Free tool
Survey your land with your phone.
On site, walk to each corner of the plot and record your phone's GPS position: the tool averages several readings, draws the outline, calculates the area and estimates its margin of uncertainty. Everything stays in your browser: no position is sent anywhere (only the background map is loaded).
Read this first: a phone is not a surveying instrument
- According to GPS.gov (the official US government website about GPS), a smartphone is typically accurate to within about 4.9 m under open sky, and less accurate near buildings, bridges and trees. Anything that blocks the sky degrades the reading, including the terrain of a deep valley.
- On a small plot, a few metres of error at each corner changes the area a lot: the tool shows it as a range.
- The result is an estimate. It is not a topographic survey, a boundary marking or an official document, and must not be used on its own for a sale, a division or legal proceedings.
On a computer? Open this page on your phone
The survey is done on the land, with the phone's GPS: a computer usually has no GPS receiver and only gives an approximate position.
- Scan this QR code with your phone's camera, or open the link below.
- On the land, tap “Turn on GPS” and allow location access.
- Go to each corner and tap “Record this corner”.
On a computer, you can also type coordinates with the keyboard (step 3, “Add a point by its coordinates”).
Turn on GPS
Turn on your phone's location (“precise” or “high accuracy” mode) and stand under open sky. Wait for the reported accuracy to settle before recording the first corner.
Live position
- Status
- GPS off
- Reported accuracy
- —
- Latitude, longitude
- —
- Last position
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Allow location on Android (Chrome)
- Check that the phone's location is on (quick settings panel, “Location” icon), preferably in precise mode.
- In Chrome, tap the icon to the left of the address (“View site information”), then “Permissions”, and allow location.
- Or: Chrome menu ⋮, “Settings”, “Site settings”, “Location”, then choose this site.
- Also check in Android settings that Chrome has the “Location” permission with precise location.
- Reload the page and tap “Turn on GPS”.
Allow location on iPhone (Safari)
- Open Settings, then “Privacy & Security”, then “Location Services”, and check that it is on.
- In the same list, tap “Safari Websites” (or your browser) and choose “While Using the App” or “Ask Next Time”.
- Turn on “Precise Location”: without it, the site only receives an approximate position, useless for a survey.
- Go back to Safari, reload the page and tap “Turn on GPS”. Menu names may vary with the iOS version.
Measurement settings
A phone usually delivers about one position per second. The longer the measurement, the steadier the average. In “number of readings” mode, the measurement stops after 5 minutes at most.
Positions less accurate than this threshold are left out of the average (they are counted as rejected).
The area is calculated in Morocco Lambert coordinates, as on land title plans. The zone is chosen automatically from the latitude of the plot.
Record the corners
Stand exactly on the corner (on the boundary marker if there is one), hold the phone still at chest height, then tap “Record this corner” and do not move until the measurement ends. Walk round the plot in order.
Blue dot: your position, circled by the reported accuracy. Numbered markers: the recorded corners; you can drag them to correct them (the point will be marked “moved by hand”). With the keyboard: select a corner with Tab and use the “Edit” button in the list.
Recorded points
Corners are joined in list order, the last one back to the first. You can rename, reorder, correct or delete each point.
Add a point by its coordinates
To complete a survey or work without GPS: GPS coordinates (decimal degrees or degrees-minutes-seconds) or Morocco Lambert (X, Y in metres).
Estimated standard deviation of the position, in metres, per coordinate. Empty: default value from the uncertainty settings.
Uncertainty settings
Standard deviations show how the positions scatter during the measurement, but often understate the real error, because successive positions tend to be wrong in the same direction. The reported accuracy is the phone's own estimate: the W3C standard defines it at 95% confidence, Android documents it at 68%; the tool reads it at 68%, which gives the larger uncertainty.
Standard deviation per coordinate, in metres, given to points without a GPS measurement. Default: 5 m, to be adapted to how the point was obtained.
Export, continue, get it measured
The CSV gives for each corner: name, latitude, longitude, Lambert X and Y, zone, reported accuracy, number of readings, standard deviations, uncertainty used and origin. Files are created in your browser.
The corners are passed in Lambert coordinates to the Area tool (angles, bearings, comparison with the area in the deed…). Once on that page, tap “Calculate the area”. The list of markers you had entered there will be replaced.
Need an area that is legally reliable?
To buy, sell, divide or register land, the area must come from a survey tied to the official system, carried out by a surveying engineer. The quote form will include this survey.
Otherwise, the quote request will give the GPS coordinates of the centre of the plot.
Your survey is kept in this browser only; nothing is sent.
On site
A good survey in five steps.
Pick the right moment
Clear sky, charged phone, location in precise mode. Avoid measuring right against a wall or under a tree: step back if needed and make a note of it.
Let the GPS settle
Turn on GPS one or two minutes before the first corner and wait for the reported accuracy to drop below the chosen threshold.
Stand on the corner
On the boundary marker if there is one, otherwise at the exact corner of the fence or wall. Hold the phone still, flat, at chest height.
Measure long enough
30 seconds per corner is a good compromise. Do not move before the end: the tool averages the readings and calculates their spread.
