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Final Report
I. INTRODUCTION
This report outlines recommendations
for capitalizing on the existence of a highly-accurate county-wide
network of stations established using the Global Positioning
System (GPS) in 1999.
This network provides the basis
for monitoring subsidence in the county with an accuracy suitable
to meet all participating agency requirements. It further provides
the basis for other spatial positioning requirements in the county
including Geographic Information System (GIS) development.
The project was the culmination
of a series of meetings and actions by agencies with a concern
about subsidence in Yolo County. A list
of the agency participants and those personnel who participated
in the project is included.
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II. BACKGROUND
The Global Positioning System
is a highly-accurate, satellite-based, spatial positioning technology.
It is capable of providing sub-centimeter horizontal positions
(coordinates) over large distances.
Because of the nature of the
GPS satellite configuration and other considerations it is not
able to provide equally accurate vertical positions (elevations).
Generally speaking, the accuracy of GPS-derived elevations is
only one-half to one-third as accurate as GPS-derived horizontal
positions (latitude and longitude), and achieving this vertical
accuracy requires a greater effort than that required for horizontal
accuracies.
The plan of the Yolo County network
was to develop a methodology to achieve the maximum vertical
accuracy in a reasonable and practical manner.
In 1994, the National Oceanographic
and Atmospheric Administration's (NOAA's) National Geodetic Survey
(NGS) began testing large multi-day GPS data sets to determine
a set of guidelines for achieving a few centimeters of accuracy
in the vertical coordinate. The data were extracted from GPS
observations obtained by the Harris-Galveston Coastal Subsidence
District in Texas and over the network established near the National
Institute of Science and Technology (formerly the National Bureau
of Standards). This latter network served as the proving ground
for GPS manufacturing companies to test their equipment against
a known standard.
After over a year of testing,
including a test project in the San Francisco Bay area, the guidelines
were established where, with a minimum of observation time and
with some more-demanding observing techniques, elevations could
be determined at the two to three centimeter level (about one
inch). This is approximately consistent with older terrestrial
leveling techniques over distances such as the station spacing
in the Yolo County network. But terrestrial leveling techniques
are more labor-intensive, require longer time to complete, and
are about five times more costly.
The network in the Bay area was
then extended eastward in 1996. It was further extended in 1997
and 1998 into the Sacramento-San Joaquin River delta area, including
the area of Yolo County along and below the Highway 80 corridor.
These succeeding projects proved
the ability of the GPS technology to meet previously unattainable
levels of accuracy over reasonably short time periods and with
reasonably short observation times per station (a minimum of
30 minutes).
In late 1998, representatives
of the U.S. Army Corps of Engineers' Topographic Engineering
Center and NGS met to discuss the use of GPS technology to measure
and monitor subsidence in other areas of the Central Valley.
Water Resources Association of
Yolo County (WRA) Executive Coordinator presented the information
to the WRA Technical Committee. Interested WRA members (the cities
of Davis, Winters and Woodland, the County of Yolo and the Yolo
County Flood Control and Water Conservation District) and the
California Department of Water Resources agreed to pursue the
establishment of a GPS network in the county suitable for monitoring
subsidence.
Reconnaissance to select existing
survey monuments and sites for new survey monuments for the Yolo
County project began in late May, 1999, and field observations
were completed in August.
A total of 50 stations were established
in the survey. After completion of the basic project stations
in July, it was decided that a station near each of two extensometers,
operated by the California Department of Water Resources, should
also be included in the project. Observations to these extensometer
sites (near Zamora and on the Conaway Ranch) were obtained in
late August.
Stations
observed in the project are listed here.
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III. DISCUSSION
While the use of GPS to determine
accurate elevations is becoming more widespread, subsiding areas
present unique concerns.
Subsidence is generally caused
by fluid withdrawal or hydro-compaction. In some areas of the
country it is caused by groundwater withdrawal (e.g., California's
Central Valley, Texas' Harris-Galveston Counties) and in others
by oil and gas withdrawal (e.g., the Port of Long Beach). Hydro-compaction
has been known to occur in some areas of the western San Joaquin
Valley.
Subsidence can be extreme. In
the Mendota area of Fresno County there has been over 27 feet
of subsidence over the history of recorded leveling, which began
in the 1920's.
In general, most of the Central
Valley of California is subject to subsidence.
Subsidence is not a phenomenon
that lends itself to prediction (of annual rates, for example),
except in an average sense, since it is a function of several
variables that vary from year to year and even from season to
season.
