To check on the right coordinates of OS active stations, see the link below http://gps.ordnancesurvey.co.uk/active.asp Conversions: http://gps.ordnancesurvey.co.uk/etrs89geo_natgrid.asp Base station data not 1 second resolution? Resample (interpolate using posgps). While this doesn't improve the data, it apparently makes it more evenly applied (no internal interpolation?). Unverified.