Data & licence
Elevation data
The land in the full-screen sky view is real land: the hills that hide the sun, the ridge a body has to clear before it is up, the slopes the moonlight falls across. The heights they are drawn from are Copernicus WorldDEM-30 — a grid of posts about 30 m apart, from the European Union's Earth observation programme.
The dataset is COP-DEM_GLO-30-DGED/2024_1, under the licence
COP-DEM-GLO-30-F, Global 30m Full, Free & Open. That licence
grants reproduction, distribution, communication to the public and
adaptation, free of charge and without limit in time. What it asks for in
return is that the data are named, that it is said what was changed, and
that two sentences are reproduced word for word.
The attribution, word for word
These two sentences belong to the licence, not to this page, so they stand here exactly as they are written and are not translated:
Produced using Copernicus WorldDEM-30 © DLR e.V. 2010-2014 and © Airbus Defence and Space GmbH 2014-2018 provided under COPERNICUS by the European Union and ESA; all rights reserved.
The organisations in charge of the Copernicus programme by law or by delegation do not incur any liability for any use of the Copernicus WorldDEM-30.
They are also on the app's own licence screen, under More → Licences, because the people owed a notice are the ones holding the app — a line on a website discharges nothing.
What was changed
The model was not used as delivered. Five things were done to it, and naming them is the second thing the licence asks for:
- Resampled — to five levels of detail, from the native 1 arcsecond down to 16, so that the near ground can be read finely and the far ground cheaply.
- Rounded — heights are whole metres, held as 16-bit integers, with a sentinel value where the survey recorded nothing.
- Differenced — a post is stored as the step from the post to its left rather than as its own height. Neighbours differ by a metre or two where the absolute values run to thousands, which is what makes a tile compress to about a quarter of its raw size.
- Compressed — raw deflate, no zlib header.
- Cut into tiles — each 226 × 226 posts, the last row and column shared with the neighbour so that no tile needs the one next door to be complete.
| Level | Post spacing | A tile spans | Per tile |
|---|---|---|---|
| 0 | 1″, ~30 m | 1/16° | ~25 KB |
| 1 | 2″, ~60 m | 1/8° | ~31 KB |
| 2 | 4″, ~120 m | 1/4° | ~38 KB |
| 3 | 8″, ~240 m | 1/2° | ~45 KB |
| 4 | 16″, ~480 m | 1° | ~51 KB |
A degree of source data is 3600 intervals across, and 3600 divides by 16, 8, 4, 2 and 1 into exactly 225 — so every level lands on the source grid without interpolating, and a tile is the same size whatever it covers. A location pulls around thirty tiles, roughly a megabyte, once.
It is a surface model
WorldDEM-30 describes the surface the satellite saw, not the bare earth beneath it. Forest and buildings are part of the land it hands over, so a ridge in a spruce forest stands where the treetops are and a rooftop can be the highest thing on a street. That is worth knowing when a terrain rise time comes out a minute late, and it is why the terrain time is offered beside the almanac time and never instead of it.
The other data Celeris carries have pages of their own: the star catalogue, the time zone boundaries and the points of interest.
The orbital elements the space station is drawn from have no page, because nothing about them is changed: they are read exactly as CelesTrak publishes them.