside the geofence are flagged or blocked, depending on the company’s settings. The app also signals mock locations and rooted or jailbroken devices; the server re-checks every upload, and the phone’s own checks are treated only as a signal.

One thing worth knowing: the app reads location only while you have the camera open and take a photo. It does not track location continuously in the background. More in security and features.

FAQ

Does phone GPS need an internet connection?

No. The phone receives signals directly from satellites. Internet only speeds up the first fix (A-GPS) and loads maps and addresses.

Why do two phones side by side show different positions?

Each has a different chip, antenna, software and view of the sky. A few metres of difference is normal.

Can GPS tell which floor I am on?

Not reliably. Vertical error is usually larger than horizontal error, and indoor signals are weak. Note the floor on the photo.

What accuracy is good enough for an inspection photo?

There is no single number. In practice, roughly 20–30 m or better is usually enough to show you were on the right site; add a note naming the work item for clarity.

If you want every site photo to carry clear coordinates and an honest accuracy figure, download SnapID Mark and check the ±m reading at your own sites.