How to measure a piece of land
- Find the place. Type an address or a place name and pick it from the suggestions, or type coordinates, paste a link from Google Maps, Apple Maps or OpenStreetMap, or a Plus Code — the code Google Maps gives a plot that has no street address, which is how a lot of rural land is known. On a phone, My location takes the map to where you are standing.
- Draw the boundary. Choose Draw land and tap each corner in turn, going round the edge. Tap the first corner again to close the shape. Its area appears in the middle of it, and the length of every side along the side.
- Put the corners right. A satellite picture is never quite where the fence is, so the page switches straight to editing: drag a corner, drag the small point halfway along a side to add a corner there, or right-click a corner to remove it. Undo takes back anything.
- Take out what is not part of it. A pond, a house plot or a road that is not in the holding: choose Draw a cut-out and draw it inside the land. Its area comes off the total. A cut-out drawn outside the land, or over another cut-out, is left out and says why, rather than quietly making the total smaller than the ground.
Several fields can be drawn at once and are added up; a field drawn next to another snaps to its corners and sides, so the two share a fence instead of a sliver of overlap. If two pieces of land do overlap, the page says which ones, because the ground they share would be counted twice.
Why the area is measured on the Earth, not on the map
A map on a screen is flat, and the ground is not. Web maps use the Mercator projection, which keeps shapes looking right by stretching everything away from the equator — and area by the most: land measured flat on the map comes out about 30% too big in Delhi and more than two and a half times too big in London. Any calculator that works out the area from the map’s own pixels carries that error into its answer.
This page never measures the map. Every corner is a latitude and a longitude, every side is the shortest line between two corners over the Earth’s surface — a geodesic on the WGS84 ellipsoid, the model of the Earth GPS uses — and the area is worked out with GeographicLib, the library behind Charles Karney’s Algorithms for geodesics. Tested on ten million polygons, GeographicLib’s area was never more than 0.0013 m² out for a boundary under 10 km long: about the size of a postage stamp. The page also works out what the flat map would have said, for the land you drew, and shows the difference, so you can see what was avoided.
How close is it?
The arithmetic is exact to a postage stamp; the picture and the taps are not. Esri’s World Imagery is “one meter or better” for most of the world’s land and 30 cm across the United States and parts of Western Europe, and it is “typically within 3-5 years of currency” — so the pictures can be a few years old, and a fence or a hedge may have moved since. Where the imagery is coarse the map lets you zoom further than the pictures go, enlarging the finest level there is, so a corner can still be placed carefully.
That is what the margin is for. Say how far off each corner could be — a metre or two is honest for a boundary read off a satellite picture — and the answer becomes the range it really is, found by moving every side out and in by that much. Walking the boundary with a phone fills the margin in from the phone’s own estimate of how far out it is.
None of this is a survey. A deed, a land record or a survey plat is what says where a boundary legally runs; this page tells you how big the ground inside a line is, wherever you put the line.
Walking the boundary with a phone
Standing on the land, choose Start walking the boundary, walk to each corner and tap Add corner here. The phone listens for five seconds at each corner and averages its readings, giving more weight to the ones it is surer of, which steadies the wander of a phone’s GPS. When every corner is in, Finish the shape. The position is used on this page only; the page asks the browser for it, and the browser asks you.
Acres, hectares and the units on a land record
An acre is exactly 43,560 square feet (NIST Handbook 44; how big an acre is, drawn to scale), which with the foot at exactly 0.3048 m is 4,046.8564224 square metres; a hectare is 10,000 square metres, about 2.471 acres. The answer comes in both, and in square feet, square metres and square yards. Land records in South Asia often use units whose size depends on the state — a bigha is 14,400 sq ft in West Bengal and 9,075 in the bigha-biswa districts of Haryana — so the links under the answer carry the area to converters that show every region’s version and the document it comes from.
The same area can go on to the garden and building pages: sod or topsoil for a lawn, gravel for a drive, pavers for a patio, or the roof over a house outline — a roof measured from above is its footprint with the overhangs, which is exactly what the roof calculator starts from.
What happens to what you draw
Opening the map fetches its pictures from Esri, so Esri receives your IP address and the part of the
world on screen, as it would for any map; the page says so before the map is opened, and nothing is
fetched from Esri until then. An address you search for goes to Esri’s search to find it;
coordinates, links and Plus Codes are read in your browser. What you draw, type as corners or open
as a file is measured in your browser and not sent anywhere. The link in the address bar keeps every shape, after the #, which browsers do not send to
any server — send it to someone and they see the same map. Files saved as KML open in Google Earth,
GeoJSON in most mapping software, and both open here again. The privacy policy
has the details.