Area units, converted
| Unit | In square metres | A way to picture it |
|---|---|---|
| 1 square metre (m²) | 1 m² | A large doormat. |
| 1 acre | 4,046.86 m² | Historically the land one ox team could plough in a day. |
| 1 hectare (ha) | 10,000 m² | A square 100 m on each side. About 2.47 acres. |
| 1 square kilometre (km²) | 1,000,000 m² | 100 hectares, or 247.1 acres. |
| 1 square mile | 2,589,988 m² | 640 acres, or 2.59 km². |
Hectares are the metric standard for land almost everywhere; acres persist in the United States, the United Kingdom and much of the Commonwealth. The convenient bridge between them is that a hectare is a touch under two and a half acres.
How the number is calculated
The area is computed geodesically — on the curved surface of the Earth rather than on the flat image on your screen. This matters because a polygon on a sphere does not obey the geometry you learned for polygons on paper. Its area depends on its angles, not only on the lengths of its sides, and a triangle drawn large enough can have three right angles.
At the scale of a garden or a field, a flat-earth calculation and a geodesic one agree to well within the error of your clicking. At the scale of a county they do not — and the failure mode is far more dramatic than most people expect.
Measure the area in Web Mercator’s own coordinates — that is, treat the map image as if it were graph paper — and the answer is wrong by a factor of 1/cos²(latitude). At the equator that factor is 1, so nothing goes wrong. In Cairo it is 1.3. In Ljubljana it is 2.1, in London 2.6, in Oslo 4.0, and in Reykjavík 5.3. A field in Iceland measured off the flat image would come out more than five times its real size, and the number would look perfectly reasonable.
This tool never does that. The area is computed from the coordinates on the globe rather than from pixels on the image, so it is unaffected by your zoom level, unaffected by the projection, and equally correct in Nairobi and in Svalbard.
How many points a curved boundary needs
Every polygon you draw is made of straight sides, so a rounded boundary — a pond, a bend in a river, a cul-de-sac — gets approximated by chords that cut across the curve. The enclosed area is therefore always too small, never too large. How much too small depends entirely on your click count.
For the worst realistic case, a perfectly circular plot:
| Points placed | You capture | Short by |
|---|---|---|
| 4 | 63.7% | 36.3% |
| 6 | 82.7% | 17.3% |
| 8 | 90.0% | 10.0% |
| 12 | 95.5% | 4.5% |
| 20 | 98.4% | 1.6% |
| 36 | 99.5% | 0.5% |
Four points around a circular field lose more than a third of it, which is the kind of error that changes a decision. Twelve points get you within 4.5%, twenty within 1.6%, thirty-six within half a percent. Past about twenty the returns are small enough that your own aim is the larger source of error.
Real boundaries are mostly straight lines with a few curved stretches, so this is a worst case rather than a typical one. The practical rule it gives you is simple: corners need one point each, and curves need roughly one point per 20° of turn.
Getting a clean trace
- Zoom in before you start. The tool records exactly where you clicked, so the precision of the result is the precision of your pointer — and a pixel at world zoom is hundreds of metres.
- Trace in one consistent direction. Doubling back on yourself creates a self-intersecting outline, and the area enclosed by a figure of eight is not a meaningful number.
- Do not close the shape yourself. The first and last points are joined automatically; clicking back on your starting point just adds a zero-length side.
Is the answer plausible?
Area is the measurement people have the worst intuition for, because it grows as the square of everything. Doubling the width of a plot quadruples it, which makes eyeballed guesses unreliable in a way that guessed distances never are. A few anchors to check a result against:
- A tennis court, doubles lines included, is about 260 m². A basketball court is 420 m².
- A full-size football pitch is roughly 7,100 m², or 0.71 hectares — a little under two acres. An American football field with its end zones is smaller, about 5,350 m².
- A hectare is a 100 m square: about one and a half football pitches, or a generous city block.
- Central Park is 3.4 km² (341 hectares, 843 acres). Monaco is 2.1 km². Manhattan is 59 km².
If you have traced what you believe is a suburban back garden and the tool reports two hectares, you have almost certainly clicked at the wrong zoom level or left a stray point behind. Checking the result against something you can picture catches that immediately.
Where this is the wrong tool
The arithmetic is exact for the shape you drew, but the shape you drew is only as good as the imagery under it — and satellite imagery is orthorectified to a tolerance, not to the centimetre. For a boundary dispute, a land registry filing, a sale, or anything where the number becomes contractual, this is a useful sanity check before you commission a licensed surveyor, not a replacement for one.