In the spite of the tremendous advances in crustal deformation measurements, space
geodetic techniques are significantly less accurate in measuring the vertical. Resolving vertical
motion using GPS may require observations over a time period twice as long as to observe a
similar horizontal signal. Over scales of less than 
30 km, the most
accurate vertical measurement
technique is still leveling. Unfortunately, it is very time-consuming and labor intensive. Reports
by Sylvester et al. [1991], Sylvester [1992],
and Holdahl and Dzurisin [1991] are representative of
recent geophysical investigations using high quality leveling observations. Savage and Pflaker
[1991] and Savage and Thatcher [1992] use tide gauges to estimate vertical uplift, but this
technique will only be useful when there is a long and reliable tide gauge record at multiple sites.
Vertical measurements using absolute gravity techniques are currently being tested by NOAA
[ Klopping et al., 1991]