227 



located on unconsolidated coastal plain sediments than for cities on 

 incompressible crystalline bedrock. This hypothesis can be tested for the 

 North American Atlantic Coast. The mean sea level trend for 24 coastal 

 plains stations between Key West, Florida, Long Island, New York, and Cape 

 Cod is 2.90 ± 0.74 mm/yr, as compared with 2.46 ± 0.59 mm/yr for 16 

 stations on crystalline bedrock between Connecticut and St. John's, 

 Newfoundland. However, the difference between the means is not significant 

 at the 95% confidence level, using Students' t-test. Removal of the long- 

 range trends does not alter this finding. Therefore, compaction effects 

 may not be the most important factor in the observed subsidence of the East 

 Coast. 



Whatever the ultimate cause of subsidence, it is desirable to isolate 

 vertical land motions from the eustatic sea level signal. Absolute sea 

 level positions must be measured relative to an ultraprecise geodetic 

 reference system, such as very long baseline interferometry (VLSI) and the 

 global positioning system (GPS) (Carter et al., 1986). VLBI will use a 

 network of fixed and mobile radio telescopes for precise geodetic 

 measurements, accurate to the 1-cm level, by the 1990s. The reference 

 point will be the earth's center of mass, as determined by satellite laser 

 ranging, over time. The VLBI network will be tied in with the GPS, which 

 employs portable ground-based radio receivers to detect satellite radio 

 transmissions. By using phase differencing, the GPS signal can be applied 

 to geodetic observations, in a manner similar to VLBI. VLBI/GPS will be 

 connected to a global network of around 100 "absolute" sea level stations 

 (GLOSS); and ancillary meteorological and oceanographic variables that 

 affect sea level will also be monitored. 



Preliminary comparisons of sea level positions from tide gages and 

 GEOSAT altimetry (Wyrtki , 1987) show a fairly good match, although only 

 covering a small area of the equatorial Pacific Ocean, over a 5 -month 

 period. This experiment illustrates the potential of satellites for sea 

 level measurements, even with current technology. This approach can be 

 extended to "calibrate" satellites for longer-term, larger-scale 

 observations over wide areas of oceans that lack tide gages. 



