apriori xyz are valid at creation date of xml instance. these are calculated using the sopac refined model, if sufficient data points (365 for semi-annual terms, 730 for annual) exist. if not, the mean xyz values are provided. these may differ from the refXyz values below, which are also mean values, since the latter may have had outliers removed. north, east, up have minOccurs="0" since insufficient data will result in no neu model terms. refXyz values are the mean of the time series and are used to calculate modeled coords at any date.