Walk round in order
Record the corners following the fence, without skipping any. Check on the map that the outline looks like the plot.
Method
How the tool calculates the area and its margin of error.
One average per corner. During the chosen time, the tool receives the phone's successive positions. Those whose reported accuracy exceeds the threshold are set aside. The corner is the average of the positions kept; the tool also calculates their standard deviation, in metres, in the Lambert plane.
The area. The corners are projected into Morocco Lambert, the projection used for land title plans, with the official parameters of the EPSG registry. The area is calculated with the shoelace (Gauss) formula, exactly as in the Area tool.
The margin of uncertainty. Each corner has an uncertainty (standard deviation of the readings or accuracy reported by the phone, as you choose; by default the larger of the two). The shoelace formula gives the effect of each corner on the area: these effects are added in squares (variance propagation) to get the standard deviation of the area, then a 95% range (± 1.96 standard deviations).
- Formula: σA² = Σ gᵢᵀ Cᵢ gᵢ, where gᵢ = ½ (yᵢ₊₁ − yᵢ₋₁ ; xᵢ₋₁ − xᵢ₊₁) and Cᵢ is the covariance matrix of corner i.
- Checks: at least 3 corners, coincident corners, crossing sides, points outside Morocco, uncertainty too large compared with the plot.
- Exports: KML (Google Earth), GeoJSON, CSV (name, latitude, longitude, X, Y, accuracy…), PDF summary, printing, and transfer to the Area tool.
The range assumes errors that are independent from one corner to the next. Yet a phone's GPS errors are often shared by readings taken a few minutes apart: the range gives an order of magnitude, not a guarantee.
Frequently asked questions
Phone surveys: common questions.
How accurate is a phone?
The official US government website about GPS (GPS.gov) states that a smartphone is typically accurate to within about 4.9 m under open sky, and that accuracy worsens near buildings, bridges and trees. Real accuracy depends on the phone, the visible sky and the moment: this is why the tool averages readings, shows the reported accuracy and calculates a range.
Can the area be used for a sale or a division?
No. It is an estimate, useful as an order of magnitude (checking an advert, preparing a visit, discussing a project). For a sale, a division, a land registration file or a dispute, the area must come from a survey tied to the official system, carried out by a surveying engineer. See the boundary marking guide and the cadastral and land title plans guide.
Why is the range so wide on a small plot?
The position error of a corner does not depend on the size of the plot: a few metres of error weigh heavily on a 200 m² plot, little on a plot of several hectares. For a small plot, measuring the sides and one diagonal with a tape usually gives a better result: use the “Field measurements” mode of the Area tool.
What does “reported accuracy” mean?
It is the accuracy estimate the phone provides with each position. The W3C Geolocation standard defines it as a radius at 95% confidence; Android's documentation defines it as a radius at 68%. It is only the device's estimate: the real error can be larger, especially near obstacles.
Why keep the screen on?
When the screen goes to sleep or you switch apps, the browser may pause location and interrupt the measurement. The tool asks the browser to keep the screen on (the “Screen Wake Lock” feature) when possible; otherwise, touch the screen from time to time.
Are my positions sent anywhere?
No. Positions are processed in your browser and kept only in its local storage, until you tap “Delete my survey”. Only the background map images are downloaded from OpenStreetMap (or the satellite imagery service), as with any online map.
I am on a computer: what should I do?
Scan the QR code shown at the top of the tool with your phone to open the same page, then carry out the survey on site. On a computer, you can also type coordinates with the keyboard, in GPS or Lambert, or use the Area tool to draw the outline on the map.
References
Sources.
Documents consulted on 30 October 2026.
GPS Accuracy
Typical smartphone accuracy under open sky (about 4.9 m) and degradation near buildings, bridges and trees. Page updated on 3 March 2022.
archive.gps.gov ↗W3CGeolocation — Candidate Recommendation Snapshot, 26 March 2026
Definition of the reported accuracy (radius at 95% confidence), of the errors (permission denied, position unavailable, timeout) and of the secure-context requirement.
www.w3.org ↗Android Developersandroid.location.Location — getAccuracy()
Estimated horizontal accuracy defined as a radius at 68% confidence.
developer.android.com ↗GoogleChrome Help — Use your location
Allowing or blocking location for a site in Chrome on Android.
support.google.com ↗AppleTurn Location Services and GPS on or off on your iPhone
Per-app location permissions and the “Precise Location” setting.
support.apple.com ↗W3CScreen Wake Lock API — Working Draft, 29 September 2026
Keeping the screen on; the lock is released when the page is hidden.
www.w3.org ↗EPSG registryEPSG:26191 — Merchich / Nord Maroc
Parameters of the Lambert projection used to calculate the area (zones I to IV: EPSG 26191, 26192, 26194, 26195).
spatialreference.org ↗EPSG registryEPSG:1136 — Merchich to WGS 84 (1)
Transformation between GPS and the Moroccan system.
epsg.io ↗Need an exact measurement?
A survey you can rely on.
We measure your land with GNSS and a total station, tied to the official geodetic network, with a signed plan.
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