WMA 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 an Accurate Geodetic Vertical Datum

WATER MISSION AREA MEMORANDUM 2020.03 – Technical Procedure

Subject: Guidance on the Display and Handling of Continuous Water-Surface Elevation Data for Streamgage, Tide, Lake and Reservoir, and Groundwater-Level Stations with an Accurate Geodetic Vertical Datum

Purpose: The procedures in this memorandum provide guidance on the display and handling of continuous water-surface elevation data for sites with an established Geodetic Vertical Datum. These procedures support Water Mission Area (WMA) policy 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.” These procedures consist of three options for displaying continuous water-surface elevation data on the National Water Information System Web interface (NWISWeb) given the current state (2019) of our computational and data dissemination systems; AQUARIUS and NWISWeb, respectively. Included with these procedures are associated considerations that must be made and minimum actions that are required for each option. The three options ensure that uniform approaches are followed that meet the varied needs of end users of continuous water-surface elevation data. Having uniform approaches will allow for easier migration of continuous water-surface elevation data into future enhanced computational and data dissemination systems.       

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 a gage height of zero flow or 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]

 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 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 Mission Area (WMA) policy 2020.02 requires all active USGS continuous-record streamgages, tide gages, lake and reservoir sites, and groundwater wells that have water-surface elevation data published using NWISWeb to be represented by the most current vertical datum with an uncertainty not to exceed 0.16 ft. The procedures outlined here support this requirement by providing guidance on the display and handling of continuous water-surface elevation data on NWISWeb following the establishment and/or conversion to the most current vertical datum, as defined by the National Oceanic and Atmospheric Administration (2018) here: https://www.ngs.noaa.gov/datums/vertical/ [link or image removed] .

Procedures:  For continuous-record streamgage, tide, lake and reservoir, and groundwater-level stations that have Geodetic Vertical Datum established, one of the following three options for displaying and handling the data must be implemented. The option selected for a particular station must consider local data-user needs. Options are presented below in no particular order. Following the three options is guidance on how to document Geodetic Vertical Datum establishment.

Option 1: Station to only report water-surface elevations in the current Geodetic Vertical Datum

No computational use or NWISWeb display of local USGS vertical datums.

• • For stream stage: Parameter code 00065 Gage height, in feet, would be reporting stage values as real-world elevations in NAVD 1988, in feet, station no longer uses local USGS vertical datum.
  • For tide gage: Parameter code 62020, Estuary or ocean water surface elevation above NAVD 1988, in feet, would be reporting water-surface values in real-world elevation, station no longer uses local USGS vertical datum.
  • For lake and reservoir elevation: Parameter code 62615 Lake or reservoir water surface elevation above NAVD 1988, in feet, would be reporting water-surface values in real-world elevation, station no longer uses local USGS vertical datum.
  • For groundwater-level: Parameter code 62611 Groundwater level above NAVD 1988, in feet, would be reporting water-level values in real-world elevation, station no longer uses local USGS vertical datum.

 

Considerations for selecting option 1:

• • Approach leads to a clean display of a single water-surface elevation time series.
  • Satisfies end users who desire, or require for simplicity, only one water-surface elevation time series. For example, a cooperator makes this request or there is a need to make the display simpler for viewing by non-technical public users.
  • If the station is likely to be affected by subsidence, then the real-world elevation of the site will likely change. In choosing this option, such sites will require a robust plan to capture and correct for elevation changes. This scenario may lead to revisions in published water-surface elevation data.
  • Creates a discontinuity in the period of record of water-surface elevation time series at the moment it is changed to real-world elevation.
  • Many required minimum actions are needed (see below)
  • The station must remain in the same datum and realization of datum in perpetuity or further datum conversions will be required in the future.

