ASCII Raster format cellsize

Question

I have a particular .asc file, with elevation data in it.
I am wondering if somebody knows, what is the unit for the "cellsize" variable in the header of the .asc file?

Here is how the header looks like:

ncols   236
nrows   121
xllcorner   2.022083333
yllcorner   41.32875
cellsize    0.000833333
NODATA_value    -32768

What unit is the "0.000833333"?
A meter, or an arc?

I found a couple of pages on internet which explain the content of the .asc files headers, but still none explain what is the length unit of the "cellsize"?

Does anyone know?

Steven Kay · Answer

Almost certainly not meters. 0.00083m is tiny, less than 1mm.

At the equator, 0.00083 degrees is around 90m, which sounds about right for a lot of the more usual DEMs (such as SRTM, GTOPO). This varies according to latitude.

1/0.00083 is roughly 1200 pixels per degree
divide again by 60 - 1200/60 = 20 pixels per arc minute
divide again by 60 - 20/60 = 1 pixel every 3 arc seconds (.33 pixels per arc second)

So it's probably a 3 arc-second DEM (like the original SRTM). You should probably check with your source for what projection they're using too

decvalts · Answer

Just to clarify the original answer - there are no standard units that cellsize has to conform to. The units are only required to be same as those for the coordinate system used to define the lower left corner (xllcorner/yllcorner). Meters would be 'valid' units for UTM, for example.

https://www.loc.gov/preservation/digital/formats/fdd/fdd000421.shtml

ASCII Raster format cellsize

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