...
- When the geopotential height of 700 hPa goes below 2500 m, it is reported 1000 m heigher than actual.
- When there is are significant levels level below 1000 hPa level (i.e. pressure > 1000), the pressure is reported 1000 hPa lowerless than actual.
Those are basically how to estimate The issues are "missing top digit of data", lost in TAC encoding, so it is not easy by nature.