CO-OPS

GPS OBSERVATIONS 

IMPLEMENTATION PLAN

January 2003 

Requirements and Development Division

Center For Operational Oceanographic Products and Services

National Ocean Service

National Oceanic and Atmospheric Administration
TABLE OF CONTENTS

1. INTRODUCTION .......................................................1

2. OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3. IMPLEMENTATION PLAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

4. REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

5. TRAINING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

6. EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

7. SCHEDULE FOR GPS OBSERVATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

8. DATABASE MANAGEMENT AND DATA SUBMISSION .....................5
CO-OPS GPS Observations Implementation Plan January 2003 1

GPS OBSERVATIONS IMPLEMENTATION PLAN

1 INTRODUCTION

In support of the National Geodetic Survey’s (NGS) Height Modernization program, the Federal

Base Network, and Cooperative Base Network, CO-OPS shall initiate a program in 2003 of

making periodic GPS observations at water level stations, as resources permit. CO-OPS

activities shall be coordinated with NGS activities for best use of available resources. 

Precise positioning of NWLON stations in a global geocentric reference framework is needed to

support NOS marine safe navigation and height modernization projects, in addition to

monitoring vertical crustal motion for absolute sea level and global climate change studies.

An ellipsoid height (h) is the distance from a point on the Earth’s surface measured along a line

perpendicular to a mathematically defined reference ellipsoid. GPS is used to measure the

ellipsoid height of a point relative to the reference ellipsoid, which presently is World Geodetic

System 1984 (WGS84). WGS 84 is an earth-centered, earth-fixed coordinate system. Refer to 

DMA Technical Report, Department of Defense World Geodetic System 1984, DMA TR

8350.2, for a definition of WGS 84 and relationships with other geodetic systems. For the

purpose of determining GPS derived orthometric heights, WGS 84 and the North American

Datum of 1983 (NAD 83) are essentially the same datum. This is important to understand when

choosing a geoid model for the orthometric height computation. An orthometric height (H) is

the distance from a point on the Earth’s surface measured along a line perpendicular to a

reference geoid. The difference between the ellipsoid height and the orthometric height is the

geoid height (N). The following simple relationship is used to determine GPS-derived

orthometric heights:

HGPS = h - N

where HGPS is the GPS derived orthometric height, h is the ellipsoid height measured with GPS,

and N is a modeled geoid height using the latest high resolution geoid model, which currently is

GEOID99. GEOID99 supports the direct conversion of NAD 83 ellipsoid heights to NAVD88

orthometric heights. Additional information regarding geoid models or access to NGS website

can be obtained from http://www.ngs.noaa.gov. Differential GPS (DGPS) static and kinematic

surveys are simply referred to as GPS surveys in this document. 
CO-OPS GPS Observations Implementation Plan January 2003 2

2 OBJECTIVES

GPS technology and procedures will be implemented in the operational plan:

 

(1) to support the development of a seamless, geocentric reference system for the acquisition,

management, and archiving of NOS water level data. This will provide a national and

global digital database, which will comply with the minimum geo-spatial metadata

standards of the National Spatial Data Infrastructure (NSDI) and connect the NOS water

level database to the NGS National Spatial Reference System (NSRS);

(2) to establish transformation functions between NOS chart datum (MLLW) and the

geocentric reference system to support NOS 3-dimensional hydrographic surveys and the

implementation of Electronic Chart Display and Information Systems (ECDIS). 

Integration of GPS procedures into CO-OPS PORTS® operations will ensure safe and

efficient navigation and cost-effective water-bourne commerce; 

(3) to support water level datum transfers by using GPS derived orthometric heights;. 

(4) to monitor crustal motions (horizontal and vertical) to support global climate change

investigations.

3 IMPLEMENTATION PLAN

3.1 Implementation of the integration of GPS procedures into CO-OPS NWLON, PORTS®,

hydrographic surveys, and special project programs shall be consistent with and in

conjunction with the existing annual Project Instructions and directives issued by those

programs. The GPS surveys should be scheduled during routine annual maintenance

trips to NWLON or PORTS® stations and during the installation of tertiary water level

stations to support hydrographic and photogrammetric surveys, U.S. Army Corps of

Engineers (USACE) projects, and special purpose surveys. CO-OPS shall continue to

coordinate the GPS occupation of water level network bench marks with NGS, USACE,

and the National Imagery and Mapping Agency (NIMA).

