Updating the International Great Lakes Datum

1

• Updating the International Great Lakes Datum
• Plan Overview
• Center for Operational Oceanographic Products and
  Services
• National Geodetic Survey
• Height Modernization Partners Meeting, April 14,
  2015
• Presented by Laura Rear McLaughlin, Mapping and
  Charting Program Manager,
• CO-OPS

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What is the International Great Lakes Datum?

• IGLD can be defined by the following three main
  attributes
• A water level reference zero
• IGLD (1955) - local mean sea level established at
  the tide gauge at Point-au-Pere, Quebec near the
  mouth of the Saint Lawrence River using
  observations from 1941-1956.   
• IGLD (1985) - the reference zero was established
  using a combination of mean sea level data from a
  time period between 1970 and 1988 (a tidal datum
  epoch) from the gauge at Point-au-Pere and a
  newer station 5 km upstream at Rimouski, Quebec.
   
• The reference zero, if any, to be used for IGLD
  (2020) has not yet been established.
• A defined time reference
• IGLD (1955) was established using water level
  observations and survey measurements for a base
  7-year time period of 1952-1958.   
• IGLD (1985) was similarly established using a
  base time period of 1982-1988.  
• IGLD (2020) is envisioned to use the base
  observational time period of 2017-2023.
• A system of elevations defined in terms of
  dynamic heights (as opposed to orthometric
  heights).
• Dynamic heights are geopotential numbers scaled
  by a constant 980.6199 gals which is normal
  gravity computed on the Geodetic Reference System
  of 1980 (GRS 80) ellipsoid at 45 degrees
  latitude. Geopotential numbers are the difference
  in gravity potential measured between the bench
  mark and the geoid.

3
Why update the International Great Lakes Datum?

• Hydrology changes
• Changing water levels over time
• Vertical Crustal Movement
• Uplift in the Northern Lakes
• Vertical land subsidence in the southern lakes
• Variation along the shorelines
• Crustal movement studies indicate vertical
  movement can be neglected over 3-5 year
  timeframe, but overall adjustments should be made
  no more often than once every 20 years
• New Technology for positioning and water level
  collection

4
The International Great Lakes Datum Project Plan

• NOAAs Center for Operational Oceanographic
  Products and Services (CO-OPS) and the National
  Geodetic Survey (NGS) has drafted an IGLD 2020
  update plan
• This plan will be presented for approval to the
  bilateral Coordinating Committee (CC) in May 2015
  
• IGLD is maintained under the auspices of Canada
  and the United States under the CC

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NGS Role in IGLD

• NGS maintains the authoritative expertise of
  geodetic datums in the Lakes
• In FY15 and FY20, collect GNSS observations at
  NWLON bench marks.
• Continue to generate and evaluate accuracy of
  scientific geoid models
• Ensure Lakes GRAV-D data is incorporated into the
  national geoid model.
• Ensure that historical surface gravity
  measurements are consistent with GRAV-D airborne
  data.

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NGS Role in IGLD

• Determine surface gravity data needs
• Collaborate on crustal movement models
• Support transformations for IGLD (1985) and IGLD
  (2020) in the VDatum software.
• Provide tools to users to obtain accurate IGLD
  dynamic heights
• Determine and disseminate a consistent set of
  heights and velocities.
• Determine how existing and future leveling will
  be used.

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CO-OPS Role in IGLD

• CO-OPS maintains water level and datum expertise
  in the Great Lakes
• Determine Low Water Datum (Chart Datum)
• Determine which existing NWLON leveling data to
  Blue book into the NGS database
• Assist NGS with recons and field campaigns for
  GNSS surveys in FY15 and FY20
• Determine the appropriate use of Hydraulic
  Correctors
• Prioritize the list of 140 potential seasonal
  gauges in small ports and harbors.
• Support VDatum transformation models between IGLD
  (1985) and IGLD (2020).

