| Guide | GDE-100 |
| Topic | GPS coordinates |
| Domain | Geospatial |
| Last tested | 4 November 2026 |
Short answer: To check GPS coordinates, convert them to decimal degrees, confirm latitude comes first and the hemisphere signs are right, then plot them on two independent maps. A dropped minus sign still gives a valid-looking point, so trust it only when both maps match the evidence.
What are GPS coordinates?
GPS coordinates are a pair of numbers, latitude then longitude, that fix a point on Earth against WGS84, the reference system GPS uses. They are written as decimal degrees (51.5080751, -0.0758275), as degrees, minutes and seconds (51°30'29.07"N 0°04'32.98"W) or as degrees and decimal minutes (51°30.48451'N 0°04.54965'W).
Latitude runs from -90 to 90 and longitude from -180 to 180. South and west values are negative in decimal form and carry S or W letters in other formats. Other grids describe the same point: MGRS for military and emergency work, the British National Grid for UK mapping, and what3words.
Photos store GPS as EXIF tags: three numbers each for latitude and longitude plus the hemisphere letters. Some cameras and phones also record GPSImgDirection, the compass bearing the lens faced.
| • | Checking where a photo's embedded GPS data places it |
| • | Converting MGRS, grid references or DMS from a report into a map pin |
| • | Testing a coordinate quoted in a post, a claim or a court filing |
| • | Catching swapped, truncated or sign-flipped coordinates before they reach copy |
How do you check GPS coordinates?
Six steps from a raw coordinate to a dated point confirmed on two maps.
We opened Google Maps, Nominatim, GeoConvert and Jeffrey's Image Metadata Viewer on 2 November 2026, and ExifTool and what3words on 4 November 2026.
Google Maps: Google Maps reads decimal degrees, degrees minutes seconds and degrees decimal minutes in its search box, and right-clicking a point shows its decimal coordinates. Free; no account needed (European visitors see a cookie choice first). Working on 2 November 2026.
OpenStreetMap and Nominatim: OpenStreetMap plots a coordinate on a community-built map, and its Nominatim service returns the nearest mapped address (reverse geocoding). Free; no account; its usage policy allows one request per second. Working on 2 November 2026.
GeographicLib GeoConvert: GeoConvert converts between decimal degrees, degrees minutes seconds, UTM, UPS and MGRS in one text box. Free; no account. Working on 2 November 2026; the same converter also runs offline once GeographicLib is installed.
ExifTool: ExifTool by Phil Harvey reads a photo's GPS tags and, with the -n option, prints them as signed decimal degrees. Free, open source and offline. We tested version 13.60, the current release (28 October 2026), on 4 November 2026.
Jeffrey's Image Metadata Viewer (discontinued): Jeffrey's Image Metadata Viewer was a web-based image metadata viewer. Its author, Jeffrey Friedl, has said on his blog that it was too expensive to maintain, and on 2 November 2026 it did not load in our test browser on two attempts; use ExifTool locally instead.
what3words: what3words converts a coordinate into a three-word address and back. Free in a browser, with a paid Pro plan; no account needed. Working on 4 November 2026 after we declined optional and advertising cookies in two consent dialogs. A three-word address cannot be recalculated by hand, so a reader can check it only with the same service: treat it as a pointer, not as evidence.
What we tested: On 2 November 2026 we took the GPS tags from a public Wikimedia Commons photo of the Tower of London (CC BY-SA 4.0, taken 2 February 2025) and from a generated test image, and ran them through our own code and three tools; on 4 November 2026 we ran the generated image through ExifTool 13.60 and its coordinate through what3words. Five tools worked; Jeffrey's Image Metadata Viewer did not load.
51.5080751, -0.0758275 dropped a pin inside the Tower of London, signed in and signed out. | ||
Reverse lookup returned the Tower of London, EC3N 4AB, labelled as a gift shop named White Tower. The swapped pair returned "Unable to geocode". | ||
The photo's degrees, minutes and seconds returned MGRS 30UYC0291910381, matching our Python result. | ||
On 4 November 2026, tags written to a generated image read back as 51.5080751, -0.0758275 with -n; the raw latitude had float noise after the seventh decimal place. | ||
Both gave 51.5080751, -0.0758275, matching Wikimedia Commons. | ||
On 4 November 2026, 51.5080751, -0.0758275 returned the three-word address taken.bliss.vent, labelled City of London. Two consent dialogs came first; we declined optional cookies and advertising consent. We did not check the square at street level. | ||
Did not load on two attempts. Its author has said on his blog that it was too expensive to maintain, so treat it as discontinued. |
One photo, one generated file, one location, two days: a snapshot, not a benchmark.
The Two-Map Rule: Never report a GPS coordinate until it lands on the same spot in two independent maps. Check the order and the hemisphere signs before you plot it.