Required minimum actions to take for converted active stations:

• • Stream Stage Sites:
    • Discuss intention to make conversion with local cooperators and local users
    • Follow 120-day National Weather Service notification procedures outlined in Rydlund and Noll (2017)
    • Notify USGS Water Alert to ensure thresholds are properly adjusted and subscribers are notified
    • Reference gages need to be reset to allow for a large round number (i.e. 10’s or 100’s) to be added to the gage reading to obtain real-world elevation gage height observations
    • Closure level run in the local USGS vertical datum which would initiate the real-world datum period
    • Update stage thresholds in AQUARIUS
    • For future rating development, duplicate a minimum of 10 of the highest discharge measurements that still represent current channel conditions:
      • Use the same measurement number with the added alpha character “Z”
      • Change the associated gage height to the equivalent real-world water surface elevation
      • Set AQUARIUS Visit dispatch options as follows:
        • Select: I want to see these data in Rating Development Toolbox to help me build curves
        • Unselect: I want to see these data in Data Correction Toolbox to help me correct my time series
        • Unselect: I want these data published to external systems
      • Re-develop the active rating curve and activate it at the time of conversion
      • Add “datum changed” qualification code to peak-stage value for the first annual peak of the real-world datum period.
      • If conversion happens at any time other than the water year boundary understand the auto selection of the annual peak stage value may be incorrect.
      • If subsidence potential exists at site, develop and adhere to strict protocols for tracking changes to real world elevations and correcting the time series.
      • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
      • If station is part of the Flood Inundation Map (FIM) program, before converting discuss this with local data users and the FIM program management
    • Tide Gage sites:
      • Discuss intention to make conversion with local cooperators and local users
      • Follow 120-day National Weather Service notification procedures outlined in Rydlund and Noll (2017)
      • Reference gages need to be reset to allow for a large round number (i.e. 10’s or 100’s) to be added to the gage reading to obtain real-world elevation observations
      • Closure level run in the local USGS vertical datum which would initiate the real-world datum period
      • Update stage thresholds in AQUARIUS
      • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
      • If subsidence potential exists at site develop and adhere to strict protocols for tracking changes to real world elevations and correcting the time series.
    • Lake and Reservoir sites:
      • Discuss intention to make conversion with local cooperators and local users
      • Reference gages need to be reset to allow for a large round number (i.e. 10’s or 100’s) to be added to the gage reading to obtain real-world elevation gage height observations
      • Closure level run in the local USGS vertical datum which would initiate the real-world datum period
      • Update stage thresholds in AQUARIUS
      • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
      • If subsidence potential exists at site develop and adhere to strict protocols for tracking changes to real world elevations and correcting the time series.
    • Groundwater-level sites:
      • Discuss intention to make conversion with local cooperators and local users
      • Close-out verification of measuring point (MP) distance above land surface
      • Reset MP to real-world elevation
      • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
      • If subsidence potential exists at site develop and adhere to strict protocols for tracking changes to real world elevations and correcting the time series.

Option 2: Station to have two water-surface elevation time series; Geodetic Vertical Datum and Local USGS Vertical Datum.

The Local USGS Vertical Datum would be used computationally and real-world elevation would be computed from that local datum. Both time series are displayed on NWISWeb.

• • For stream stage: Parameter code 00065 Gage height, in feet, would be reporting stage values in the local USGS vertical datum, and in the contiguous U.S. and Alaska, Parameter code 63160 Stream Level above NAVD 1988, in feet, would be reporting real-world water-surface elevation.
  • For tide gage: Parameter code 00065 Gage height, in feet, would be reporting stage values in the local USGS vertical datum, and in the contiguous U.S. and Alaska, Parameter code 62020, Estuary or ocean water surface elevation above NAVD 1988, in feet, would be reporting water-surface values in real-world elevation.
  • For lake and reservoir elevation: Parameter codes 00065 Gage Height, in feet, or 00062 Elevation of reservoir water surface above datum, in feet, would be reporting water-surface elevations in the local USGS vertical datum, and in the contiguous U.S. and Alaska, Parameter code 62615 Lake or reservoir water surface elevation above NAVD 1988, in feet, would be reporting real-world water-surface elevation.
  • For groundwater-level: Parameter code 72019 Depth to water level, feet below land surface would be reporting water level in the Local USGS vertical datum, and in the contiguous U.S. and Alaska, Parameter code 62611 Groundwater level above NAVD 1988, in feet, would be reporting water-level values in real-world elevation.

