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Grid cell lookup

DWD's radar products are grids, not point forecasts. The integration reports the value of the one grid cell that contains your location.

The grid​

All products are on DWD's "DE1200" grid: 1100 × 1200 cells of 1 km × 1 km in a polar stereographic projection, covering Germany and a band beyond its borders. Where the grid reaches beyond what DWD's radars see, its cells hold no data. Each cell holds one value per file: an amount, a rate or a class.

So the sensors describe a square of about 1 km around your location. Two places a few hundred metres apart usually read the same cell; a neighbour 1 km away can see a summer shower differently.

Finding the cell​

Your latitude and longitude are projected onto the grid, and the integration takes the cell your point falls into, not the one whose centre happens to be nearest. That matches how DWD defines the grid, so the value is the one DWD's own tools show for your location.

The integration reads the grid each file describes in its header, rather than a copy of the grid kept in the code. If DWD ever moves or resizes a grid, the integration keeps reading the cell that holds your location. A file on a grid it cannot interpret is refused instead of being read at the wrong cell.

Which locations are covered​

  • At setup, a location is accepted when it lies within a box around the grid (latitude 45.6° to 56.3°, longitude 1.4° to 18.8°). This box is larger than the grid, since no file is at hand during setup.
  • When a product is read, a location off its grid is reported in the log: The configured location is not on the grid of the … release: DWD's radar does not cover it. Set the integration up again with a location inside Germany.
  • Near the borders, the grid reaches further than the radars see. A cell without radar coverage holds "no data", and its sensors are unknown. The log then warns that there is no radar data for your location right after setup.

No data for a cell​

Even inside Germany, a cell can be without data for a while: when a radar is out of service, DWD builds the composite without it, and the cells only that radar covered are empty. The sensors are then unknown, and the log names the missing radar when it can tell. See Availability and staleness.