Conventional
Measurement Method
Also called
“geodetic differential
leveling”, this initial form of
measurement originally consisted
of the establishment of
permanent benchmarks. Included
in these benchmarks were precise
elevations, latitudes and
longitudes for each point.
As the land
surface began to subside due to
groundwater withdrawal, the need
to relevel benchmarks became
necessary. Over the years, new
benchmarks were added (for a
total of more than 2,500) and
“relevelings” were conducted in
1978, and again in 1987. And
although this measurement method
provided excellent spatial
subsidence data, the cost of the
releveling procedure for a
single epoch prohibited us from
accessing up-to-date data at a
rate necessary to sufficiently
monitor the sometimes monthly,
weekly, or even daily effects of
subsidence. It was time to take
advantage of new technologies
that could provide us with the
same accuracy, yet allow us to
constantly monitor subsidence in
a cost-effective way. In 1987,
in conjunction with the
conventional releveling, an
experimental GPS releveling was
initiated.
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