I haven't been able to find how the level data is formatted in the files and hopefully somebody knows.

The files have an asc identifier and data is typically formatted as follows (from file tq2515_DSM_1M.asc):

   ncols 1000    
   nrows 1000    
   xllcorner    525000    
   yllcorner    115000    
   cellsize     1    
   NODATA_value -9999
   33.182 33.232 33.322 33.392   etc (1000 values ?)
       V  (1000 rows)

My best assumption is they contain ground level data at 1m intervals for a 1000m grid. For instance in the file above the bottom left OS grid reference is 525000, 115000 and then the level data for each point is listed from left to right and top to bottom ie data starts at GR 525000, 116000 and finishes at 526000, 115000. There are a thousand lines of data in the file and (I assume) 1000 values in each line.

Can anybody confirm as I have checked the levels at each corner against the contours on the 1:25,000 OS maps and (in one instance) the level was not within the contour interval?

I am not a computer geek and only starting to learn GIS, but this seemed an ideal way to refine the ground levels from the 5m contour interval shown on OS mapping (UK).

  • I don't understand why you are attempting to review the raw ASCII file? As @IanTurton says this is a basic format readable by nearly every GIS software. Simply add it to your map.
    – Hornbydd
    May 13, 2021 at 16:10
  • Historic England has a guide historicengland.org.uk/content/docs/research/…
    – Mapperz
    May 13, 2021 at 19:57
  • Thanks, I'll check out the guide.
    – Clive
    May 14, 2021 at 16:50
  • I had looked at Historic England but didnt find the detail I needed. I am not a GIS user (a complete novice) but realised level data suitable for my purpose was “available” without an expensive commercial purchase. For some time I have appreciated that GIS is a brilliant tool but just need to invest the time to learn the basics.
    – Clive
    May 15, 2021 at 6:58

1 Answer 1


They are an Esri ASCII raster file that can be opened by most GIS software and tools.


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