Line positions are one of the fundamental parameters for any line emitting plasma and is one thing that high resolution spectra can determine accurately.
In general line fitting will determine the line position along with the other parameters needed to fit the data. To determine the line position accurately some of the same considerations for determining the line flux come into play. In the case that the instrumental arf has significant structure or a slope then forward folding a model is the most accurate method to determine the line position. If the spectral data has a significant continuum slope then the line centroids from fitting or from forward folding may be systematically shifted without an accurate model for the background. The level at which the user needs to determine the accuracy of the line position determines the degree to which these concerns need to be addressed.
Once the line is fit using either RMF or without, using a simple Gaussian for example, the line center is given in the fit parameters. To determine the uncertainty for the line center,
isis> (low, high) = vconf(param_index);
where the param_index depends on the order in which the fit function is set up. A simple way to determine this is to use the isis command