3.2 RDD shall evaluate the GPS work already performed by NGS at water level stations, and

subsequently provide FOD a list of specific stations which shall be observed during the

next inspection.

3.3 GPS-derived orthometric heights can be accurately determined and used for water level

datum transfers according to (a) the established guidelines for 3-D precise relative

positioning to measure ellipsoid heights, (b) properly connecting to several NAVD88

bench marks, and (c) using the latest high-resolution modeled geoid heights for the area

of interest. 

In many remote locations, the use of GPS-derived orthometric heights for datum transfer
CO-OPS GPS Observations Implementation Plan January 2003 3

will be more efficient (timely) and more cost-effective than the use of conventional

differential surveying techniques and may, under certain circumstances, preclude the

installation of additional water level stations to establish a datum.

4 REQUIREMENTS

CO-OPS static GPS observations shall be performed in accordance with the CO-OPS User’s

Guide for GPS Observations, January 2003, for performing GPS observations at water level

stations. NOAA Technical Memorandum NOS NGS-58, Guidelines for Establishing GPSDerived Ellipsoid Heights (Standards 2 cm and 5 cm), Version 4.3 shall also serve as reference

for further details. Rapid static surveys shall be conducted in accordance with the 2 cm

standards outlined in NGS-58 document.

These guidelines are written for establishing GPS derived ellipsoid heights accuracies standards

2 cm for all NWLON, PORTS®, hydrographic surveys, and special project applications.

4.1 Station and Bench Mark Priorities

Priority shall be given to the type of water level station connected with GPS, as well as the type

of mark observed at each station, as provided below.

4.1.1 Static GPS surveys shall be conducted on a minimum of one bench mark at each water

level station, according to the priority levels below. 

1. National Water Level Observation Network (NWLON) and 

PORTS® stations.

2. Long term operating secondary water level stations.

3. New tertiary hydro and special project stations. 

4. Historical subordinate water level stations with an accepted MLLW value

on the current official tidal datum epoch.

4.1.2 Static GPS surveys shall be conducted at water level stations concurrently with the

occupation of NAVD 88 marks, if possible, to accomplish water level datum transfers

using GPS-derived orthometric heights.

4.1.3 The station bench mark selected for GPS observations shall have stability code either A

or B. GPS observations on the PBM are preferred if the PBM has the stability code of A

or B and also if it is suitable for satellite observations. Stability code C and D bench

marks shall not be used for GPS observations unless there are other issues such as

security issues, access, or required bench marks with stability codes A or B are not

available. 

4.1.4 Generally once a mark is selected for GPS observations, future GPS observations shall be
CO-OPS GPS Observations Implementation Plan January 2003 4

done on the same mark. In many states, CO-OPS has provided NGS with lists of selected

marks suitable for GPS observations at water level stations, and NGS has completed

observations on these marks. Some tidal marks designated as Federal Base Network

(FBN) or Cooperative Base Network (CBN) marks may be stability code C. It may be

necessary to select new GPS marks, or set new marks, at some stations to ensure stability

over time.

4.1.5 Additional GPS suitable marks shall also be connected during the static survey using

rapid static GPS procedures to verify bench mark stability, if time and personnel 

resources are available, or for special projects. Priority shall be given to connecting to

the NSRS, particularly to the North American Vertical Datum of 1988 (NAVD 88) bench

marks.

5 TRAINING

CO-OPS shall institute a GPS training program regarding the principles of GPS operations, use

of various manufacturers’ GPS systems for data collection, and at a later date, post-processing

GPS data with in-house and commercial software. The program may be a combination of handson training conducted by NGS and CSDL/HTP, university, and USACE personnel and formal

classroom training conducted by universities, private companies, and GPS manufacturers, as

appropriate.

Data collection training shall include static, rapid static, and real-time kinematic surveying

procedures. Software training shall include static and kinematic solutions using NGS’ PAGES

and KARS, respectively, and commercial software where available.

