Download Diffa map in ASCII Grid format
A map built from OpenStreetMap data, ready to download
Customize the map
Open this place's area in the constructor and adjust everything to your needs
Open in constructor →Map passport
What this map contains by default
This is how the map is built as-is — every setting can be changed in the constructor.
About ASCII Grid
Map of Diffa in ASCII Grid — an export of real OpenStreetMap data for the selected area.
ESRI ASCII Grid is the common text format for an ELEVATION GRID: a six-line header (NCOLS, NROWS, XLLCORNER, YLLCORNER, CELLSIZE, NODATA_VALUE) followed by space-separated numbers. Every cell value IS the elevation in METRES above sea level with one decimal — not a colour, not a shadow, not an image pixel. Heights come from the global Copernicus DEM GLO-30: about a 30 m step, vertical accuracy ~2–4 m. The grid is clipped exactly to the area you ordered, written in the coordinate system you chose, and empty cells carry NODATA_value -9999. Delivery is a ZIP holding exactly two files: map.asc and map.prj with the coordinate system (the .asc itself has nowhere to declare it). An honest caveat: over the sea GLO-30 returns exactly 0 m rather than emptiness — that is the behaviour of the elevation source, not a fault of the export. Text is bulkier than a binary raster, so the grid has a ceiling of 4 million cells: a larger area is coarsened just enough to fit, and the new step is stated as a number. It opens in a plain text editor, ArcGIS, QGIS, GRASS, GDAL, R, rasterio, Surfer and Civil 3D. This is pre-design, GIS and visualisation — not a land survey.
Which layers are in the file
This file for the selected area contains:
- Roads
- Buildings
- Water
- Greenery
- Points of interest
Coordinate systems
WGS 84 (EPSG:4326), selectable in the wizard
Units
Meters (projected in the chosen coordinate system)
| Category | GIS |
| File extension | .asc |
| Tier | Open Beta |
How to use ASCII Grid for Diffa
- Unpack the archive — keep map.asc and map.prj in the same folder and with the same name, otherwise the GIS will not find the coordinate system.
- QGIS — Layer → Add Raster Layer → pick the .asc; the grid lands in its place, because the extent and the cell size are written in the header.
- GDAL — gdalinfo map.asc shows the header and the extent, and gdal_translate converts the grid into GeoTIFF if you need a compact binary version.
Programs that open ASCII Grid
FAQ: ASCII Grid for Diffa
Is this an elevation/terrain (DEM) file?
Yes. Every cell value is the elevation in metres above sea level from Copernicus DEM GLO-30 (about a 30 m step, vertical accuracy ~2–4 m). For the same terrain as a binary file take dem-geotiff; for “X Y Z” lines or CSV take elevation-xyz.
Why is the sea zero instead of empty cells?
That is how the source behaves: Copernicus GLO-30 returns exactly 0 m over water rather than NODATA. We neither rewrite nor hide it — a zero over water means “the source said 0”, while -9999 marks places with no coverage at all.
Where is the coordinate system stored?
There is nowhere to store it inside the .asc — that is why map.prj with the system you chose travels next to it in the archive. Keep map.asc and map.prj together, like .shp and .prj in a Shapefile.
Why does a large area arrive coarsened?
A text cell costs more than a pixel, so the grid has a ceiling of 4 million cells (roughly 60×60 km at a 30 m step). A larger area is coarsened just enough to fit, and the new step in metres is stated in a warning attached to the order.
Why did I download a ZIP instead of an .asc file?
There are two files inside the archive: map.asc with the grid and map.prj with the coordinate system. The .asc format cannot store the projection inside itself, so it travels in a separate companion file — the same way as with Shapefile.
