California Geodetic Control Committee

Mr. Gregory A. Helmer, PLS, Chairperson
c/o RBF Consulting
email: <gahelmer@rbf.com>

Table of Contents

CALIFORNIA SPATIAL REFERENCE SYSTEM

(HORIZONTAL COMPONENT)

A PROPOSAL

FEBRUARY 1994

(Draft HTML Version)

Prepared by:

Alvin (Skip) Christensen, PLS
Lawrence R. Fenske, PLS
Kari Launen, PLS


California Spatial Reference System
Horizontal Component
Version 5.0, February1994

TABLE OF CONTENTS

ABSTRACT

INTRODUCTION

BACKGROUND

ISSUES

RECOMMENDATION (PROPOSAL)

CSRS RECOMMENDATION DETAILS

APPENDIX A, "Summary & Status, California Spatial Reference System"

APPENDIX B, "Requested Data Sheet Revisions (i.e., Additions)"


ABSTRACT

Table of Contents

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.

INTRODUCTION

Table of Contents

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.)

BACKGROUND

Table of Contents

EXISTING GEODETIC CONTROL:

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.

Note: The NAD83 datum is referenced to the ellipsoid of the Geodetic Reference System of 1980 (GRS80); whereas, the reference ellipsoid for GPS is the World Geodetic System of 1984 (WGS84). The two ellipsoids are effectively the same. (R. B. Langley, GPS World, February 1992; M. C. Grunthal, GPS World, April 1992; A. Leick, GPS Satellite Surveying, 1990; C.R. Schwarz, North American Datum of 1983, NOAA Professional Paper NOS 2, December 1989.) For more information, refer to these and other technical articles/publications.

CALIFORNIA HIGH-ACCURACY NETWORK:

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.

CRUSTAL MOTIONS IN CALIFORNIA:

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.

NGS's ROLE - PAST, FUTURE:

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.

CALIF. GEODETIC CONTROL COMMITTEE:

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.

ISSUES

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The critical horizontal spatial referencing issues challenging California's professional surveyors are as follows:

  1. 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.

  2. 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.

  3. 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.

  4. 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).

  5. 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.

RECOMMENDATION (PROPOSAL)

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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.

CSRS RECOMMENDATION DETAILS

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)

Table of Contents

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.

  1. Name: California Spatial Reference System (CSRS), Horizontal Component.

  2. Definition: The CSRS horizontal competent is a high-accuracy, monumented geodetic spatial-reference network consisting of ...

  3. Status:
  4. Datum: NAD83.
  5. Standards & Specifications (Minimum):
  6. Reference Control Network:
  7. Survey Method: GPS only.
  8. Stations: See Appendix A for a tabulation of the HPGN and proposed HPGN densification stations.

ESTABLISHMENT:

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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.

  1. 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.

  2. Standards, Specifications, Procedures: See "Description", above and the other applicable subsections in this section, "CSRS Recommendation Details".

  3. Who Initiates: Any state/local agency, academic/research institution, or private firm may initiate a HPGN densification survey.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. Non-CSRS, High-Accuracy GPS Surveys: Some high- accuracy GPS surveys will be completed that meet or exceed the requirements for CSRS horizontal control surveys, but the survey data will not be formatted ("blue booked") and submitted to NGS for inclusion in the NGRS (either the Federal or Cooperative Base Network). For these surveys, the California Geodetic Control Committee will encourage the surveyor to file a record of survey so the horizontal control information will be available for use by other surveyors.
  9. References:

DATA PUBLICATION/DISTRIBUTION:

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  1. Epoch Dates:

  2. Data Publication and Distribution:
  3. Crustal Motion Computer Program: NGS shall continue to develop, maintain/update, and distribute (sell) to California surveyors a stand-alone computer program for estimating the effects of crustal motions. This program, which is named "Horizontal Time Dependent Positioning" (HTDP), estimates updated coordinate and/or observation values, horizontal crustal motion velocities, and station displacements. The HTDP program shall be updated (refined) as data from new surveys become available.

    The California Geodetic Control Committee, in cooperation with NGS, will develop guidelines for using the HTDP program.

MAINTENANCE:

Table of Contents

  1. Coordination: The California NGS State Geodetic Advisor shall coordinate the CSRS maintenance efforts.

  2. 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

  3. 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.

  4. 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.

