Frequently Asked Questions


To access the list of common IGLD queries, please access the website for the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data.

  1. What is the International Great Lakes Datum?
    1. IGLD is the official reference system, also known as a vertical datum, used to measure water level heights in the Great Lakes, connecting channels, and the St. Lawrence River system. This common reference system used by the United States and Canada is required to collect, compile, and disseminate accurate data related to hydraulics, hydrology, and water levels.
  2. Why does the current IGLD need to be updated?
    1. There are a number of reasons to update the current IGLD (1985). First, the IGLD needs to be updated approximately every 25-30 years to account for movements in the earth’s crust. This movement is caused by glacial isostatic adjustment, which is causing uplift in northern areas of the region and subsidence in the southern areas of the region. Second, an updated IGLD will also allow for the use of new technology to determine the reference surface (i.e. the equipotential surface to which all heights are referenced). Previous iterations of the IGLD (1955 and 1985) relied on geodetic leveling to define the reference surface, which is time consuming, cost-prohibitive, susceptible to accumulation of systematic errors, and makes the datum accessible only where leveling bench marks exist. Third, an update to the IGLD will also allow for better alignment with the new North American-Pacific Geopotential Datum of 2022 (NAPGD2022). NAPGD2022 will rely upon the geoid to calculate its reference surface, which is available throughout the United States as well as Canada and is more consistent and accurate at cm-levels.
  3. Why does the IGLD matter?
    1. Harmonious and bi-national use of the Great Lakes and St. Lawrence River resources require a water level gauge network for measuring water level depths, flows, and volumes. This water level gauge infrastructure is a key component of transportation networks in ports and inland waterways to facilitate trade and recreational boating, hydroelectric and nuclear power generation, domestic and industrial water use, as well as monitoring of the largest freshwater ecosystem in the world.
  4. Who is revising the IGLD?
    1. U.S. Partners:
      • National Oceanic and Atmospheric Administration
      • U.S. Army Corps of Engineers
      • U.S. Geological Survey
    2. Canadian Partners
      • Canadian Hydrographic Service
      • Canadian Geodetic Survey
      • Environment and Climate Change Canada
  5. I am a professional mariner or recreational boater. Will my navigational products change after the IGLD update?
    1. Yes, most likely. A new IGLD will have impacts on navigation. Low Water Datums (chart datum) will need to be changed on nautical charts from IGLD (1985) to IGLD (2020). Additionally, chart depths may need to be changed and under-keel clearance may be affected. Low Water Datums at connecting channels and the St. Lawrence River gauging stations, along with those locations used for dredging, will need to be re-determined.
  6. I am a shoreline use planner. Will the new IGLD impact my work?
    1. Shoreline use permits in both the United States and Canada will need to be referenced to IGLD (2020) because lake levels, station bench marks, and high water marks will be based upon the new datum. This means that monthly water level bulletins and weekly water level forecasts published by the U.S. Army Corps of Engineers, the Environment and Climate Change Canada, and the Canadian Hydrographic Service will also be based on the new datum.
  7. How will the new IGLD impact water managers in the region?
    1. The IGLD will necessitate the:
      • Update of historic water level records and lake level forecasting products to new datums
      • Update of stage-discharge rating equations and other supporting models/tools used to calculate lake outflows, lake and connecting channel hydrodynamic and routing models
      • Update of hydroelectric rating tables
      • Update of water supply information
      • Update of heights of power entities’ and Seaway authorities’ water level gauges for flow determination and regulation planning
      • Update of water level information in publications and other communications
      • Determination of effects on infrastructure such as municipal water intakes and nuclear power station water cooling systems
  8. When will IGLD (2020) be released?
    1. IGLD (2020) is expected to be adopted in the U.S. by 2027.
  9. What is the difference between IGLD and LWD?
    1. International Great Lakes Datum (IGLD) is the vertical reference for water levels in the Great Lakes and connecting channels.
    2. Low Water Datum (LWD) is chart datum that is referenced to IGLD.
    3. To learn more about IGLD and LWD, please visit the Coordinating Committee website here.