1. International Great Lakes Datum Overview Presented at a Height Modernization Program meeting January 9, 2014 by
David Conner
Geodetic Advisor to the State of Ohio
National Geodetic Survey, NOAA

2. NGS and the NSRS Who are those guys? And what is CO-OPS?
NOAA's National Geodetic Survey (NGS) defines and manages a National coordinate system which includes NAVD88 and NAD83. This network, the National Spatial Reference System (NSRS), provides the foundation for transportation and communication; mapping and charting; and a multitude of scientific and engineering applications.
NGS was known as the US Coast and Geodetic Survey (USC&GS) until 1970 when it was renamed to become part of the newly formed National Oceanic and Atmospheric Administration (NOAA). NOAA is within the US Dept of Commerce. NGS is within NOAA’s National Ocean Service (NOS).
NOS’s Center for Operational Oceanographic Products and Services (CO-OPS) is responsible for monitoring tides, water levels and currents. They maintain a network of 53 water level gauges throughout the Great Lakes and their connecting channels. They are responsible for determining heights associated with the International Great Lakes Datum (IGLD), which they do in cooperation with NGS and our International partners in Canada.
Prior to the formation of NOAA the U.S. Lake Survey (USLS) was responsible for the network of water level gauges in the Great Lakes. USLS was established in 1841 and operated under the supervision of the US Army Corps of Engineers (USACE), first out of Buffalo, NY, then Detroit, MI. When NOAA was formed in 1970 many of the USLS functions were transferred to it.
U.S. Lake Survey timeline
1970 – Nixon White House announces Reorganization Plan No.4 of 1970 establishing NOAA. On October 3, 1970 the Lake Survey was redesignated the Lake Survey Center and officially transferred to NOAA, becoming part of the National Ocean Survey, the former Coast and Geodetic Survey.
Certain Lake survey elements remained with COE … lake regulation and hydraulic branches of the Engineering Division transferred to Detroit District COE
1974 – NOAA Great Lakes Environmental Research Laboratory (GLERL) takes over Lake Survey limnology and research operations .

3. IGLD … what is it?
The IGLD is a dynamic height system. Dynamic heights, by definition, define a water level surface. They are the base height system used for IGLD 1985 in the Great Lakes. Dynamic heights are the reference system of choice for large hydraulic systems everywhere.
NAVD 88 heights are typically given as orthometric heights, although they can also be expressed as dynamic heights.
Orthometric heights generally do not define a water level surface.
IGLD heights are corrected NAVD 88 dynamic heights.
Hydraulic correctors (HC) are applied to NAVD 88 dynamic heights to make them consistent with observed water levels on the Great Lakes. HC’s are the difference in the mean water level for the reference period (7 year) between a subordinate gauge and the primary gauge on a lake.
Hydraulic correctors are typically less than a few cm but can exceed 10 cm ( cm on Lake Superior, cm on Lake Michigan).
USLS … The US Lake Survey was established in 1841 and operated under the supervision of the US Army Corps of Engineers (USACE).
When NOAA was formed in 1970 many of the USLS functions were transferred to it. The USACE continues to play a major role

4. IGLD … “I” is for International!
Management of these waters is governed by International Treaty administered by the International Joint Commission
The Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data meets twice each year and recently held its 95th meeting. It is made up of US and Canadian scientific agency representatives.
Published water level data are coordinated by government agencies in both countries to ensure there is agreement.
Coordinated basic hydraulic and hydrologic data are required to solve numerous international problems related to joint use of International waters
Management of these waters is governed by the Boundary Waters Treaty of 1909 between the US and Canada. This treaty is administered by the International Joint Commission which has representatives from both countries.

5. IGLD … Why is it important?
The Great Lakes are a vast hydraulic system with water levels and flows influenced by engineered channels and control structures.
The entire region is tilting due to post-glacial rebound (aka glacial isostatic adjustment … GIA)
The absolute rate of change at a given location must be considered along with the relative rate of change across the region. The Great Lakes basin is tilting at a rate of about 60 cm
(~ two feet) per century.
The Great Lakes-St Lawrence River System reaches almost halfway across the North American continent.