Subsidence is not uniform over
even relatively small areas. Differential subsidence (the amount
a particular area subsides with respect to a neighboring area)
can occur over points separated by as little as a mile, and perhaps
less. For this reason, subsidence can only be monitored accurately
if it is related to areas of stability, and this is where GPS
technology is of particular benefit.
Prior to the mid-1990s the generally
accepted method of measuring and monitoring subsidence was by
conventional leveling techniques. These techniques were extremely
labor-intensive and costly. Because of the differential nature
of subsidence a denser network of leveling was required, driving
the cost up higher yet.
By 1996 GPS had proven itself
capable of achieving similar coverage with comparable accuracies
at only a fraction of the cost. But it did not obviate the need
to tie areas of subsidence to known areas of stability, sometimes
tens of kilometers distant (as in the case of Yolo County).
A complicating factor in the
use of GPS is that the coordinates obtained, both horizontal
and vertical, are related to a mathematical surface, expressed
in Cartesian (x, y, and z) coordinates. These coordinates then
need to be converted to the geodetic system.
In North America the geodetic
system is composed of two parts; the horizontal part is the North
American Datum of 1983 (NAD83) and the vertical part is the North
American Vertical Datum of 1988 (NAVD88). It is a relatively
straightforward process to translate the GPS-derived Cartesian
coordinates into the NAD83 and NAVD88 geodetic coordinates.
The accuracy of this translation
depends on the physical stability of the points being measured.
Subsiding areas present unique problems that make accurate coordinate
conversion problematic. To overcome these problems it is necessary
to perform simultaneous GPS observations in the subsiding area
and stations with known NAVD88 elevations in non-subsiding (stable)
areas. This was accomplished in the Yolo County network.
View the
results of terrestrial leveling that was performed between some
of the network stations.
The use of GPS eliminates the
need for significantly more expensive and labor-intensive terrestrial
leveling techniques.
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IV. THE
YOLO COUNTY NETWORK
The Yolo County network consists
of 47 stations inside the county and three stations outside the
County.
The stations outside the county
serve to connect the network to areas of stability to the east
and to the north. An existing stable site in western Yolo County
was also used. Two of the stations in the network are Continuously
Operating Reference Stations (CORS). These two stations are part
of the Bay Area Regional Deformation (BARD) network monitored
by the University of California, Berkeley Seismological Laboratory
in cooperation with the University of California, Davis. The
BARD network provides monitoring of the network of CORS sites
throughout northern California. Coordinates (latitude, longitude
and elevation) for these sites are monitored continuously.
The Yolo County network, a combination
of existing and new survey monuments, was observed according
to guidelines established by the National Geodetic Survey. The
GPS observations took place in July and August, 1999.
A copy of the project map is
included in the envelope at the back of the printed report. For
a more complete discussion of the network observations and related
information see "The Yolo County 1999 GPS Subsidence Project:
Field Operations Report", also by Don D'Onofrio and Jim
Frame.
The network, for its county-wide
coverage and accuracy, is still only a combination of discrete
points. It is simply the framework to which other stations, perhaps
more dense and employing other technologies, can be related.
It is also important to note
that the network coordinates represent positions at the time
of the observation. Since subsidence is a dynamic process, differing
from location to location, the coordinates are susceptible to
change over time. These issues are discussed in the recommendations,
(Section VI).
The Yolo County project was accomplished
under contract with Frame Surveying & Mapping, a Davis-based
firm, which retained Don D'Onofrio for his geodetic expertise.
All observations were performed with support from a number of
participating public agencies. These agencies included:
· U.S. Bureau of Reclamation
· California Department of Transportation
· Yolo County Planning and Public Works Department
· City of Woodland
Personnel from these agencies
were provided at no cost to the project.
This support accounted for about
70 person-days of effort. This is both a significant contribution
and an outstanding example of cooperative effort among different
levels of government.
The overall project was managed
and administered by the City of Davis Public Works Department.
The value of these staff contributions is approximately $25,000.
In addition, GPS survey equipment-usage
contributions were provided for the duration of the project by
the following agencies:
· U.S. Bureau of Reclamation
· California Department of Transportation
· University of California, Davis
The GPS and related equipment
was provided to the project at no cost. The value of the equipment
contributions is approximately $15,000.
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V. SUBSIDENCE
IN OTHER AREAS
At least two areas of the United
States have established ongoing GPS monitoring of subsidence:
the Harris-Galveston Coastal Subsidence District in Texas and
the Port of Long Beach in California.
