California Geodetic Control Committee
Mr. Gregory A. Helmer, PLS, Chairperson
c/o
RBF Consulting
email:
<gahelmer@rbf.com>
FEBRUARY 1994
(Draft HTML Version)
Prepared by:
California Spatial Reference System
Horizontal Component
Version
5.0, February1994
APPENDIX A, "Summary & Status, California Spatial Reference System"
APPENDIX B, "Requested Data Sheet Revisions (i.e., Additions)"
Global Positioning System (GPS) technology and the expanding use of spatial information for geographic information systems (GISs) and other new uses of survey data are causing momentous changes in surveying. These changes have created a critical need for a new, statewide high-accuracy horizontal geodetic network for referencing spatial data. The existing network, the National Geodetic Reference System (NGRS), has served the nation and California well, but currently it is not adequate for the horizontal referencing needs of the 21st century.
To meet California's long-term spatial referencing needs, it is proposed that a portion of the existing NGRS be upgraded to establish a new, high-accuracy horizontal geodetic network consisting of about 1,100 stations. This new network will be the horizontal component of the planned "California Spatial Reference System" (CSRS) and will be designated as California's official horizontal reference system for all surveying activities. After December 31, 1999, it will be the only legal reference system for California Coordinate System coordinate values. The proposed CSRS horizontal network is to be established and maintained through cooperative efforts involving federal, state, and local agencies, academic/research institutions, private firms, and individuals.
Today, the surveying profession is undergoing tremendous changes; not only in technology (technically), but also in data usage. The primary causes of these changes are ...
These changes are demanding, and will continue to demand, the use of one, consistent/accurate statewide, horizontal spatial reference system for all surveying activities. The use of unrelated local datums of varying accuracy standards and pre-GPS-established horizontal reference networks is rapidly becoming inadequate (and unnecessary). Evidence of these demands and changes are already apparent: (a) various local agencies have, or are considering, regulations which require recorded surveying documents to be referenced to a specific network/datum; (b) interest and use of the California High Precision Geodetic Network (HPGN) is expanding; (c) the Governor's GIS Task Force has recommended that the California HPGN be used as the "foundation" for all future geographic information production (GIS data); (d) at the national level, efforts are underway to establish High-Accuracy Reference Networks as part of a National Spatial Reference System, and (e) an increasing number of continuously operating GPS reference stations of geodetic quality are becoming operational throughout the state, particularly in southern California.
To be effective, a statewide horizontal spatial reference system must meet the following essential criteria.
The needs for a clearly-defined and well-maintained horizontal spatial reference system are especially critical in California where crustal motions are prominent (a factor that is insignificant in most states). Because of crustal motions, it is impractical to maintain the existing horizontal control networks with over 18,000 stations to GPS accuracy standards. Instead, a small (in number of stations), well-maintained, high-accuracy network that is suited for GPS surveys is required.
This document describes the horizontal component of a new "California Spatial Reference System" and proposes that it be systematically established and maintained to meet California's horizontal referencing needs of today and throughout the 21st century. (Note: Another California Geodetic Control Committee document will address the vertical-position referencing needs. Possibly, the same stations will serve both horizontal and vertical positioning needs.)
Currently, the recognized horizontal geodetic control network (system) within California is the National Geodetic Reference System (NGRS) which has 18,000 California stations (9,000 of which are first or second order). This nationwide horizontal control system is the result of many years of effort, dating back to 1807, by the National Geodetic Survey (NGS) and its predecessor agencies to establish, maintain, and improve the national geodetic network. The NGRS, which was established basically through the use of traditional survey methods (mostly triangulation for horizontal positioning), has served the nation and California well for many years.
In the 1986, NGS redefined the horizontal datum for the NGRS, replacing the North American Datum of 1927 (NAD27) with the North American Datum of 1983 (NAD83). When the new datum was established, the entire network also was readjusted to improve the horizontal accuracy of the stations. Thus, changes in station coordinates from NAD27 to NAD83 were caused by both a datum change and a new adjustment. Although the NAD83 readjustment removed many of the distortions and errors in the historical NGRS network, this control network currently remains inadequate (in general) for many GPS surveys and today's expanding spatial information needs. The reasons are discussed under "Issues", below.