  5. 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).

  6. Crustal Motion Resurveys:

  7. CSRS "Adopt A Monument" Program: To aid in the maintenance efforts for the CSRS, the California Geodetic Control Committee will explore the feasibility of an "Adopt A Monument" program in which agencies, academic/research institutions, firms, and individuals can voluntarily assist in maintaining one or more CSRS stations. The responsibilities of the sponsor for a CSRS station will be as follows:

ADJUSTMENTS:

Table of Contents

The general goal for the CSRS is to minimize the number of changes in published coordinates and epochs.

  1. 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.

  2. 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:

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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:

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.

  1. 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.

  2. In general, CORSs included in the CSRS shall ...

  3. Generally, when CORSs are used to determine the position of unknown points, the unknown positions shall be determined based on the ....
  4. Efforts have been initiated by the CGCC to inventory the CORSs in California. The Committee plans to publish and maintain a document that lists these stations with the applicable information for each station.
  5. In support of the expected expanding use of CORSs, the CGCC will compile information and encourage research into the following:

STATUTES:

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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.

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.

CURRENT STATUS (1/94):

  1. 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.

  2. 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.

  3. 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.

EDUCATION:

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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.


SUMMARY & STATUS

CALIFORNIA SPATIAL REFERENCE SYSTEM - HORIZONTAL

(HPGN and HPGN Densification)

Table of Contents

COUNTY

HPGN
Stations

HPGN-D
Stations

Total
Stations

Aera
sq. mi.

Station
Spacing

Alameda

2

10

12

825

8.3

Alpine

0

5

5

727

12.1

Amador

2

5

7

601

9.3

Butte

2

11

13

1,665

11.3

Calaveras

0

7

7

1,036

12.2

Colusa

1

7

8

780

8.4

Contra Costa

3

8

11

780

8.4

Del Norte

4

6

10

1,003

10.0

El Dorado

2

9

11

1,805

12.8

Fresno

7

32

39

5,998

12.4

Glenn

2

6

8

1,319

12.8

Humbolt

4

22

26

3,600

11.8

Imperial

8

18

26

4,597

13.3

Inyo

11

18

29

10,098

18.7

Kern

10

45

55

8,170

12.2

Kings

2

10

12

1,436

10.9

Lake

2

8

10

1,327

11.5

Lassen

7

18

25

4,690

13.7

Los Angeles

7

47

54

4,079

8.7

Madera

2

9

11

2,147

14.0

Marin

3

7

10

588

7.7

Mariposa

1

5

6

1,461

15.6

Mendocino

5

25

30

3,510

10.8

Merced

2

10

12

2,008

12.9

Modoc

6

10

16

4,340

16.5

Mono

7

12

19

3,103

12.8

Monterey

11

19

30

3,324

10.5

Napa

1

5

6

797

11.5

Nevada

2

7

9

992

10.5

Orange

3

15

8

785

6.6

Placer

4

6

10

1,507

12.3

Plumas

4

8

12

2,618

14.8

Riverside

13

38

51

7,243

11.9

Sacramento

1

8

9

1,015

10.6

San Benito

0

7

7

1,397

14.1

San Bernardino

23

61

84

20,164

15.5

San Diego

9

42

51

4,281

9.2

San Francisco

1

0

1

91

9.5

San Joaquin

3

12

15

1,436

9.8

San Luis Obispo

5

27

32

3,326

10.2

San Mateo

1

6

7

531

8.7

Santa Barbara

7

19

26

2,745

10.3

Santa Clara

4

22

26

1,316

7.1

Santa Cruz

1

5

6

440

8.6

Shasta

8

16

24

3,850

12.7

Sierra

0

7

7

959

11.7

Siskiyou

10

18

28

6,318

15.0

Solano

2

5

7

872

11.2

Sonoma

3

17

20

1,598

8.9

Stanislaus

3

9

12

1,521

11.3

Sutter

1

4

5

607

11.0

Tehama

2

10

12

2,976

15.7

Trinity

3

14

17

3,223

13.8

Tulare

5

17

22

4,845

14.8

Tuolumne

2

9

11

2,293

14.4

Ventura

3

18

21

1,864

9.4

Yolo

1

7

8

1,034

11.4

Yuba

0

4

4

639

12.6

TOTAL

238

832

1,070

158,693

12.2