NavList:

I have drawn a diagram and I think that what I was trying to calculate was the intercept from the actual position. Because I know the actual position (GPS) this intercept should be zero. In other words it was to be a sanity check. However members have performed the sanity check by working out the intercept from he actual position.

I may come back to this calculation but at the moment I am more interested in using Raper's log half sine squared tables and the hav-cosine formula to calculate the altitude.  Raper does not provide natural versions of this function but there is a way around it with additional calculations. The altitude is currently 8' in error.  If I use the cosine formula the altitude agrees with  HD486 so I have probably made some numerical erros.

Thanks for your help.