In 1991/92, a high-accuracy (B Order, 1:1,000,000), horizontal geodetic control network, consisting of 238 stations, was established throughout California by GPS survey methods. This network is referred to as the "California High-Precision Geodetic Network" (HPGN). (Similar networks in other states are often called "High- Accuracy Reference Networks" or HARNs.)
The California Department of Transportation (Caltrans) sponsored (funded) the establishment of the HPGN. The actual survey efforts were accomplished as a cooperative NGS and Caltrans project with assistance from various local agencies, universities, and private firms. HPGN stations generally are located along transportation corridors and are spaced about 40 miles apart on a grid-like network.
The HPGN is not a new datum; the datum remains NAD83. The HPGN is a new adjustment of the HPGN stations (on NAD83) at the epoch date of the HPGN surveys. The epoch date represents the mean date of the GPS data collection effort and is 1991.35 for the HPGN. The final HPGN adjustment was performed by NGS and Scripps Institution of Oceanography in 1992. Thus, the adjustment has a time tag of NAD83(1992). About 100 stations in the HPGN had existing NAD83(1986) coordinates. Coordinate shifts for these stations from NAD83(1986) to NAD83(1992), which resulted from the high-accuracy GPS survey, range from zero to 1.3 meters with an average shift of 0.3 meters.
NGS is in the process of adjusting the 18,000 historical NGRS stations to the HPGN.
For much of California, the earth's surface (crust) is moving in a complicated manner relative to the other continental states. The crustal motions are caused primarily by the northwest movement of the Pacific tectonic plate relative to the North American plate. Two types of motions occur: (a) secular, which is a relatively constant movement; and (b) episodic, which is the sudden movement caused by an earthquake.
Fortunately for California surveyors, the earth scientists have been studying California's crustal motions for a number of years and have developed considerable knowledge regarding these motions. Schematic maps have been developed which show the estimated secular crustal motions throughout California. In some areas, the secular motion exceeds five centimeters per year (a meter in 20 years). The scientists also have learned that, in general, an earthquake must exceed a magnitude of six or seven to cause significant episodic crustal motion. In addition, studies have shown that displacements caused by earthquakes dissipate relatively quickly as the distance from the epicenter increases. However, the 7.3 magnitude Landers earthquake in June 1992 displaced nearly all HPGN stations in southern California to some extent. One HPGN station was displaced nearly two meters by the Landers earthquake.
California GPS surveyors must learn how to "live" with crustal motions; i.e., to be knowledgeable as to when it is necessary to apply appropriate crustal motion adjustments and when not, and how to determine these adjustments when applicable.
For most local surveys, the effects of crustal motions usually can be ignored (and in practice, are ignored). But, crustal motions can become an important consideration when the local survey data is used for other purposes; e.g., the data is (a) incorporated into geographic information data base, (b) used as control for another survey, (b) merged with data from another survey, and/or (d) used for current/historical data comparisons. The importance of considering crustal motions will increase as survey records and data bases mature and new data is mixed with historical data.
In addition, the effects of crustal motions must be considered for high-accuracy GPS surveys covering large areas or long corridors, especially east/west surveys.
Until recently, NGS actively provided all required geodetic control needs for the surveying profession. Over 285,000 horizontal control stations have been established nationwide and are included in the NGRS. Data for these stations is readily available from NGS's National Geodetic Information Center; but, as mentioned previously, the historical NGRS is currently inadequate for the needs of today and in the future.
Unfortunately, resources at the Federal level are not available to establish and maintain a new horizontal reference system of sufficient accuracy, density, and timely availability to fulfill today's GPS surveying and spatial referencing needs. Thus, NGS is formulating various policies to guide the development of a modern National Spatial Reference System. Key policies, as expressed in the NGS August 4, 1993, "Mission, Vision, and Goals" document are summarized below.
In late 1992, members of the Advanced Technologies Subcommittee of the California Land Surveyors Association began to discuss the need to develop standards and specifications for high-production-type GPS surveys. To initiate action, the subcommittee held an informal meeting in January 1993 at the annual California State University, Fresno Surveying Engineering Conference. Various GPS surveying issues were discussed, including geodetic reference networks. As a result of this meeting, a second meeting was held in March 1993 to continue discussions. At the March meeting, the group established a formal California Geodetic Control Committee (CGCC) consisting of 17 members from various public and private organizations. The Committee includes a broad representation of California surveyors (GPS experience, organizational size, geographic location, etc.).
