Watermills — how this layer is made

What you are looking at

Each point is a watermill — a feature tagged `man_made=watermill` in OpenStreetMap. 14,485 of them worldwide, drawn as a dot sized by the footprint of the building. At the closest zooms about a third are joined by their outline as it is mapped.

Click any mill to see what OpenStreetMap records about it — who looks after it, where a mapper noted that. Where it has a Wikipedia article, a short summary is fetched live and shown in your own language; and every mill links to the OpenStreetMap object it came from.

The building, not the waterworks

This layer is the mill itself. The things that drove it — the weir, the leat, the mill pond, the wheel — are mapped as separate objects in OpenStreetMap and are not on this map. A mill site whose building has gone but whose channel survives is usually not here at all.

Mapped, not modelled

Each mill is an object a mapper drew, from survey, local knowledge or aerial imagery; nothing here is modelled or estimated.

Where they are — and why you should not trust this map's shape

This map is very unevenly distributed, and here the unevenness is mostly not real.

Set against population, Germany has roughly 300 times as many mapped watermills as India, on the same rough count that gives windmills a ratio of about 330. The two maps look much alike. But they should not, and that is what makes this layer worth reading carefully.

The windmill as a building genuinely was a north-west European and Mediterranean development, so its concentration is a fact about the world. The watermill is not. Water mills were described by Vitruvius in the first century BC, and were built and used for two thousand years across China, Persia, India, the Islamic world, Africa and the Americas. There is no sense in which they were a European technology.

So what this map is mostly recording is not where watermills were. It is:

which mills survived, as buildings, into the age of photography and heritage protection — and in much of Europe old mills were deliberately conserved, converted or restored, while elsewhere they were replaced, rebuilt or simply fell down;
who has mapped them, which is the ordinary OpenStreetMap caveat, and a strong one for a subject that mostly interests local historians;
who uses this tag at all, since a working mill converted to a house or a restaurant may be recorded as either.

Please read an empty area as "not recorded here" rather than "none were ever built". It is a fair guess that watermills were once close to universal wherever water runs downhill and people needed to grind grain.

One colour, and why

Most layers on HueMaps colour their features by some property. This one does not, because OpenStreetMap records nothing about a watermill that could carry a legend — there is no well-used tag for the type of mill, what it milled, or whether it still turns.

Size carries the information instead.

Size

Each symbol is sized by the footprint of the mill, in square metres measured on the globe. Symbol area is proportional to that (radius to its square root), so a mill four times the area draws twice as wide.

Three in five watermills are mapped as an outline rather than a bare point, and among those the median footprint is 123 m² — about three times a windmill's, which is the honest difference between a building that houses a wheel, a millstone floor and usually a dwelling, and one that is essentially a tower. The remaining two in five are recorded as a single point with no shape; those are drawn at a nominal small size rather than at the minimum, because being mapped as a point usually says something about the mapper's time rather than about the mill.

This layer is better recorded than windmills on both counts that matter: 61% carry a name and 62% are drawn as a shape, against 43% and 53%.

A few of the largest are not watermills

The largest features here are whole industrial or utility sites whose entire grounds carry `man_made=watermill` — a water reservoir plant at Kalyani in West Bengal, an industrial flour mill in Burgundy, a set of livestock watering troughs in Kenya, a Bunge mill in Brazil, an Italian milling works. They are drawn very much larger than any real watermill.

Twenty-eight features exceed 5,000 m² against a median of 123, so this is a tail rather than a pattern; and no rule that removed them would spare a genuine mill complex, which is why they are left in and named here rather than quietly filtered.

A picture of the present

Today's map, not a history. OpenStreetMap is edited continuously; this copy is refreshed periodically. There is no year control, and a mill demolished some time ago may still be mapped until a mapper updates it.