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WMA 2020.02: Policy on Accurate Geodetic Vertical Datum Establishment and/or Conversion for Continuous Water-Surface Elevation Data for Streamgage, Tide, Lake and Reservoir, and Groundwater-Level Stations
WATER MISSION AREA MEMORANDUM 2020.02 – Technical Policy
Subject: Policy on Accurate Geodetic Vertical Datum Establishment and/or Conversion for Continuous Water-Surface Elevation Data for Streamgage, Tide, Lake and Reservoir, and Groundwater-Level Stations
Purpose: The purpose of this memorandum is to require that all active USGS continuous-record streamgages, tide gages, lake and reservoir sites, and groundwater wells be represented by the most current vertical datum. As of the release of this policy memorandum, the North American Vertical Datum of 1988 (NAVD88) is the most current vertical datum in the contiguous United States and Alaska. Hawaii and United States Territories have other independently defined current vertical datum representations. The desired uncertainty for datum establishment and a timeline to achieve compliance are presented to assure consistency, preparation, and transferability to the future vertical datum identified as the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).
Scope: The scope of this policy is to establish a reliable and consistent process for determining true geodetic elevations for all active USGS continuous-record stations that measure water-surface elevation that is reported and published using the National Water Information System (NWIS) Web interface (NWISWeb). The policy is intended to be in effect for the foreseeable future and it is applicable to all Water Science Centers (WSCs) and Water Mission Area (WMA) Divisions. Determining geodetic elevations at active USGS continuous-record stations shall begin with the release of this policy memorandum and be completed by September 30, 2022. Conformity and execution of this policy is reliant upon WSC Data Chiefs and Field Office Chiefs, WMA Branch Chiefs, and applicable Project Chiefs and Project Managers.
Definitions:
Vertical Datum is a surface of zero elevation to which heights of various points are referenced.
Local USGS Vertical Datum represents a fixed starting point loosely based on the river bed for measuring water levels in riverine systems, often referred to as gage datum, and land surface for measuring groundwater levels.
Geodetic Vertical Datum represents a coordinate system coupled with a reference surface used to provide known locations to begin surveys and create maps.
Current Vertical Datum is the accepted most current representation of land surface elevations as defined by the National Geodetic Survey https://www.ngs.noaa.gov/datums/vertical/ [link or image removed]
Geodetic Vertical Datum Definitions for the Contiguous United States and Alaska (NOAA, 2018b)
National Geodetic Vertical Datum of 1929 (NGVD29) (formerly the Sea Level Datum of 1929) represents a 1929 adjusted vertical-control datum defined by a surface passing through mean sea level at specified points to which elevations determined by leveling are referred.
North American Vertical Datum of 1988 (NAVD88) represents a North-American continent vertical-control datum established in 1991 composed of adjustments of leveling observations from a single origin.
Geodetic Vertical Datum Definitions for Hawaii (NOAA, 2018b)
Local Mean Sea Level (LMSL) tidal datum is used for vertical control.
Geodetic Vertical Datum Definitions for U.S. Territories (NOAA, 2018b)
American Samoa Vertical Datum of 2002 (ASVD02) consists of a leveling network on the island of Tutulia (territory of the America Samoa) affixed to a single origin point.
Guam Vertical Datum of 2004 (GUVD04) consists of a leveling network on the island of Guam affixed to a single origin point.
Northern Marianas Vertical Datum of 2003 (NMVD03) consists of a set of three (3) independent leveling networks on the islands of Rota, Saipan and Tinian (in the commonwealth of the Northern Mariana Islands) affixed to a single origin point on their respective islands referenced to the Saipan tide station.
Puerto Rico Vertical Datum of 2002 (PRVD02) consists of a leveling network on the island of Puerto Rico affixed to a single origin point.
Virgin Islands Vertical Datum of 2009 (VIVD09) consists of a set of three (3) independent leveling networks on the islands of St. Croix, St. John and St. Thomas (in the United States Virgin Islands). Each of these leveling networks is affixed to a single origin point on their respective island.
