Optimal measurement of ice-sheet deformation from surface-marker arrays

Surface strain rate is best observed by fitting a strain-rate ellipsoid to the measured movement of a stake network,or other collection of surface features, using a least-squares procedure. Error of the resulting fit varies as (LAt/7T)~ , where L is the stake separation, At is the time period between initial and final stake survey, and n is the number of stakes in the network. This relation suggests that if n is sufficiently high, the traditional practice of re-visiting stake-network sites on successive field seasons may be replaced by a less costly single-year operation. A demonstration using Ross Ice Shelf data shows that reasonably accurate measurements can be obtained from 12 stakes after only 4 days of deformation. The least-squares procedure may also aid airborne photogrametric surveys in that reducing the time interval between survey and re-survey could permit better surface-feature recognition.