I computed a vertical sounding at position (40.24, -3.40) with the FNL archived data. In the text results, the PRSS and the MSLP are 923.3 and 1021 mb, respectively. Assuming a hydrostatic atmosphere the model terrain height at this point can be estimated as 977 MASL. But in the text an Estimated Surface Height of 634 MASL is also reported, which is very close to the actual terrain height of 667 MASL. What should I consider as the model terrain height ASL at this point, 977 or 634 MASL?
For some datasets we have enough information to compute the surface height more acurately than just using the difference between the PRSS and MSLP. The FNL dataset happens to be one of those datasets. When we have a surface temperature and a surface pressure, we can compute the surface height by “building down” from the first pressure level above the surface using the Hypsometric equation, which you can find in a meteorology textbook. This value should be more accurate than the first method, if available.