The measurement uncertainty will be established by analysing all sources of measurement uncertainty and by combining them to one single error bar for each measurement point. The investigation of all sources of uncertainty will be a time consuming and iterative process and will require a detailed understanding of the instrumentation that is being used as well as a careful consideration of the operational influences that contribute to the measurement uncertainty. Error bars during the early stages of GRUAN are expected to be somewhat crude. As the understanding of the instrumentation improves and as the instrumentation itself improves, uncertainty estimations are expected to improve as well. Thus, it is expected that there will be different versions of GRUAN data, which differ mostly in their uncertainty estimate.