Although the Committee officially is comprised of 17 members, all interested California surveyors are encouraged to participate in the activities of the Committee. In fact, a number of other surveyors actively assist the Committee in developing consensus positions, preparing policy/procedural documents, and other Committee efforts.
The California Geodetic Control Committee is assigning various tasks (issues) to specific subcommittees. This document (proposal) is the result of the efforts by the Horizontal Geodetic Reference System subcommittee.
The critical horizontal spatial referencing issues challenging California's professional surveyors are as follows:
Inadequate Network Accuracy: Although the historical
NGRS was established through exemplary survey efforts, most horizontal stations
within the NGRS are of insufficient accuracy for today's GPS survey methods
(except the HPGN stations). Routinely, GPS surveys exceed the accuracy of
the controlling NGRS stations which means high-accuracy GPS surveys are distorted
by adjusting them to less-accurate control.
Also, GPS methods allow
baselines to be determined (measured) that were impossible to determine with
historical surveying methods; e.g., across mountain ranges. As a result,
GPS survey methods detect errors in horizontal control networks that could
not be discovered (or corrected) prior to GPS.
Poor Station Location: The survey methods used to establish
the NGRS required "lines of sight" between stations; thus, many stations
are located on mountain peaks, tops of tall buildings, or where towers are
required for use. As a result, numerous NGRS stations are inaccessible and
unusable, or at best, inconvenient to use.
In addition, many existing
control stations are unsuited for GPS survey methods because they lack the
required visibility to the sky. At these stations, nearby trees, buildings,
or other structures block the satellite signals from reaching the GPS receiver
antenna. Other NGRS stations are unsuitable because they are located near
reflective surfaces (e.g., buildings) or transmission lines or towers that
can cause GPS data distortions and multipathing errors.
Finally, most
existing NGRS stations were primarily located to meet the survey needs rather
than for convenient public access. Thus, many stations are located on private
lands which, at times, makes accessibility difficult or impossible.
Limited Network Maintenance: For many years, NGS had an
active program to maintain the NGRS stations. However, in recent years,
budget constraints have eliminated these maintenance efforts. This reduced
maintenance effort, together with normal construction activities and other
events, has resulted in many stations being lost or destroyed.
For
California, the accuracy of GPS survey methods has introduced a new maintenance
issue. Crustal motions along the central and southern coast, and to a lesser
extent in other regions, can distort (over time) the monumented network so
that it no longer meets GPS-survey accuracy capabilities and requirements.
(See "Crustal Motions in California", above, for details.) Currently,
comprehensive policies and procedures have not been developed and published,
at either the national or state level, for maintaining (updating) the existing
NGRS for the effects of crustal motions within California.
No Established Crustal Motion Procedures: In addition
to a maintenance issue, California's crustal motions require the development
and use of specific procedures for data publication, coordinate notation,
and survey adjustments. Today, these procedures are not
established.
Because of crustal motions, the positions of control
stations will change both "in actual fact" and by published value (the latter,
if the network is maintained properly). Thus, publication and coordinate
notation procedures, such as dating coordinate values, are necessary to (a)
ensure a clear, undisputable understanding of the basis for the information
(data) presented, (b) facilitate the efficient exchange of geographic information
for GISs and other uses, (c) enable the reliable use of data obtained at
different time periods, and (d) accommodate the needs of large and high-accuracy
GPS surveys.
The effects of crustal motions create a number of options
for adjusting surveys. To ensure consistency in record data, procedural
guidelines are required; e.g., selection of the survey adjustment date
(epoch).
Reduced NGS Assistance: NGS does not have the resources to establish and maintain a new (modern) horizontal, spatial reference system that meets the needs for local and regional surveys. Resources at the national level will be focused on a framework network (station spacing 100 kilometers). The individual states, in cooperation with NGS, are to develop, establish, and maintain a densified network that meets the needs of the local surveyor. Refer also to "NGS's Role - Past, Future", above.
To meet California's horizontal spatial referencing needs of today and throughout the 21st century, it is recommended that a California Spatial Reference System (CSRS) horizontal network, consisting of about 1,100 B Order or better monumented stations, be developed, established, and maintained, through cooperative efforts involving NGS, state/local agencies, academic/research institutions, and the private sector; and that this network be designated as California's official, and only, horizontal spatial reference system for all land surveying activities.