Background: Water-level data products associated with USGS streamgages, tide gages, lake and reservoir gages, or wells, typically are referenced to Local USGS Vertical Datum that consists of local benchmarks or reference marks that represent a surface used as a zero point for stage, or gage-height measurements. Similarly, USGS groundwater-level information has historically been derived and disseminated as depth below land surface. While these cases represent a local datum expression, other USGS water-level data are based on a reference surface collated with benchmarks and a coordinate system known as a geodetic vertical datum. The two most commonly referenced geodetic vertical datums in the contiguous U.S. and Alaska are the National Geodetic Vertical Datum of 1929 (NGVD29) and the North American Vertical Datum of 1988 (NAVD88). Differences between NGVD29 and NAVD88 range from -1.3 to 4.9 ft. across the contiguous U.S. and 3.1 to 7.9 ft. in Alaska (NOAA, 2018a)
Dissemination of water-level information referenced to a known geodetic vertical datum has become increasingly important in managing the Nation’s waters and minimizing the loss of life and property from natural disasters. As a result, it has become necessary to provide an accurate and current geodetic datum expression that benefits external Federal, State, and Local agencies, along with engineers, academics and others that desire this universally-based datum framework to reference water-level information. Because our data is multi-use by nature, a consistent and accurate datum framework is needed even though some local projects might not see the benefit.
Policy: All active USGS continuous-record streamgages, tide gages, lake and reservoir sites, and groundwater wells that have water-surface elevation data published using NWISWeb must be represented by the most current vertical datum with an uncertainty not to exceed 0.16 ft (0.05 m). This uncertainty is the 2 sigma vertical precision of a Level II survey as outlined by Rydlund and Densmore (2012, table 11) and should be able to be obtained anywhere in the Nation. Compliance with this policy is required to be achieved within three years of the release date of this policy memo (by January 27, 2023). Procedures outlined in Rydlund and Densmore (2012) and Rydlund and Noll (2017) are required to be followed for geodetic vertical datum establishment. Historic locations that are no longer operated are exempt from this policy. However, sites reactivated after the release of this policy will be required to establish the most current geodetic vertical datum.
This policy assures execution of the 1993 directive issued by the Federal Geodetic Control Subcommittee (FGCS) to affirm NAVD 88 as the official civilian vertical datum for surveying and mapping performed or financed by the Federal Government for the contiguous U.S. and Alaska (NOAA, 1993). In addition, this policy assures consistency, alignment, and association with guidance on use and documentation of horizontal and vertical datums in the USGS publication series information products FSP Toolbox, 2016 [link or image removed]
This policy integrates techniques and methods established by the following USGS manuals:
References
Kenney, T.A., 2010, Levels at gaging stations: U.S. Geological Survey Techniques and Methods 3-A19, 60 p. https://pubs.usgs.gov/tm/tm3A19/ [link or image removed]
National Oceanic and Atmospheric Administration [NOAA], 1993, Affirmation of vertical datum for surveying and mapping activities: Federal Register, v. 58, no. 120, p. 34245 [Docket No. 930650-3150].
National Oceanic and Atmospheric Administration [NOAA], 2018a, National Geodetic Survey, VERTCON v2.1, https://www.ngs.noaa.gov/PC_PROD/VERTCON/ [link or image removed]
National Oceanic and Atmospheric Administration [NOAA], 2018b, National Geodetic Survey, Vertical Datums, ngs.infocenter, https://www.ngs.noaa.gov/datums/vertical/ [link or image removed]
Rydlund, P.H., Jr., and Densmore, B.K., 2012, Methods of practice and guidelines for using survey-grade global navigation satellite systems (GNSS) to establish vertical datum in the United States Geological Survey: U.S. Geological Survey Techniques and Methods, book 11, chap. D1, 102 p. with appendixes. https://pubs.usgs.gov/tm/11d1/ [link or image removed]
Rydlund, P.H., Jr., and Noll, M.L., 2017, Vertical datum conversion process for the inland and coastal gage network located in the New England, Mid-Atlantic, and South Atlantic-Gulf hydrologic regions (ver. 1.1, July 2017) U.S. Geological Survey Techniques and Methods, book 11, chap. B8, 29 p., https://doi.org/10.3133/tm11B8. https://pubs.er.usgs.gov/publication/tm11B8 [link or image removed]
Related Procedures
Water Mission Area Procedure Memorandum 2020.03 Guidance on the Display and Handling of Continuous Water-Surface Elevation Data for Streamgage, Tide, Lake and Reservoir, and Groundwater-Level Stations with a Geodetic Vertical Datum [link or image removed]
Appendix
Appendix A – Figure 4, The Datum Conversion Decision Tree and Survey [link or image removed]
Subject: Policy on Accurate Geodetic Vertical Datum Establishment and/or Conversion for Continuous Water-Surface Elevation Data for Streamgage, Tide, Lake and Reservoir, and Groundwater-Level Stations
Purpose: The purpose of this memorandum is to require that all active USGS continuous-record streamgages, tide gages, lake and reservoir sites, and groundwater wells be represented by the most current vertical datum. As of the release of this policy memorandum, the North American Vertical Datum of 1988 (NAVD88) is the most current vertical datum in the contiguous United States and Alaska. Hawaii and United States Territories have other independently defined current vertical datum representations. The desired uncertainty for datum establishment and a timeline to achieve compliance are presented to assure consistency, preparation, and transferability to the future vertical datum identified as the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).
