The California Surveyor - CLSA

California's New
High Precision
Geodetic Network
HPGN What is it?
A high precision geodetic network
(HPGN) or similarly a high accuracy
reference network (HARN) is a
group of stations uniformly spaced
usually 25 to 100 km (15 to 60 miles)
from one another, whose horizontal
positions relative to each other and
to the NAD83 reference coordinate
system, are known to a very high
degree of accuracy. The California
HPGN is only one of several already
existing HPGN's which have been
established in other states.
By Michael Stephens
What is the impetus for
HPGN Why do it?
The two main problems addressed
and mitigated by the existence and
utilization of an HPGN for satellite
surveying purposes are: (1) the difficulties
associated with the use of
existing NGRS (National Geodetic
Reference System) stations for satellite
data collection, and (2) the manifest
difficulties in trying to adjust a
high accuracy GPS survey to a less
accurate existing NGRS network
established by conventional methods.
Two main complaints that have
been made by users of the existing
NGRS reference stations with respect
to GPS survey network design and
data collection have been: (1) the lack
of easy accessibility of many of the
stations, and (2) the unsuitability of
many of the stations for use because
of surrounding obstructions. The
establishment of an HPGN alleviates
these problems by making all
stations that are a part of the
network easily accessible to road
vehicles and by placing them in
areas that have a 360 degree unobstructed
view of the sky from about
15 degrees above the horizon on up
allowing incoming satellite signals to
reach the receiver without being
blocked, deflected, or distorted.
Difficulties are continually arising
when GPS survey networks are adjusted
to NGRS reference stations.
One of the main reasons for this is
because of the level of error in
NAD83 positions of existing reference
network stations. It is not
uncommon to find errors of up to a
few decimeters in some of the
NAD83 stations in California. This
has often led to problems when
performing a least squares adjustment
of the highly accurate GPS
derived baselines holding 1st or 2nd
order NGS stations. Numerous least
squares adjustments have often been
required to determine whether poor
statistical solutions within an adjusted
network (i.e., high residuals
on individual baselines or a high
overall variance factor) were a result
of unsatisfactory coordinate positions
of the NGRS station(s) being
held fixed as control, or whether they
were a result of the data collected on
or design of the survey network
being adjusted. Furthermore, if the
poor solutions were a result of unsatisfactory
control then additional
adjustments and analysis is of the
survey network were often required
to determine which NGRS station(s)
had an unsatisfactory position.
Since the underlying objective of
an HPGN is to eliminate as much as
possible any existing distortion in
the positions assigned to NAD83
reference network stations many of
the current problems associated with
analyzing and adjusting a GPS survey
into an existing network will be
minimized.
Relation To NGS Control
The existence of an HPGN does not
supersede the current NGS Triangulation
Network for control (also
known as the National Geodetic
Reference System—NGRS). What will
eventually happen (perhaps within
the next year or two) is that the
current NGS network will be adjusted
into the HPGN network. This
will be done to minimize the amount
of distortion in the NGS net. This
will slightly change the currently
published NAD83 coordinate values
assigned to some of the stations.
Stations of the
California HPGN
The California HPGN consists of 244
stations spaced somewhat evenly
approximately 40 miles apart
throughout the entire length of the
state.
Most of the stations are located
off of major state or interstate highways
and are easily accessible by car
(see Figure 1).
Most of the set HPGN stations are
a modification of the NGS three
dimensional Class A monument. The
HPGN stations set this way basically
consist of a 2" dia aluminum monument
cap stamped with the station
name and date set. The aluminum
cap is set on top of an aluminum rod
driven to refusal and is surrounded
by a 6" PVC casing and concrete
collar and covered by an aluminum
frame and top. A few of the HPGN
monuments consist of a cap set directly
in solid rock or existing concrete.
The remaining HPGN stations
consist of existing NGRS network
monuments near proposed station
locations which met the HPGN
project criteria (see Table 1).
10 The California Surveyor Fall 1992