Important Notice: This section outlines certain NGS responsibilities. Although a consensus has been developed with NGS on many issues, full concurrence has not been reached on all issues. This is a proposal.
DESCRIPTION:
(DEFINITION, STANDARDS,
SPECIFICATIONS)
Note: Although it is planned that the CSRS will serve both horizontal and vertical positioning needs, the following is limited to the CSRS horizontal control component.
Name: California Spatial Reference
System (CSRS), Horizontal Component.
Definition: The CSRS horizontal competent
is a high-accuracy, monumented geodetic spatial-reference network consisting
of ...
Since the California HPGN (epoch 1991.35) has been established,
the following is limited to the establishment of the HPGN densification stations.
These stations will be established on a survey-by-survey basis as interest
develops within an area and resources become available.
Coordination/Review: The California NGS State
Geodetic Advisor shall coordinate the HPGN densification surveys and perform
the required "on site" NGS reviews. The Advisor also will provide assistance
(advice) for densification surveys.
Standards, Specifications, Procedures: See
"Description", above and the other applicable subsections in this section,
"CSRS Recommendation Details".
Who Initiates: Any state/local agency,
academic/research institution, or private firm may initiate a HPGN densification
survey.
Who Accomplishes: The HPGN densification surveys
shall be accomplished through cooperative efforts involving NGS, state/local
agencies, academic/research institutions, and/or private firms. The field
efforts for the densification surveys shall be performed at the local level
(state/local agencies, academic/research institutions, private firms, and
individual surveyors). Also, see subsection "Adjustments", below.
Planning: Prior to planning the station locations,
the lead agency/firm should contact others who are interested in geodetic
control and determine their interest and support for the proposed densification
survey. Contacts should include various state/local agencies, academic/research
institutions, local professional surveying organizations, earth scientists,
and local geodetic surveyors.
NGS Submittal: All HPGN densification surveys
(CSRS surveys) shall be submitted to NGS, in the required format, for inclusion
in the National Geodetic Reference System as part of either the Federal or
Cooperative Base Network.
NGS On-Site Reviews: At a minimum, the following
on-site reviews shall be made by the California NGS State Geodetic Advisor
for each HPGN densification survey.
DATA PUBLICATION/DISTRIBUTION:
Epoch Dates:
Coordination: The California NGS State Geodetic
Advisor shall coordinate the CSRS maintenance efforts.
Standards, Specifications, Procedures: All survey
efforts related to maintenance of the CSRS stations shall be performed in
accordance with the standards, specifications, and procedures that were used
to originally establish the given station(s) and the procedures in this section,
"CSRS Recommendation Details".
Note: Some crustal motion
resurveys will establish new coordinates, having a new epoch, for all stations
included in the resurveys. However, the CSRS goals are to keep the number
of epochs is use throughout California, at any one time, to a minimum and
also to avoid statewide readjustments (and thus, new statewide epochs) for
as long as feasible. See policies under "Adjustments", below
Who Accomplishes: The CSRS maintenance efforts
shall be accomplished through cooperative efforts involving NGS, state/ local
agencies, academic/research institutions, private firms, and individual
California surveyors. See also "Adopt A Monument" program, below.
Annual Inspections: Each CSRS station shall be
inspected annually and a report made to the California NGS State Geodetic
Advisor. This effort shall be accomplished at the local level (state/local
agencies, academic/research institutions, private firms, and individual
surveyors).
The California Geodetic Control Committee will encourage
NGS to develop a user-friendly, computerized recovery note system for reporting
station conditions and, when required, revising station descriptions.
Lost or Damaged Stations: Efforts to replace lost
or damaged CSRS stations should be initiated and completed as soon as feasible
after the need is discovered. The field effort required to replace lost
and damaged stations shall be accomplished at the local level (state/local
agencies, academic/research institutions, private firms, and individual
surveyors).
Crustal Motion Resurveys:
The general goal for the CSRS is to minimize
the number of changes in published coordinates and epochs.
The California HPGN: Resurveys of HPGN stations
will be necessary to reestablished HPGN stations that are lost, damaged,
or displaced by earthquakes, and for periodic readjustments to eliminate
accumulated secular crustal motions. All HPGN resurveys shall be adjusted
as determined by NGS.
