Download - CLSA

Continued from previous page
A Reflection on the San Diego County High Precision Geodetic Network
and the Start of the Statewide HPGN Survey
A Trimble 4000 SST GPS receiver borrowed from MIT at station SDGPS
15, a station set to replace the mark on Volcan Mountain which was
inaccessible due to snow.
Two of the NGS operators who were assigned existing
NAD83 network stations on mountaintops were late
getting started on the first day after having to slog thru
the snow to get to their respective stations.
One of the County of Orange surveyors was delayed
due to a helicopter crash on the freeway near his
assigned station.
One of the borrowed Trimble receivers was set to log
to an external data collector, so no data was collected
for that station on Friday. A call to Trimble on Saturday
morning corrected the problem.
The crew at a station by the Old Point Loma
Lighthouse, found themselves answering questions
from 10,000 tourists and 200 2 nd graders.
On Saturday and Sunday, the County Party Chief
occupying a station in a desert campground east of
Borrego Springs found himself surrounded by participants
in what is the annual Peg Leg Liars Contest.
When he told them he was making measurements to
satellites, they did not really believe him and thought it
an elaborate setup for a contest entry.
Mike Binge at station Junction Azimuth mark near the Tecate border
crossing during the 1991 San Diego-HPGN survey.
which is really too close together to achieve “A” order relative
accuracy (1:10,000,000 over 20km is smaller than the
expected setup error), however every effort was made to
achieve the highest accuracy possible. Not only were all 34
stations observed simultaneously, the observations were
repeated on 3 days, meteorological observations were
observed at most stations several times during the sessions
and plumb-bobs were used to verify optical plummet
tribrachs. The schedule for operations was set for Friday,
Saturday and Sunday, April 5, 6, & 7, 1991 to minimize
impacts on existing projects and deadlines. Miraculously,
the rain stopped as this start date approached.
With all of the careful planning and preparation, the
actual field operations went off relatively trouble free. Of
course, there were some issues, such as:
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Once all of the data was downloaded and backed-up to
multiple 3.5” diskettes (I still have memories of formatting
hundreds of diskettes for hours on end…the County IT
department saved 6 cents apiece purchasing unformatted
diskettes), the data was then delivered to Yehuda Bock’s
group at Scripps Institution of Oceanography (SIO) for processing.
Dr. Bock is one of the developers for the GAMIT
software (GPS at MIT) along with scientists at the
Massachusetts Institute of Technology. The GAMIT software
is a true multiple baseline processor in that instead of
looking at one base and rover GPS receiver pair at a time
to determine a vector or baseline, GAMIT looks at all
receivers and double difference combinations available to
determine unknown station coordinate parameters in one
least-squares solution. With 34 simultaneous data sets,
that makes for a lot of double difference combinations and
requires a fair amount of computational power.
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