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John, Mandatory levels below surface report height, but shouldn't have temperature, dewpoint, and winds. When gridding the data, the height values exist at those levels and will be taken in to account. TMPC, DWPC, SPED, DRCT should be -9999. so are not used. When displaying levels, even from models like the RUC which have gridpoint values below ground, it is possible to construct a 1/0 mask (or 1/-9999) to multiply the data values by so that the below ground levels are effectively removed from the display. I showed how to do this several times in the GEMPAK email archives (search on "mask" in the support archive)- http://www.unidata.ucar.edu/glimpse/gempak/1427 In the case in the URL above, I was using the surface hght grid from the eta, but you can grid level=sfc, parm=hght in aobsnd to create a surface hght grid as well. Steve Chiswell Unidata User Support On Fri, 5 Feb 1999 address@hidden wrote: > Steve, > > How does oabsnd handle levels that are below the lowest observed > level (e.g., 1000 mb at DEN)? > > Thanks, > John C. >