Updating the International Great Lakes Datum Plan Overview

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

2. 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

5. 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.

6. 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.

7. 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).

8. 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?

9. 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

10. 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

11. 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.

12. 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?

13. 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

14. 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);

15. The International Great Lakes Datum Possible U.S. Seasonal Gauges

16. 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?

17. QUESTIONS? Laura.Rear.McLaughlin@noaa.gov 301-713-2981 x 128