Scope: The scope of this policy is to establish a reliable and consistent process for determining true geodetic elevations for all active USGS continuous-record stations that measure water-surface elevation that is reported and published using the National Water Information System (NWIS) Web interface (NWISWeb). The policy is intended to be in effect for the foreseeable future and it is applicable to all Water Science Centers (WSCs) and Water Mission Area (WMA) Divisions. Determining geodetic elevations at active USGS continuous-record stations shall begin with the release of this policy memorandum and be completed by September 30, 2022. Conformity and execution of this policy is reliant upon WSC Data Chiefs and Field Office Chiefs, WMA Branch Chiefs, and applicable Project Chiefs and Project Managers.
Definitions:
Vertical Datum is a surface of zero elevation to which heights of various points are referenced.
Local USGS Vertical Datum represents a fixed starting point loosely based on the river bed for measuring water levels in riverine systems, often referred to as gage datum, and land surface for measuring groundwater levels.
Geodetic Vertical Datum represents a coordinate system coupled with a reference surface used to provide known locations to begin surveys and create maps.
Current Vertical Datum is the accepted most current representation of land surface elevations as defined by the National Geodetic Survey https://www.ngs.noaa.gov/datums/vertical/ [link or image removed]
Geodetic Vertical Datum Definitions for the Contiguous United States and Alaska (NOAA, 2018b)
National Geodetic Vertical Datum of 1929 (NGVD29) (formerly the Sea Level Datum of 1929) represents a 1929 adjusted vertical-control datum defined by a surface passing through mean sea level at specified points to which elevations determined by leveling are referred.
North American Vertical Datum of 1988 (NAVD88) represents a North-American continent vertical-control datum established in 1991 composed of adjustments of leveling observations from a single origin.
Geodetic Vertical Datum Definitions for Hawaii (NOAA, 2018b)
Local Mean Sea Level (LMSL) tidal datum is used for vertical control.
Geodetic Vertical Datum Definitions for U.S. Territories (NOAA, 2018b)
American Samoa Vertical Datum of 2002 (ASVD02) consists of a leveling network on the island of Tutulia (territory of the America Samoa) affixed to a single origin point.
Guam Vertical Datum of 2004 (GUVD04) consists of a leveling network on the island of Guam affixed to a single origin point.
Northern Marianas Vertical Datum of 2003 (NMVD03) consists of a set of three (3) independent leveling networks on the islands of Rota, Saipan and Tinian (in the commonwealth of the Northern Mariana Islands) affixed to a single origin point on their respective islands referenced to the Saipan tide station.
Puerto Rico Vertical Datum of 2002 (PRVD02) consists of a leveling network on the island of Puerto Rico affixed to a single origin point.
Virgin Islands Vertical Datum of 2009 (VIVD09) consists of a set of three (3) independent leveling networks on the islands of St. Croix, St. John and St. Thomas (in the United States Virgin Islands). Each of these leveling networks is affixed to a single origin point on their respective island.
Background: Water-level data products associated with USGS streamgages, tide gages, lake and reservoir gages, or wells, typically are referenced to Local USGS Vertical Datum that consists of local benchmarks or reference marks that represent a surface used as a zero point for stage, or gage-height measurements. Similarly, USGS groundwater-level information has historically been derived and disseminated as depth below land surface. While these cases represent a local datum expression, other USGS water-level data are based on a reference surface collated with benchmarks and a coordinate system known as a geodetic vertical datum. The two most commonly referenced geodetic vertical datums in the contiguous U.S. and Alaska are the National Geodetic Vertical Datum of 1929 (NGVD29) and the North American Vertical Datum of 1988 (NAVD88). Differences between NGVD29 and NAVD88 range from -1.3 to 4.9 ft. across the contiguous U.S. and 3.1 to 7.9 ft. in Alaska (NOAA, 2018a)
Dissemination of water-level information referenced to a known geodetic vertical datum has become increasingly important in managing the Nation’s waters and minimizing the loss of life and property from natural disasters. As a result, it has become necessary to provide an accurate and current geodetic datum expression that benefits external Federal, State, and Local agencies, along with engineers, academics and others that desire this universally-based datum framework to reference water-level information. Because our data is multi-use by nature, a consistent and accurate datum framework is needed even though some local projects might not see the benefit.