In both cases their monitoring
efforts go back to the 1950s. The former began GPS monitoring
in 1995 and the latter about 1998.
Other areas in California have
been surveyed for subsidence, including the Santa Clara Valley,
Coachella Valley and the California Aqueduct.
Representatives involved in these
efforts have been contacted for subsidence monitoring recommendations.
Appendix D features a list of those
contacted and a sample of the letter sent to each. A review of
the responses combined with input from county and local agencies
should help to develop a sound policy for future monitoring of
the Yolo County network.
Any policy developed for these
monitoring efforts must include the ability to relate Yolo County
survey monuments to those stable survey monuments outside the
County.
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VI. RECOMMENDATIONS
FOR MONITORING AND USE OF THE YOLO COUNTY NETWORK
A first step in capitalizing
on the county-wide network is to ensure that the survey monuments
that make up the network are preserved.
Survey monuments are susceptible
to destruction from construction activities, inadequately planned
or improperly executed maintenance activities, and vandalism.
Occasionally a monument will be in the way of highway construction
and cannot be preserved. Establishment of a new station monument
and transfer of elevation prior to the destruction of the original
will preserve the value of the monument to be destroyed and the
associated subsidence history.
Recommendation
1. Inform public and private
agencies involved in construction, utilities management, public
works and related activities in the county about the network
and the location of all stations. Information about the project's
web site should be included in this information. There is no
cost, per se, associated with this recommendation. Information
about the network, including station listing, description, and
a variety of related information are included in the project
web site.
Recommendation
2. Task a single county entity
with visiting each monument in the network annually to assess
the integrity of the individual monuments. Any discrepancies
in monument description or condition should be brought to the
attention of interested County parties and to the National Geodetic
Survey (NGS). Follow proper formats for reporting such discrepancies.
We estimate that Recommendation
2 will require about one person-week of effort per year. This
includes visiting all sites and preparing reports on any discrepancies
noted.
Recommendation
3. Identify stations in imminent
danger of destruction and replace them in advance, following
National Geodetic Survey guidelines. (A copy of these guidelines
may be obtained from the NGS California State Geodetic Advisor,
Marti Ikehara - <marti_ikehara@dot.ca.gov>). A station
destroyed before replacement represents a permanent break in
the subsidence history of that station.
We estimate that Recommendation
3 will require about three person-days of effort for each station
destined to be destroyed plus the cost of materials to establish
the new station. This includes selecting the location of the
new site (in the vicinity of the destroyed station), resetting
the new station monument, performing a terrestrial leveling tie
between the two monuments, and preparing the necessary documentation.
NOTE: Recommendation
4. Re-observe the entire
network in three years (2002). Depending on the results of this
re-observation the county can better determine the time period
for subsequent re-observations.
We estimate that Recommendation
4 will require somewhat less time and cost than the original
(1999) project since there will be few, if any, new survey monuments
to be set. This assumes the project will be accomplished in the
same manner as the 1999 project, i.e., observation personnel
and equipment provided by participating agencies. It is possible
that changes in the use of the GPS technology may allow the project
to be accomplished in a shorter time frame, and thus at a further
reduced cost.
Recommendation
5. Investigate the benefits
of more frequent re-observation of particular areas of the county.
We have no estimate for the cost of such a project because of
the uncertainty of the size and scope of such a project. Subsidence
rates and amounts differ throughout the county. Areas where smaller
amounts of subsidence are critical to infrastructure maintenance
might lend themselves to more frequent re-observation.
We can provide estimates if there
is interest in considering a project in a particular area.
Recommendation
6. Investigate densification
of the network in areas of particular interest. We have no estimate
for the cost of such a project because of the uncertainty of
the size and scope of the project. As discussed above, differential
subsidence can occur over relatively small areas. The basic network,
which provides an excellent framework for the county as a whole,
might not provide the spacing and location of monuments for a
more in-depth investigation in concentrated areas, e.g., the
cities of Davis or Woodland or, the levees along Cache Creek.
Using modified GPS observing
techniques, specifically Real Time Kinematic (RTK), can provide
a reasonably cost-effective and timely means to accomplish an
accurate survey over city-sized areas. Conventional terrestrial
leveling techniques might also be suitable over these short distances.
Recommendation
7. Provide continuing nonfinancial
support for the Continuously Operating Reference Station (CORS)
at the University of California, Davis. This site can be of significant
value in ongoing subsidence measurement operations.
The UCD CORS site is referenced
to other CORS outside the county and can help to provide real-time
elevations at the site in the NAVD88 reference frame. County
agencies can use this site in ongoing subsidence measurement
operations.