6 EQUIPMENT

CO-OPS presently has 8 GPS receivers - four Ashtech Z-Surveyors and four Javad Legacy - all

dual frequency GPS receivers and choke ring antennas, with precise GPS fixed height antenna

tripods, as of December 2002. An additional 4 complete GPS systems are needed (total inventory

of 12 systems) to implement this plan and to ensure adequate equipment is available to support

the stated objectives. This equipment shall be allocated to FOD and RDD as necessary to

accomplish the planned work.
CO-OPS GPS Observations Implementation Plan January 2003 5

7 SCHEDULE FOR GPS OBSERVATIONS

CO-OPS conducts scheduled annual maintenance visits to NWLON stations and routine

operation and maintenance visit to PORTS® sites. These maintenance visits typically require

about 2 to 3 days onsite, depending on the extent of maintenance needed. In addition, CO-OPS

personnel provide field support during the installation and removal of tertiary water level

stations for NOS hydrographic and photogrammetric surveys, USACE maintenance dredging

projects, special purpose tide and tidal current projects, and marsh restoration projects. These

type of field operations usually require 3 to 4 days onsite.

CO-OPS plans to conduct the GPS observations either in conjunction with these project

activities, or to perform a series of observations at selected stations independent of the annual

maintenance or installation/removal schedule. Use of contractors may also be considered, as

resources permit. Once a GPS receiver is set up on a bench mark, GPS observations will be

collected automatically and no manual intervention is required. The field crew can perform

other activities such as laser or optical leveling to other bench marks, upgrading the software,

changing DCP boards, and diving, etc, while the GPS receiver is collecting data.

FOD shall consider all the factors regarding selection of a proper bench mark for GPS

observations such as the safety of the GPS receivers (equipment theft), access, stability code of

the bench mark, and non-obstructions of GPS satellites, time available for GPS observations, etc.

To meet the standards for 3-D positioning precision, it may require an additional 1 to 2 days

initially be added to the field party's schedule to complete the GPS work in conjunction with

annual scheduled maintenance visits. As CO-OPS personnel gain experience with GPS data, no

additional time may be needed to do the GPS observations on maintenance visits. In some cases

time and personnel resources limitations may make the independent approach, i.e. excluding the

maintenance visit and hydrographic surveys or special projects install/remove schedule -

regarding GPS observations more practical.

The majority of the NWLON and subordinate station bench marks occupied with GPS have

been accomplished through close coordination with NGS Headquarters and NGS state Geodetic

Advisors. CO-OPS will continue to maintain this invaluable partnership. In fact, the plan

recognizes NGS as the primary means of obtaining GPS training for CO-OPS personnel.

8 DATABASE MANAGEMENT AND DATA SUBMISSION

The raw GPS data collected by CO-OPS personnel during CY 02 has been submitted to NGS for

processing. This arrangement is subject to change if CO-OPS personnel become proficient at

processing the data, or if NGS is no longer able to process CO-OPS data. The following

information on GPS data is therefore provided only as a reference.
CO-OPS GPS Observations Implementation Plan January 2003 6

Data shall be submitted to NGS for archiving in accordance with the Data Submission Section of

NGS-58, Version 4.3 and the Input Formats and Specifications of the National Geodetic Survey

Data Base. It is emphasized that NGS specifications must be fulfilled if the data is to be

processed and accepted into the NGS database.

Conducting GPS observations generates a tremendous volume of data and information. For

example, during a 24-hour static survey using a 30 second epoch update and tracking an average

of 8 satellites, over 2 MB’s of raw GPS data are collected. GPS data are collected in a receiver

manufacturer’s format. The receiver formatted data can be processed if using the manufacturers

software. However, when using NGS PAGES software it is necessary to download the data

from the receiver and convert it to Receiver Independent Exchange Format (RINEX) for

processing. RINEX is a universal GPS format developed to provide easy exchange of GPS data. 

NGS archives both receiver format and RINEX data.

The processed GPS data produces coordinates in a geocentric coordinate system. GPS

coordinates generated from CO-OPS projects shall be latitude, longitude, and ellipsoid heights

and X, Y, and Z in NAD 83 and ITRFxx, respectively. The xx in ITRF refers to the epoch year

of the ITRF coordinates. These data shall be submitted to NGS for approval, and if accepted,

placed in the NSRS. CO-OPS shall archive the ellipsoid heights in the Data Management System

(DMS) data base.