Dear Cathy,
thanks for your reply, the data is stored in a spreadsheet.
However I got an answer via another way (translated from Dutch; thanks to A.Schipper RU):
There are (at least) 2 ways:
1. if the coordinates of each measuring point are known - make a .txt file with these x, y-coordinates and the measured values (one point on eacht row). make a shp file (ArcCat) with these points and convert it to a raster in ArcMap.
2. with 'ASCII to Raster' (ArcMap) you can convert an ASCII, or .txt, file directly into a raster. Make sure that each value is on the correct place in the file, like you can do with a spreadsheet.
The first 6 rows must have the following info:
ncols XXX
nrows XXX
xllcorner XXX
yllcorner XXX
cellsize XXX
nodata_value -9999
I haven't used either way yet, that's for the student to do (and learn from)!