Windmills — how this layer is made

What you are looking at

Each point is a windmill — a feature tagged `man_made=windmill` in OpenStreetMap. 12,351 of them worldwide, drawn as a dot sized by the footprint of the building. At the closest zooms the largest 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.

Not wind turbines

This is the word's older meaning: a building that grinds, saws or pumps using sails turned by the wind. A modern wind turbine is a different thing, is tagged differently in OpenStreetMap, and is not on this map — there are some 463,000 of those, and they would swamp the 12,351 mills entirely.

If you came looking for wind farms, this is not the layer. One is planned.

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 the map looks like this

This map is very unevenly distributed, and unlike most of the layers here that unevenness is real.

Set against population, Germany has roughly 1,900 times as many mapped windmills as India. On almost every other layer on HueMaps a ratio like that would be a warning that you were looking at where OpenStreetMap's mappers live rather than where the things are, and we would say so. Here it is the subject itself. The windmill as a building was developed and built at scale in a particular part of the world — the Low Countries, England, northern Germany, Denmark, Iberia and the Aegean — over several centuries, and it was never the technology of choice across most of South and East Asia. The Netherlands really does have more surviving windmills than India.

So the concentration is geography and history, not a mapping artefact, and the honest thing is to draw it and tell you what it is.

That said, two ordinary caveats still apply. Mapping effort is uneven everywhere, so within the windmill-building regions the density partly reflects how thoroughly each country has been surveyed. And a mill that has lost its sails or been converted to a house may or may not still be tagged as a windmill, which varies by mapper and by country.

One colour, and why

Most layers on HueMaps colour their features by some property. This one does not.

The tempting candidate is the tag recording the type of mill — tower, smock, post, and several more. It is a genuine and interesting distinction, and there are two problems with colouring by it. It is recorded on fewer than one mill in ten, so nine of every ten dots would be grey. And the vocabulary is north-west European: it has no term that describes the windmills of La Mancha, or the vertical-axis asbad of eastern Iran, which is very probably where the technology began. A legend built from it would quietly assert that the Dutch classification is the world's.

So the layer is a single colour, and the 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.

These are the smallest features on HueMaps. Half the mills are mapped as an outline, and among those the median footprint is 39 m² — about the base of a modest tower — against a stadium's 40,000 and a golf course's 600,000. The other half are mapped as a single point with no shape recorded at all; 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.

Footprint is a fair measure here even so. A great Dutch tower mill really does cover several times the ground of a small post mill, and it is the one dimension the data records consistently.

A few of the largest are not windmills

The four largest features on this map are industrial mills — flour and rice mills in Chile, Colombia, Uzbekistan and Spain — whose entire site has been tagged as a windmill in OpenStreetMap. They are drawn very much larger than any real windmill.

There are eighteen features above 1,000 m² in a layer whose median is 14 m², so this is a handful 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.

Only about four mills in ten carry a name, so many of these dots are recorded as windmills and nothing more.