HPGN crustal motion resurveys (secular and
episodic) shall be referenced to stable (no crustal motion), high-accuracy,
horizontal control on the North American tectonic plate; i.e., B Order or
better National Geodetic Reference System control. New coordinates, based
on the resurvey results, will be published for all stations in the resurvey,
including those not affected by crustal motion (except the controlling stations
on the North American tectonic plate). The epoch of the new coordinates
shall correspond to the mean date of the data collection. Resurveys and
adjustments for secular crustal motion will be delayed as long as feasible
to avoid epoch changes. See also "Crustal Motion Resurveys", above.
HPGN resurveys to replace lost or damaged stations, shall be adjusted to
adjacent B Order, Class I HPGN stations as specified for HPGN densification
surveys in the next subsection. The name of the replaced station will be
changed.
NGS will perform all final data processing and adjustments
for HPGN resurveys.
HPGN Densification Surveys: In general, HPGN
densification surveys shall be referenced to adjacent B Order, Class I HPGN
stations. During the adjustment process, corrections will be applied to
the observations for the estimated secular crustal motions, as directed by
NGS. The epoch of the densification surveys shall be the epoch of the
controlling HPGN stations.
If secular crustal motions have degraded
the B Order, Class I HPGN control to the extent that a satisfactory adjustment
cannot be performed using the HPGN control, the densification survey shall
be adjusted in the same manner outlined above for HPGN resurveys; i.e.,
referenced to geodetic control on the North American tectonic plate.
Densification stations displaced by crustal motions (secular and episodic)
shall be reestablished as part of the HPGN resurvey for the same area (see
above), or such stations shall be reestablished through readjustment methods
involving the HPGN resurvey, data from continuously operating GPS reference
stations, an updated HTDP program, and/or advice from earth
scientists.
Resurveys for HPGN densification stations that are lost
or damaged shall be accomplished and adjusted in the same manner as the original
densification surveys; i.e., referenced to adjacent B Order, Class I HPGN
stations. The name of the replaced station will be changed.
NGS
will perform the final data processing and adjustments for HPGN densification
surveys, unless NGS approves the execution (performance) of these adjustments
by others; e.g., the Department of Transportation (Caltrans), academic
institutions, other federal agencies, etc.
CONTINUOUSLY OPERATING GPS STATIONS:
Continuously operating GPS reference stations (CORSs) are facilities that collect GPS data on a continuous (24-hour) basis. Often, the data collected is made available to others through electronic data transfer means. A fee is charged for data from some facilities.
The primary benefits of CORSs are:
Some CORSs can serve as GPS receivers at known points
for geodetic surveys. Potentially, this could significantly reduce GPS equipment
needs (costs) for GPS surveys.
CORSs provide data which can improve the accuracy of GIS-
type GPS surveys, "freeing up" geodetic-quality field receivers for other
surveys. Similarly, the accuracy and functionality of hand-held GPS receivers
can be improved.
Because CORSs collect data continuously, an instantaneous
position can be computed for the station at scheduled intervals or as needed.
This capability provides data to accurately determine secular motions.
It also enables the earth scientists to monitor crustal motions for earthquake
research efforts.
In addition, the instantaneous-positioning-capability permits
the position of a CORS to be reestablished (new coordinates established)
immediately after an earthquake. Resurveys are unnecessary since sufficient
data is available soon after an earthquake to reference the station to other
CORSs on the North American tectonic plate (and throughout the world).
The California Geodetic Control Committee (CGCC) recognizes the benefits and expanding use of CORSs. The CGCC has initiated efforts to establish standards, procedures, and guidelines for this evolving GPS survey methodology. Preliminary concepts and planned efforts are listed below.
The CGCC will encourage owners of CORSs to make
their stations, CSRS stations. Efforts to provide the required supplemental
information (e.g., leveling data) for submittal to NGS might be a cooperative
effort, involving the station owner and others.
In general, CORSs included in the CSRS shall ...
Another California Geodetic Control Committee document is
being prepared, by another subcommittee, to detail the recommended legislation
actions necessary to establish, maintain, and promote a California Spatial
Reference System. To be an effective system, the following key actions will
be required.
Add statutes which define the National Geodetic Reference
System and the California Spatial Reference System.
Add a statute which names the horizontal component of
the CSRS as the official California horizontal spatial reference system.