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The International Great Lakes Datum

• FY15 Approach
• CO-OPS to work on three research questions
• How do we prioritize seasonal gauge locations?
• How do we determine Low Water Datum, it has been
  static since 1933?
• Are hydraulic correctors necessary?

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How do we prioritize seasonal gauge locations?

• Approach
• Use vetted prioritization scheme to rank seasonal
  gauges
• Proximity to international nodal points
• Residing in geodetically dynamic area
• Residing in hydraulic corrector dynamic area as
  indicated by Milbert model (NGS)
• Locations identified by USACE high priority
  commercial port
• Locations identified by USACE high priority
  recreational port/harbor of refuge
• Existing infrastructure (ie docks, piers).
• Proximity to NGS leveling line (BMs, tie into
  line to blue book).
• Proximity to high population density

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How do we determine Low Water Datum?

• Low Water Datum (Chart Datum) was determined in
  1933 and has been held constant limited
  documentation on how it was determined
• Elevations assigned to chart datum were updated
  in 1955 and 1985
• LWD will need to be updated relative to IGLD
  (2020) 
• During the recent record low water conditions
  (2012-2013), water levels were often below the
  existing LWD elevations for Superior, Huron and
  Michigan
• Do we redefine Low Water Datum or just assign it
  new numbers based on the IGLD 2020 update?
• Communicate Low Water Datum changes

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Will Hydraulic Correctors be necessary for IGLD
2020 and how would they be calculated

• IGLD (1985) was based on a massive surveying
  effort.
• HCs were used to correct for systematic survey
  errors.  
• Methods of calculating HCs for seasonal stations
  are undocumented.
• NGS Milbert interpolation model is a source for
  HC.
• The Milbert model is known to have inaccuracies.
• IGLD (2020) will rely on a GEOID model GPS
  reference system instead of surveying and may
  remove the need of HCs.
• If HC is required for IGLD (2020), is there a
  better spatial Interpolation model to replace the
  NGS Milbert model for calculating HC.

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The International Great Lakes Datum

• NGS will work on three research questions
• How accurately can Dynamic Heights be computed
  without new leveling or new surface gravity in
  order to support IGLD (2020)
• How accurately can specially-designed,
  high-accuracy GNSS campaign-style surveys
  estimate crustal movement rates in the Great
  Lakes region.
• Can GNSS and geoid models replace leveling for
  water management of the Great Lakes and St.
  Lawrence Seaway?

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The International Great Lakes Datum

• FY15 Field Campaigns
• GPS
• NGS to conduct Coordinated GPS
• campaign in FY15 and FY20 with Canada
• GPS measurements will occur at bench mark
  locations at permanently operating water level
  gauges
• GPS measurements/leveling also at seasonal water
  level gauging sites

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The International Great Lakes Datum

• Water Level Field Campaigns
• Permanent Water Level Gauges
• 53 U.S. water level stations and 34 Canadian
  stations are foundation for IGLD update
• Seasonal Gauges
• CO-OPS has identified about 140 additional
  potential locations for seasonal water level
  gauging
• Funding will determine actual number
• Gauges are deployed for a single four
• month period (June-September) will take
• seven years to complete (2017-2023)

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The International Great Lakes Datum
Possible U.S. Seasonal Gauges
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Other research topics

• What is the reference zero, if any?
• Are Gravity measurements needed at the master
  stations?
• What the Master Station and Nodal Points for each
  Lake/Lake Pair change?
• Will we separate Huron/Michigan they are now
  treated as one Lake pair?
• What does the comparative analysis of past GNSS
  campaigns implicate?
• How will anomalous water level variations at
  Green Bay and Saginaw be handled?
• How will Interconnecting Channel LWD steps be
  handled?
• How will VDatum models in the Lakes be
  incorporated into IGLD?
• Will Lidar and ultrasound altimetry be used for
  estimating water surface in the Great Lakes
  region?
• Will Satellite altimetry be helpful in
  determining the lake levels?

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QUESTIONS?
Laura.Rear.McLaughlin_at_noaa.gov 301-713-2981 x 128