Pseudo code (IDL format) for mapping from fine to coarser pressure grid

Function make_map, pfine, lev_coarse
Pfine = array of fine pressure levels
Lev_coarse = array of indices for coarse pressure levels
Nfine = # fine levels
Ncoarse  = # coarse levels
 
map = dblarr(nfine,ncoarse)

for i = 0,ncoarse-2 do begin
    for j = lev_coarse(i),lev_coarse(i+1)-1 do begin
        xdelta_p = alog(pfine(lev_coarse(i+1))) -  alog(pfine(lev_coarse(i)))
        xcoeff = 1. - (alog(pfine(j)) -  alog(pfine(lev_coarse(i))))/xdelta_p
        map(j,i) = xcoeff
    endfor
endfor

for i = 1,ncoarse-1 do begin
    for j = lev_coarse(i-1)+1,lev_coarse(i) do begin
        xdelta_p = alog(pfine(lev_coarse(i))) -  alog(pfine(lev_coarse(i-1)))
        xcoeff = 1. - (-alog(pfine(j)) +  alog(pfine(lev_coarse(i))))/xdelta_p
        map(j,i) = xcoeff
    endfor
endfor

return,map