Recommendation
8. Investigate the establishment
of a CORS site in the north county area. We estimate the cost
of a CORS installation to be about $30,000 plus annual monitoring
and maintenance costs; there are entities outside the county
that may be willing to cooperate with the county in sharing these
costs.
The 20-plus CORS sites that have
been established in northern California have been established
in areas to support earthquake hazard research. The site at UC
Davis is the only one that adequately serves Yolo County for
subsidence purposes. Our experience with establishing CORS sites
as part of California's Earthquake Hazards Reduction Program
indicate that obtaining land use permits for CORS establishment
can be very time-consuming.
Experience in southern California
has shown that educational institutions frequently make good
partners for such an installation. A CORS site established in
Dunnigan, couple with the two existing extensometers monitored
by the California Department of Water Resources (in Zamora and
on the Conaway Ranch), would provide an excellent basis for indications
of general subsidence trends in the county.
Recommendation
9. Consider the merits of
encouraging the Federal Emergency Management Agency (FEMA) to
adopt the results of the project in its flood plain mapping efforts.FEMA
flood plain maps are required to be based on NAVD88 elevations
supplemented by data from other Federal, state and local agencies.
The 1999 YOLO County GPS Subsidence
Network will provide the most accurate and up-to-date NAVD88
elevations in the county. Affected personnel and agencies will
have to fully understand the issues of flood plain mapping in
subsiding areas. Subsidence could place previously unaffected
properties in the flood plain, and could similarly result in
the removal of some currently affected areas from requiring flood
plain insurance.
Recommendation
10. Investigate other supporting
technologies as an adjunct to the GPS Subsidence Network within
Yolo County. (See Section VII.)
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VII. OTHER
SUPPORTING TECHNOLOGIES
There are additional technologies
that can provide information about change detection (subsidence).
These are: Synthetic Aperture
Radar (SAR) and Light Detection And Ranging (LIDAR). Both technologies
provide areal coverage, rather than discrete point coverage as
with GPS. Both LIDAR and SAR have other uses beyond those for
use as detectors of vertical changes.
Synthetic Aperture Radar (SAR)
is a technology suited for change detection. The technology employs
aircraft- or satellite-based sensors that monitor the ground
and in successive passes can determine the change in elevation
over large areas to about five centimeter accuracy. There are
some issues that need to be resolved, especially in agricultural
areas. The technology cannot distinguish between the ground and
the tops of vegetation. In an extreme case, successive passes
over a field lying fallow at the first observation and with a
full mature crop of corn at the second would indicate the change
of perhaps a couple of meters!
Light Detection And Ranging (LIDAR)
is the optical analog of radar. The energy source for the system
is laser. The system serves as a range finder, measuring the
distance from the LIDAR platform, generally an aircraft, to the
ground. It otherwise provides information and data similar to
SAR for the purposes of monitoring vertical changes.
It should be remembered that
both SAR and LIDAR are change detection technologies. They can
apparently record the relative differences in an area over time,
but cannot measure the absolute change without some ground-truth
mechanism. That is where GPS is needed, whether or not SAR or
LIDAR are employed. There are costs associated with each of these
technologies (SAR and LIDAR).
In the early phases of their
development and use, there may be opportunities for further partnering
efforts and cost-sharing.
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VIII. CONCLUSION
A county-wide, unified, dense,
and highly accurate network of monumented geodetic control stations
sufficient to meet most spatial positioning requirements is now
in place in Yolo County.
The network will support subsidence
monitoring, its primary function, but will also meet most surveying,
engineering design and Geographic Information System (GIS) technology
needs into the future.
Full benefit from use of the
network for monitoring subsidence can only be realized if the
network is preserved and re-measured periodically.
The coordinates (latitude, longitude
and elevation) determined in the 1999 GPS Subsidence Project
are consistent with those used by other Federal and State agencies.
Accurate measurements of subsidence
can only be achieved if the network is related to stable (non-subsiding)
areas, which are primarily outside the county. Densification
(closer spacing of monitoring points) in some critical areas
in the county is recommended.
Participating agencies should
also keep abreast of emerging technologies, i.e., SAR and LIDAR,
to determine how they might be used in the subsidence monitoring
process.
Respectfully submitted:
Don D'Onofrio, Geodetic Consultant
7228 Willowbank Way
Carmichael, CA 95806
(916) 944-7879
Jim Frame, Frame Surveying &
Mapping
609 A Street
Davis, CA 95616
(530) 756-8584
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