ESRI ASCII Grid (.asc)
Open BetaESRI ASCII Grid is a text raster format with a six-line header (NCOLS, NROWS, XLLCORNER, YLLCORNER, CELLSIZE, NODATA_VALUE) and a grid of space-separated numbers. It opens in any text editor and is read by ArcGIS, QGIS, GRASS, GDAL, R, rasterio. To be honest: our export first renders OSM vector layers to an intermediate GeoTIFF, then converts it to .asc via the GDAL AAIGrid driver. So the output is a rasterized map of the area as a text grid, not an elevation/terrain model. For actual relief use HGT or an external DEM. The file is delivered as a ZIP archive: the .asc itself plus a .prj projection (CRS) file so GIS software knows the coordinate system — like a Shapefile.
Key Features
About ASCII Grid Format
ESRI ASCII Grid is the simplest of the raster formats: six header lines (the number of columns and rows, the coordinates of the lower left corner, the cell size, the NODATA value) and then a grid of numbers separated by spaces. The file reads in any text editor and is parsed by five lines of code in any language, which is why it is still used in academic pipelines and for long-term storage.
The format is chosen when what matters is not efficiency but portability and transparency: a supplement to an article, a dataset in a repository, an input grid for GRASS, R or rasterio, an exchange with a colleague who has no GIS. Everything in the file is visible to the eye — nothing is hidden in a binary structure.
The output is a ZIP archive with two files: map.asc with the grid of values and map.prj with the description of the coordinate system. It is done that way because the .asc itself does not store the projection, and without it a GIS will not put the grid in its place. The cells hold the rasterized map of the selected area — the OSM vector layers are first drawn into an intermediate GeoTIFF, from which the first band is taken; empty cells get the value -9999.
What Program Opens ASCII Grid Files
The following programs and applications can open, edit, and work with ESRI ASCII Grid (.asc) (.asc) files exported from OSM2CDR:
Format specifications
- Open standard
- 2D only
- Lossless (no quality loss)
- No attributes
- Output: 1 file
- Plain-text AAIGrid: a six-line header plus a grid of cell values rasterized from the OSM vector layers via an intermediate GeoTIFF, NODATA -9999.
Who Uses ASCII Grid Maps
How to Export ASCII Grid
How to Open After Export
- 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.
- R and Python — terra::rast() or rasterio.open() read the format directly; if you like, the grid can be loaded with plain numpy, skipping the six header lines.
- A text editor — Notepad++ or any other will open the file to check the header; for large areas pick an editor that handles big files.
Popular maps in ASCII Grid
Get ESRI ASCII Grid (.asc) map exports
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