  This paper describes the accuracy and the errors of water vapour content
measurements in the atmosphere using optical methods, especially
starphotometer. After the general explanations of the used expressions for the
star-magnitude observations of the water vapour absorption in section 3 the
absorption model for the water vapour band will be discussed. Sections 4 and 5
give an overview on the technique to determine the model parameters both from
spectroscopic laboratory and radiosonde observation data. Finally, the sections
6 and 7 are dealing with the details of the errors; that means errors of
observable magnitude, of instrumental extraterrestrial magnitude, of
atmospheric extinction determination and of water vapour content determination
by radiosonde humidity measurements. The main conclusion is: Because of the
high precision of the results the optical methods for water vapour observation
are suited to validate and calibrate alternative methods (GPS, LIDAR,
MICROWAVE) which are making constant progress world-wide in these days.
