Observatories — how this layer is made

What you are looking at

Each point is a place where people look at the sky — a feature tagged `man_made=observatory`, `man_made=telescope` or `amenity=planetarium` in OpenStreetMap. 9,355 of them worldwide, drawn as a dot coloured by which of the three it is and sized by the footprint of the site.

Click any feature to see what OpenStreetMap records about it — the institution that runs 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 feature links to the OpenStreetMap object it came from.

A count of features is not a count of observatories

This is the most important thing to understand about this layer, and it is a consequence of what the three tags mean:

Observatory — 3,353 features. The site or the building.
Telescope — 5,116 features. A single instrument: one optical tube, or one radio dish.
Planetarium — 916 features. A dome that projects the sky for an audience. A public building, not an instrument at all.

A large radio array is therefore mapped dish by dish. The Very Large Array in New Mexico is twenty-seven features. MeerKAT in South Africa is sixty-four. A university dome with one telescope inside it may be a single feature — or two, if somebody tagged both the building and the instrument.

The visible result is that the densest view of this layer anywhere on earth is the Karoo in South Africa, where 556 features sit in a single screen around MeerKAT and the Square Kilometre Array site. That is one observatory. The Atacama is second, at 233. No city of domes and planetariums comes close.

Colouring the instruments apart from the sites is the best this map can do about it: a swarm of light blue is one array, a scatter of dark blue is many separate observatories.

The one layer here that is not a map of Europe

Most HueMaps layers built from OpenStreetMap lean heavily towards western Europe, and the honest reason is usually a mixture of what exists and who maps. This layer is the exception, and it is worth stating plainly.

Counted crudely — features inside a bounding box, per million people — Germany has about 11 and India 0.12, a ratio of roughly 90 to 1. On the ambulance stations layer that same ratio is over 800, and on the windmills layer around 330. In absolute numbers China holds more features than the United States (1,524 against 1,480), which is true of almost nothing else on this site.

Astronomy is sited where the sky is dark, dry, high and stable — the Atacama, Hawaii, the Canaries, the Karoo, the Tibetan plateau — and that geography has little to do with where OpenStreetMap's contributors live. Where a subject really is global, this map shows it. That is worth knowing when you look at the layers where it does not.

Mapped, not modelled

Each feature is an object a mapper drew, from survey, local knowledge or aerial imagery; nothing here is modelled or estimated, and no astronomical register has been merged in.

What is in and what is out

`man_made=telescope` includes instruments that are decommissioned or are museum pieces, and OpenStreetMap carries no reliable tag separating a working telescope from a retired one. They are all here. Historic observatories that are now only museums are here too, since they still carry the tag.

Amateur observatories in gardens are here where someone mapped them, which will be inconsistently. Satellite ground stations and communications dishes are sometimes tagged `man_made=telescope` by mappers who read the tag loosely; a few are on this map.

Size

Each symbol is sized by the footprint of the feature, in square metres measured on the globe. Symbol area is proportional to that (radius to its square root).

This layer has the widest size range on HueMaps. About half are mapped as an outline, and among those the median footprint is 101 m² — one dome — while the 95th percentile is 11,167 m² and the largest single feature is the Australian Square Kilometre Array Pathfinder's reserve at 25.4 square kilometres. Three orders of magnitude separate the ends, so the size scale is set differently at far and close zoom to keep both readable.

The other half are recorded as a bare point with no shape, and are drawn at a nominal small size rather than at the minimum.

58% carry a name. Many single dishes inside an array do not, which is why the operator row in the details panel is often the only thing distinguishing one from its neighbours.

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 an instrument dismantled some time ago may still be mapped until a mapper updates it.