Policy: All active USGS continuous-record streamgages, tide gages, lake and reservoir sites, and groundwater wells that have water-surface elevation data published using NWISWeb must be represented by the most current vertical datum with an uncertainty not to exceed 0.16 ft (0.05 m). This uncertainty is the 2 sigma vertical precision of a Level II survey as outlined by Rydlund and Densmore (2012, table 11) and should be able to be obtained anywhere in the Nation. Compliance with this policy is required to be achieved within three years of the release date of this policy memo (by January 27, 2023). Procedures outlined in Rydlund and Densmore (2012) and Rydlund and Noll (2017) are required to be followed for geodetic vertical datum establishment. Historic locations that are no longer operated are exempt from this policy. However, sites reactivated after the release of this policy will be required to establish the most current geodetic vertical datum.
This policy assures execution of the 1993 directive issued by the Federal Geodetic Control Subcommittee (FGCS) to affirm NAVD 88 as the official civilian vertical datum for surveying and mapping performed or financed by the Federal Government for the contiguous U.S. and Alaska (NOAA, 1993). In addition, this policy assures consistency, alignment, and association with guidance on use and documentation of horizontal and vertical datums in the USGS publication series information products FSP Toolbox, 2016 [link or image removed]
This policy integrates techniques and methods established by the following USGS manuals:
-
- TM 3-A19 [link or image removed], Kenney, T.A., 2010, Levels at gaging stations.
- TM 11-D1 [link or image removed], Rydlund, P.H., Jr., and Densmore, B.K., 2012, Methods of practice and guidelines for using survey-grade global navigation satellite systems (GNSS) to establish vertical datum in the United States Geological Survey.
- TM 11-B8 [link or image removed], Rydlund, P.H, Jr., and Noll, M.L., 2017, Vertical datum conversion process for the inland and coastal gage network located in the New England, Mid-Atlantic, and South Atlantic-Gulf hydrologic regions (ver. 1.1, August 2017).
References
Kenney, T.A., 2010, Levels at gaging stations: U.S. Geological Survey Techniques and Methods 3-A19, 60 p. https://pubs.usgs.gov/tm/tm3A19/ [link or image removed]
National Oceanic and Atmospheric Administration [NOAA], 1993, Affirmation of vertical datum for surveying and mapping activities: Federal Register, v. 58, no. 120, p. 34245 [Docket No. 930650-3150].
National Oceanic and Atmospheric Administration [NOAA], 2018a, National Geodetic Survey, VERTCON v2.1, https://www.ngs.noaa.gov/PC_PROD/VERTCON/ [link or image removed]
National Oceanic and Atmospheric Administration [NOAA], 2018b, National Geodetic Survey, Vertical Datums, ngs.infocenter, https://www.ngs.noaa.gov/datums/vertical/ [link or image removed]
Rydlund, P.H., Jr., and Densmore, B.K., 2012, Methods of practice and guidelines for using survey-grade global navigation satellite systems (GNSS) to establish vertical datum in the United States Geological Survey: U.S. Geological Survey Techniques and Methods, book 11, chap. D1, 102 p. with appendixes. https://pubs.usgs.gov/tm/11d1/ [link or image removed]
Rydlund, P.H., Jr., and Noll, M.L., 2017, Vertical datum conversion process for the inland and coastal gage network located in the New England, Mid-Atlantic, and South Atlantic-Gulf hydrologic regions (ver. 1.1, July 2017) U.S. Geological Survey Techniques and Methods, book 11, chap. B8, 29 p., https://doi.org/10.3133/tm11B8. https://pubs.er.usgs.gov/publication/tm11B8 [link or image removed]
Related Procedures
Water Mission Area Procedure Memorandum 2020.03 Guidance on the Display and Handling of Continuous Water-Surface Elevation Data for Streamgage, Tide, Lake and Reservoir, and Groundwater-Level Stations with a Geodetic Vertical Datum [link or image removed]
Appendix
Appendix A – Figure 4, The Datum Conversion Decision Tree and Survey [link or image removed]