DATA
Vertical Land Motion from GPS Networks: MIDAS Robust Trend Estimates: IGS08 Reference Frame
Please contact us if there are any discrepancies or inaccuracies found below.
DOI

Short Name
Vertical_Land_Motion_IGS08
Description
Automatic estimation of velocities from GPS coordinate time series is becoming required to cope with the exponentially increasing flood of available data, but problems detectable to the human eye are often overlooked. This motivates us to find an automatic and accurate estimator of trend that is resistant to common problems such as step discontinuities, outliers, seasonality, skewness, and heteroscedasticity. Developed here, Median Interannual Difference Adjusted for Skewness (MIDAS) is a variant of the TheilSen median trend estimator, for which the ordinary version is the median of slopes vij = (xj  xi)/(titi) computed between all data pairs i > j. For normally distributed data, TheilSen and least squares trend estimates are statistically identical, but unlike least squares, TheilSen is resistant to undetected data problems. To mitigate both seasonality and step discontinuities, MIDAS selects data pairs separated by 1 year. This condition is relaxed for time series with gaps so that all data are used. Slopes from data pairs spanning a step function produce onesided outliers that can bias the median. To reduce bias, MIDAS removes outliers and recomputes the median. MIDAS also computes a robust and realistic estimate of trend uncertainty. Statistical tests using GPS data in the rigid North American plate interior show +/0.23 mm/yr rootmeansquare (RMS) accuracy in horizontal velocity. In blind tests using synthetic data, MIDAS velocities have an RMS accuracy of +/0.33 mm/yr horizontal, +/1.1 mm/yr up, with a 5th percentile range smaller than all 20 automatic estimators tested. Considering its general nature, MIDAS has the potential for broader application in the geosciences. MIDAS Velocity file README can be found at http://geodesy.unr.edu/velocities/midas.readme.txt
Version
1
Measurement

Processing Level

Coverage
Global
Northernmost Latitude
90.0
Southernmost Latitude
90.0
Westernmost Longitude
180
Easternmost Longitude
180
Time Span(s)
January 1, 1996 to
Present
Resolution

Projection

Platform/Sensor
GPS
Project

Data Provider
Nevada Geodetic Laboratory