I have FNL meteorological files, which I use to compute trajectories with HYSPLIT. I need to extract the numerical value of some fields (geopotential at 700 hPa at each i,j grid point). I ran your PROGRAM EXAMPLE which extracts the first field (precipitation), but I do not know how modify the program to extract geopotential at 700 hPa.
You need to tell the program specifically which record to read. The first record in the file (as in the program, REC=1) is an index record. The second record (IREC=2, REC=IREC) is the surface pressure (PRSS). You can figure out the record number by looking at the table just above “PROGRAM EXAMPLE” at https://www.ready.noaa.gov/fnl.php. Note that 700 hPa is level 5. Counting the number of fields starting at level 1 (PRSS=1, MSLP=2), then level 2 and so on, the 700 hPa geopotential is #36. With the index record as REC=1, the 700 hPa geopotential is REC=37. This gives the data at the first time period. After the last record (level 14 TEMP) the data for the next time period starts (index record and fields), and so on.
You can display a plot of the data from https://www.ready.noaa.gov/READYamet.php with the program “Analysis Maps”. Although there is some smoothing when the contours are created, this should be a good way to check your output of the data values.