Considerations for selecting option 2:

• • Multiple displays of the same water-surface elevation data, can be somewhat confusing to public
  • Provides display of real-world elevation time-series data with minimal extra work
  • For streamgages “measured stage” symbol for gage heights associated with a discharge measurement only shows up on gage datum time series as it is tied to Parameter code 00065.
  • While the entire period of record can be computed and shown in real-world elevation time series, unless there is some specific reason or data request to provide real-world elevation time-series data prior to the date the high-accuracy datum was established, it is recommended to only provide real-world elevation data from the date that the datum was established forward.
  • Need to develop a procedure for approval of the real-world elevation time series; if subsidence potential exists this is best to be done from one datum check to another (i.e. when benchmarks are re-occupied). If subsidence potential does not exist, approval of elevation time series can be approved at the same rate as the gage height time series
  • This approach provides some flexibility to account for past and future datum realizations (i.e. NAD83 CORS96 Epoch 2002 vs. NAD83 2011 Epoch 2010, vs. NAPGD2022). However, it is not recommended to change realizations.

Required minimum actions to take for active stations:

• Stream Stage sites:
  • Create water-surface elevation time series in AQUARIUS, Parameter code 63160 Stream Level above NAVD 1988, in feet, computed from the stage time series via either a “reference datum conversion” processor, or a dependent parameter rating and publish this to NWISWeb. Configure NWISWeb display to show this new water-surface elevation time series along with Parameter code 00065 Stage, in feet.
  • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
  • If subsidence potential exists at site develop and adhere to strict protocols for tracking changes to real-world elevations and correcting the elevation time series.
• Tide Gage sites:
  • Create water-surface elevation time series in AQUARIUS, Estuary or ocean water-surface elevation above NAVD 1988, in feet, computed from the stage time series via a “reference datum conversion” processor, and publish this to NWISWeb. Configure NWISWeb display to show this new water-surface elevation time series along with Parameter code 00065 Stage, in feet.
  • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
  • If subsidence potential exists at site develop and adhere to strict protocols for tracking changes to real-world elevations and correcting the elevation time series.
• Lake and Reservoir sites:
  • Create water-surface elevation time series in AQUARIUS, Parameter code 62615 Lake or reservoir water surface elevation above NAVD 1988, in feet, computed from the local USGS vertical datum time series, and publish this to NWISWeb. Configure NWISWeb display to show this new water-surface elevation time series along with currently used Parameter code, likely either 00065 Gage Height, in feet, or 00062 Elevation of reservoir water surface above datum, in feet.
  • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
  • If subsidence potential exists at site develop and adhere to strict protocols for tracking changes to real-world elevations and correcting the elevation time series.
• Groundwater-level sites:
  • Create water-surface elevation time series in AQUARIUS, Parameter code 62611 Groundwater level above NAVD 1988, in feet, computed from the local USGS vertical datum time series, and publish this to NWISWeb. Configure NWISWeb display to show this new water-surface elevation time series along with Parameter code 72019 Depth to water level, feet below land surface.
  • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
  • If subsidence potential exists at site develop and adhere to strict protocols for tracking changes to real-world elevations and correcting the elevation time series.

Option 3: Station to only report water-surface elevations in Local USGS Vertical Datum. Water-surface elevation data will only be displayed in Local USGS Vertical Datums. A statement describing how a conversion factor is to be applied to the local USGS vertical datum to obtain water-surface elevations in the Current Geodetic Vertical Datum must be provided on the NWISWeb station page containing the water-surface elevation time series.

• • For stream stage: Parameter code 00065 Gage height, in feet, would be reporting stage values in local USGS vertical datum.
  • For tide gage: Parameter code 00065 Gage height, in feet, would be reporting stage values in local USGS vertical datum.
  • For lake and reservoir elevation: Parameter codes 00065 Gage Height, in feet, or 00062 Elevation of reservoir water surface above datum, in feet, would be reporting water-surface elevations in the local USGS vertical datum.
  • For groundwater-level: Parameter code 72019 Depth to water level, feet below land surface, would be reporting water-level values in local USGS vertical datum.