Clarify and revise existing California Coordinate System
(CCS) statutes so that CCS coordinates must be based on direct ties to at
least two permanently-established stations listed below, after December 31,
1999. (For GPS survey methods, a direct tie means a non-trivial baseline
determination.)
Notes: Retain last sentence of Section 8813. In Item "c", above, second-order is consistent with current statute, but inconsistent with today's technology; revise to first?
Prior to January 1, 2000, the California Geodetic Control Committee and, hopefully, other professional surveying organizations will promote the use of the horizontal geodetic control specified above.
Add a CCS provision which requires that documents and electronic
files using CCS coordinates shall state the epoch (date) that the coordinate
data is valid; i.e., the basis of the coordinate data shown.
Add a CCS provision which requires that documents and
electronic
files using CCS coordinates shall state the order of accuracy, as defined
by the FGCS or CGCC standards and specifications, for all included CCS control
points having an order of accuracy of Second Order or better.
Add a CCS provision which requires that the epoch for
a
given survey shall be the latest NGS published epoch of the controlling stations
for the survey. If the latest published epochs of the controlling stations
are not the same, the epoch of the controlling station having the latest
epoch shall be used.
Appropriate adjustments shall be made to
controlling stations
having earlier epochs so that the coordinates of the controlling stations
are consistent.
Add a CCS provision which states that adjustments for crustal motions shall be made in accordance with NGS-published procedures and values.
CURRENT STATUS (1/94):
The California HPGN: The California High-Precision Geodetic
Network survey established 238, B Order horizontal control stations that
are distributed throughout California (see Appendix A). Field efforts for
the HPGN were completed in 1991, and the station Data Sheets (final coordinates)
were published in August 1992 by NGS.
Since the HPGN observations
were performed, several earthquakes
have displaced a number of stations. To date, the known areas affected by
earthquakes are in the vicinity of Ferndale, Humbolt County (about eight
stations), Landers, San Bernardino County (44 stations), and Northridge,
Los Angeles county (?? stations).
The Ferndale and Landers areas
have been resurveyed. The
Landers resurvey data processing and adjustment are completed and the data
is available from NGS. Completion of the Ferndale resurvey data processing
and adjustment is scheduled to follow soon. It is anticipated that the resurveys
for the Northridge earthquake will be performed in mid 1994.
HPGN Densification Surveys: In addition to the HPGN stations,
the proposed CSRS will include about 850, first-order horizontal geodetic
control stations (see Appendix A). A number of surveys have already been
initiated to establish these stations. To date, surveys are underway for
approximately 400 HPGN densification stations. For additional information,
refer to Appendix A.
Continuously Operating GPS Reference Stations: A recent
survey initiated by the California Geodetic Control Committee identified
at least 11 geodetic quality, continuously operating GPS reference stations
that are in operation today throughout California; another 10 stations are
planned in the near future. The Committee will continue to work cooperatively
with the owners of continuously operating stations, and also will continue
to encourage them to incorporate these stations into the National Geodetic
Reference System; and thus, the CSRS.
At this time, the largest
network of CORSs is operated by
Scripps Institution of Oceanography at the University of California, San
Diego. Currently, this network, the Permanent GPS Geodetic Array (PGGA),
has 10 stations. Dual-frequency data is collected on a continuous basis
at a 30-second sampling rate. The data is available to PGGA subscribers
through a personal-computer-based bulletin board.
Successful implementation and usage of the California Spatial Reference System will require an effective education program. The California Geodetic Control Committee is establishing an education subcommittee for this effort. However, to achieve an official California horizontal spatial reference system (the CSRS) - in "law" by legislation and "in fact" by acceptance and usage - will require the educational support of California's professional surveying organizations, State universities, NGS, state/local public agencies, and others.
As part of the education program, the CGCC will encourage NGS to develop, in cooperation with California surveyors, a comprehensive document which reviews the horizontal geodetic control issues in California and the application (use) of the available information and data. Possible topics should include a general discussion of California's crustal motions, application of the "Horizontal Time Dependent Positioning" computer program, definition and use of epoch data, historical listings of CSRS station shifts (by epoch), and other related material.
CALIFORNIA SPATIAL REFERENCE SYSTEM - HORIZONTAL
(HPGN and HPGN Densification)
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