The California Surveyor - CLSA

determine the amount of adjustment
to be made to the remaining existing
network stations. The results of the
adjustment (i.e., the newly arrived
at coordinate values for the remaining
NGRS stations) may not be published
for a few years.
Survey Specifics of
HPGN System
The California HPGN was a joint
project developed and completed by
both the California Department of
Transportation (CALTRANS) and
the National geodetic Survey (NGS).
The way in which the survey was
planned out and completed may best
be explained if broken down into
three components consisting of the
network design, project reconnaissance,
and data acquisition.
Network Design
The initial network design for the
HPGN was planned out on a California
state highway road map by
the Headquarters Surveys office of
CALTRANS in Sacramento. Since
one of the main reasons for establishing
an HPGN was to allow easy
accessibility to control stations, the
initial approximate locations for the
stations were selected (for the most
part) within state highway rights-ofways
along state highway corridors.
The stations were plotted out on
the road map at approximately 30 to
40 mile spacing in a grid pattern
throughout the entire state. The final
network layout provided for
about 170 new stations excluding ties
to existing horizontal and vertical
control within California or near its
borders.
Project Reconnaissance
Copies of the road map consisting of
the plotted locations of the proposed
new HPGN stations were given to
each of the 12 CALTRANS districts
so that each could go out into the
field and select specific points in the
vicinity of the station locations indicated
on the map. Each district was
responsible for selecting station sites
within their own district boundaries.
In addition to the new stations being
set along the California state Highway
rights-of-ways, network design
guidelines and project reconnaissance
called for the inclusion of 5 other
existing groups of stations. These
groups were as follows:
1) All California stations in the
National Geodetic Survey's
National Crustal Motion Network
(NCMN). These are order A stations
(10 times more accurate than
HPGN stations) and were used as
control for the HPGN adjustment.
2) Ties to an existing NGRS horizontal
control station in each one
degree by one degree latitude and
longitude block to ensure an interrelationship
between the HPGN and
the existing 16,000 horizontal control
stations.
3) Ties to the existing NGRS
vertical control network to provide
information for orthometric height
determinations throughout the
network.
4) Ties to tidal bench marks to
provide an accurate interrelationship
between the geodetic and tidal
datums.
5) Ties to the southern tier of
stations in the Oregon HPGN project
near the California-Oregon border.
Data Acquistion
GPS satellite data was collected by
DISTRICT
01
02
03
04
05
06
07
08
09
10
1 1
12
TOTAL
both CALTRANS and NGS crews
using two primary types of dual
frequency receivers. These were the
Ashtech L XII and the Trimble 4000
SST. Additionally, data collected
from four fixed receiver sites located
at the Mojave CIGNET station, JPL
in Pasadena, Scripps Institution at
La Jolla, and at Pinyon Flat was
incorporated into the project.
Satellite data was collected on all
stations in the network for 6 hours;
and each station was occupied on at
least two different days. All NCMN
stations were occupied on three separate
days and approximately one
fourth of the total number of HPGN
stations excluding the NCMN
stations were occupied three times.
Data for the project began to be
collected on April 5,1991 and it ended
about 4 months later on August 13,
1991. Baseline data reductions were
performed by NGS personnel in San
Diego and in Sacramento by in house
NGS written software (OMNI). The
baselines were then sent back east to
Maryland where they were reprocessed
using a precise ephemeris for
additional accuracy. A least squares
adjustment was then performed on
all the baselines by NGS and the final
coordinate values released in mid
May 1992.
Table 1
California HPGN Network Stations
CALTRANS
STATIONS
15
25
9
12
13
15
6
16
7
12
15
2
147
CALTRANS
NCRS
STATIONS
0
4
3
2
0
3
0
6
10
0
0
0
28
NETWORK
(NCRS)
TIES
2
6
3
2
5
6
0
6
1
3
4
1
39
NCMN
STATIONS
0
2
0
1
2
0
3
2
2
1
5
0
18
TIDAL
BENCH
1
0
0
1
2
0
1
0
0
0
0
0
5
1 2 The California Surveyor Fall 1992