Where do the time and GPS on a stamped photo actually come from, and can you trust them?
Short answer: the time usually comes from the phone's own clock, and the coordinates from whatever position the phone had at that moment: a satellite fix, a Wi-Fi estimate, or a fix from a minute ago. The street address is not measured at all. The phone's geocoding service (Apple's on iPhone, usually Google's on Android) looks it up from the coordinates, and it can be off by a building. Most stamps do not say which source they used.
No app can make a phone's clock or GPS infallible. An app can check the clock against a time server, read the accuracy figure the phone reports, notice when a fix is old or marked as mock, and print the source of each value on the stamp. Otherwise you have to do those checks yourself.
Where does the time on the stamp come from?
A phone has one clock that apps read, and the owner can set it by hand. If it is wrong, every app is wrong too. The remedy is to ask a time server over the internet. NIST, the US time authority, runs servers that answer in the Network Time Protocol (NTP), "the most commonly used Internet time protocol". The app compares the answer with the phone clock.
Two limits apply. The check needs a connection; an app that has never been online cannot have checked anything. And network time is close, not exact: NIST says the accuracy of a time message received over the network "will generally be no better than a few milliseconds". For a photo that is irrelevant; a clock a day off is not.
Time zones cause more trouble than wrong clocks. "14:05" is a different moment in Miami than in Warsaw; "14:05 -05:00" is not. A stamp without a zone or UTC offset leaves you guessing how the phone was set.
Where do the coordinates and the "±" accuracy come from?
Phones do not run on GPS alone. Android apps normally ask the fused location provider, which "manages the underlying location technologies, such as GPS and Wi-Fi", and get one position without being told which produced it. GPS.gov states that "GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky", but "their accuracy worsens near buildings, bridges, and trees". Indoors, the position often comes from Wi-Fi or cell towers and can be hundreds of meters off.
The "±" figure is the phone's own doubt. Android defines it as the "estimated horizontal accuracy radius in meters of this location at the 68th percentile confidence level". Apple's wording: "The radius of uncertainty for the location, measured in meters." So ± 5 m is a street-level fix and ± 300 m is a neighbourhood.
A fix can also be stale. Android's guidance warns that the quick "last known location" call "might be out of date, if no other clients have actively used location recently". Shoot within a second of opening the camera and the position may be from wherever the phone last looked. A stamp that flags a fix older than about 30 seconds closes this gap.
On Android the position can also be faked on purpose. Developer options include a setting to select a mock location app, which Google's developer-options page describes as providing "a fake GPS location of the device" for testing. Android marks such fixes as coming from "a test location provider", which "may not be related to the actual location of the device". An app can read that flag and warn; it cannot block the setting.
Where does the street address come from?
Nothing on the phone measures an address. The app sends coordinates to a geocoding service and gets text back. Apple says its geocoder "works with a network-based service to look up placemark information" and that "a live network connection must be present in order for a geocoding request to succeed". The result is the nearest address the service knows, which on a lot line or in a dense block can be the neighbour's. Offline there is no street address; Apple's geocoder can still name the country. The coordinates are the measurement; the address is a label.
| Value on the stamp | Where the phone gets it | How it can be wrong | What to look for |
|---|---|---|---|
| Date and time | Phone clock; some apps compare it with a time server (NTP) | Clock set by hand; no connection to check; wrong time zone | A source note ("checked against time server" / "not checked") and a UTC offset |
| Coordinates | Satellite fix, or Wi-Fi/cell estimate from the fused provider | Indoors or between buildings; stale last-known fix; mock location (Android) | The ± figure, the age of the fix, a mock-location warning |
| Accuracy (±) | The phone's own error radius in meters (Android: 68% confidence; Apple: confidence level not stated) | On Android, one time in three the true spot is outside the circle | Single digits = street level; hundreds = network estimate |
| Street address | Reverse geocoding by the phone's geocoder (Apple on iPhone, usually Google on Android) over the network | Nearest known address, not the measured one; blank when offline | A "from map" note; compare with the coordinates |
| Heading | Magnetometer (compass), true or magnetic north | Steel, vehicles and phone cases disturb the compass | True or magnetic north |
What do Solocator and Timemark say about their sources?
Solocator's iOS settings describe the stamp as "GPS position and ± accuracy", with time as "Local Date & Time/Local Date Only, UTC Date & Time" and the time zone "On/Off". Its Android page says "having a clear view of the sky often improves accuracy". Time server, satellite versus Wi-Fi, and the source of the Street Mode address: not stated.
Timemark makes a specific claim about time: "Timemark gets the photo time from Timemark's servers, not from your phone's clock." It adds: "If your phone's clock has been changed, Timemark corrects it automatically, so the photo still shows the real time", and "It gets the time zone from your phone's location." Without a connection since install or restart, "Timemark shows -- when it cannot confirm the real time." Those are Timemark's statements, not tested here. The page calls the location "GPS" but does not say how it is obtained; no accuracy figure is given.
JotCam puts the limit plainly: "Coordinates come from the device and are intended as field context. They are not survey-grade measurements." That is true of every phone app. The real difference is whether the stamp names the source of each value and flags doubts: clock disagrees with the time server, fix older than 30 seconds, mock location, address is a lookup. That is what an app like Asof prints on the stamp, and the first thing to look for when you compare timestamp apps. See also what to check in an Android timestamp camera.
How to check a stamped photo's values yourself
- Read the stamp for source notes. Look for "GPS", "from map", "UTC", "checked", or a ± figure. If there are none, assume phone clock and unknown position source.
- Open the file's EXIF data. Use the details panel on a computer or the photo info screen on the phone and compare its date and time with the stamp. Pixels and EXIF are separate records and both can be edited, so agreement is a sign, not a settlement.
- Resolve time-zone differences first. A gap of whole hours is a zone or daylight-saving difference. A gap of minutes or days is a clock problem.
- Paste the coordinates into a map. Check that the pin lands on the place in the picture within the ± radius. If the printed address is the building next door, that is a geocoding result; the coordinates decide.
- Read the accuracy and any warnings. ± 3 m under open sky is normal; ± 500 m means a network estimate. A mock-location or stale-fix warning means the coordinates should not be relied on. If stamp and file disagree, ask for the original file.
Questions people ask
Is the time on a stamped photo the phone's clock or real time?
Unless the app says otherwise, it is the phone's clock, which the owner can change by hand. Some apps compare it with a time server; Timemark says it takes the time from its own servers. That needs a connection. Offline, an app falls back to the clock or shows nothing.
What does the ± number next to the coordinates mean?
The phone's own estimate of its error: on Android the radius within which the true position falls with 68% probability, on Apple the radius of uncertainty. ± 5 m is a street-level fix. ± 300 m usually means Wi-Fi or cell towers, not satellites.
Can someone fake the GPS on a stamped photo?
On Android, a Developer options setting lets a separate app feed the phone a fake GPS position. Android marks such fixes as mock locations, so a camera app can flag them but cannot stop them. A stamp that flags a mock location beats one that prints coordinates with no comment.