Measuring on a map · 4 min
Measure the area of a field, roof or plot
Square metres, hectares and acres, and why a roof is bigger than it looks from above.
Measuring on a map
Trace it, read it
To measure an area on a map you trace its outline, corner by corner, and close the shape. The tool works out the area inside.
Click the corners where the edge actually turns. A field with ten corners needs ten clicks, not a rough rectangle round it.
Lesson notes
Everything from the cards above, on one page to come back to.
Trace it, read it
To measure an area on a map you trace its outline, corner by corner, and close the shape. The tool works out the area inside.
Click the corners where the edge actually turns. A field with ten corners needs ten clicks, not a rough rectangle round it.
The units people use
Small areas are square metres (m²) or square feet. A hectare is 10,000 m², a square 100 metres on each side: a little bigger than a football pitch.
An acre is about 4,047 m², so a hectare is about 2.47 acres. Farmers and estate agents in the US, UK and Ireland still use acres.
A plot 50 m × 20 m = 1,000 m² = 0.1 hectares ≈ 0.25 acres
The roof trap
A map looks straight down, so it measures a roof's footprint: the flat area it covers. A sloping roof's real surface is bigger, because it is tilted.
For a roof pitched at 30°, the surface is about 15% more than the footprint. Roofers and solar installers work from the sloped figure.
Footprint 100 m², pitch 30°: 100 ÷ cos(30°) ≈ 115 m² of roof
Questions
How many square metres are in a hectare?
10,000. A hectare is a square 100 metres on each side, about 2.47 acres.
Is a map's area measurement accurate?
MapGO works the area out on the curved Earth, so it does not depend on the map's projection. The accuracy comes from how carefully you trace the corners.