Considerations for selecting option 3:

• • Water-surface elevation time series not provided in real-world datum
  • End users have to do their own computations to obtain water-surface elevation in real-world datum by following the recommended statements below.

Required minimum actions to take for converted active stations:

• • Stream Stage sites:
    • Add statement to NWISWeb banner on station page where time-series data is displayed. Example statement provided:
      • “The elevation of this station has been determined to be XXXX.XX feet above NAVD 1988. To obtain stream water-surface elevation values in NAVD 1988 add XXXX.XX feet to gage height values.”
    • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
  • Tide Gage sites:
    • Add statement to NWISWeb banner on station page where time-series data is displayed. Example statement provided:
      • “The elevation of this station has been determined to be XXXX.XX feet above NAVD 1988. To obtain tide water-surface elevation values in NAVD 1988 add XXXX.XX feet to gage height values.”
    • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
  • Lake and Reservoir sites:
    • Add statement to NWISWeb banner on station page where time-series data is displayed. Example statement provided:
      • “The elevation of this station has been determined to be XXXX.XX feet above NAVD 1988. To obtain reservoir water-surface elevations in NAVD 1988 add XXXX.XX feet to gage height values.”
    • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document
  • Groundwater-level sites:
    • Add statement to NWISWeb banner on station page where time-series data is displayed. Example statement provided:
      • “The elevation of this station has been determined to be XXXX.XX feet above NAVD 1988. To obtain groundwater elevation values in NAVD 1988 add XXXX.XX feet to water-level values.”
    • Update Station Description “Origin and History of Datum" element and the datum statement in the Manuscript document

Documentation Requirements: 

1. 1. Populate Altitude History table in GWSI as per Section 9 of the Site File and Groundwater Site-Inventory Coding Instructions of the User’s Manual for the National Water Information System of the U.S. Geological Survey (USGS, 2016). Section 9 is provided in Appendix A.
   2. Populate / Update GWSI site information (see Rydlund and Noll, 2017)
      • Altitude
      • Accuracy
      • Method
      • Datum
   3. Update Station Description “Origin and History of Datum” element.
      • Example wording:
        • “Datum of gage is 59.92 ft above NAVD of 1988. The NAVD of 1988 datum was determined by a level 2 GNSS survey (Reference Frame NAD_83 (2011) (EPOCH:2010.0000), GEOID12B) by USGS on January 15, 2015 and tied to RM 2. Prior to January 15, 2015, the datum of gage was 62 ft above NGVD of 1929 as determined via topographic map by USGS on October 3, 1965.”
      • For further details on the required information and examples for different scenarios, see Appendix B.
   4. Archive important information associated with the determination of real-world elevation for the site. This should be addressed in Center’s Quality Assurance and / or Data Management Plans. See Appendix A for recommended archival procedures.
   5. If during a datum conversion an old, previously used datum is found to be in error (outside of the assigned accuracy of that datum as defined in the sitefile in GWSI) a revision of the old datum and associated revision statement must be added the station manuscript.

References

National Oceanic and Atmospheric Administration [NOAA], 2018, 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]

U.S. Geological Survey, 2016, User’s Manual for the National Water Information System of the U.S. Geological Survey: Groundwater Site-Inventory System (ver. 5.2): Open File Report, 307 p. http://nwis.usgs.gov/nwisdocs5_2/gw/GWSI_complete.pdf [link or image removed]

Appendixes [link or image removed]

Appendix A - GWSI: 2 -- Section 9. Altitude Datum Components and their History

Appendix B - Recommended Documentation and Archival Procedures for Datum Establishment and Conversions at Continuous-Record; Streamgages, Lake and Reservoir sites, and Groundwater wells

Appendix C - Archived Materials for 04240300

Appendix D - Archived Materials for 04224000

Appendix E - Archived Materials for 043205040

Appendix F - Archived Materials for 04235440