6. The Great Lakes-St Lawrence River System reaches almost halfway across the North American continent.

7. National Spatial Reference System (NSRS)
Cleveland Water Level Gauge and GPS CORS
The leveling network and water level observation network intersect at the gauges, increasingly co-located with GNSS CORS
U.S. = 53 gauges, 13 with CORS
Canada = 33 gauges, 9 with CORS
Why co-locate a GNSS CORS with a water level gauge? See next slide …
First NOAA CORS at a Great Lakes WLG … BFNY 2002, HBCH 2003

8. Vertical rate of change due to Post-Glacial Rebound
AKA Glacial Isostatic Adjustment (GIA). Line of zero change is approx at Lake Erie. North of zero there is uplift, south of zero there is downward movement.
Datum transformation software NAVD88 – IGLD
The above image is from “Present-day tilting of the Great Lakes region based on water level gauges” by André Mainville and
Michael R. Craymer ftp://geod.nrcan.gc.ca/pub/GSD/craymer/pubs/grlakes_gsab2005.pdf
International Great Lakes Datum of 1985 (IGLD 1985), IGLD 2020 is coming

9. IGLD … Why is it useful?
It is a dynamic height system which can be used to relate various data to a common water level surface
It provides a mechanism for the US and Canada to reference heights to a common datum throughout the Great Lakes region
Each country has adopted other vertical datums for general use.
- The US vertical datum is NAVD 88.
- The Canadian vertical datum was CGVD 28 until it adopted CGVD 2013 in November 2013
Various other vertical datums have been used in the past …
CGVD28, NAVD88, Mean Tide at New York, Mean Tide New York 1929 ?, MSL, NGVD29, USLS 1903, USLS 1935 IGLD55, IGLD 1985, IGLD 2020?
More info at “North American Vertical Datum and International Great Lakes Datum:
They Are Now One and the Same: by David Zilkoski … with particular attention to pages 4-7 “HISTORY OF GREAT LAKES VERTICAL CONTROL NETWORKS”

10. IGLD … When/where is it used?
It is used for basic hydraulic and hydrologic data related to engineering and scientific works on or near Great Lakes waters in both the US and Canada
These works include things related to navigation and management of water … charts, dredging, erosion, marine boundary determination, bridges, locks, dams, sewage and drainages systems, nuclear power plant water intakes and much more.

11. IGLD … Why is there more than one?
The IGLD is revised about every 30 years to remove the effect of GIA from published heights.
IGLD 1985 is the current datum. It was preceded by IGLD 1955, USLS 1935, USLS 1903 and various other datums
Goal: IGLD 2020 released to the public in 2025
GIA is the most significant source of vertical change in North America, WRT size of the affected area.

12. IGLD… How are IGLD heights made available?
Dynamic heights are published by NGS on all bench mark datasheets
IGLD 1985 heights are not displayed on NGS’ datasheets
Hydraulic correctors are determined by CO-OPS
NOS provided software can be used to convert NAVD88 heights to IGLD 1985 heights. This software applies a hydraulic corrector when appropriate.
IGLD 1985 software *
VDatum software
Note: Vdatum IGLD 1985 function currently has a bug, with a fix in the works
IGLD water levels are found on the CO-OPS water level web page at
IGLD 55 heights are not readily available. There is no datum transformation software for IGLD 55, same for USLS 1903 and USLS 1935
Data requests including IGLD 55, USLS 1903 and USLS 1935 should be directed to CO-OPS
NOAA, NOS, Center for Operational Oceanographic Products and Services
User Services: (301)

13. The lower portion of this page is the output from the IGLD 1985 datum transformation software

14. IGLD 1985 heights… important details
Hydraulic correctors are applied to dynamic heights at locations on or very near the lake, which is assumed to be a flat surface
Hydraulic correctors are not applied on rivers and connecting channels, which are sloping surfaces
On sloping water surfaces the IGLD 1985 height and NAVD 88 dynamic height are one and the same

15. IGLD 2020 planning
IGLD 2020 will be done after adoption of a new geoid-based US vertical datum in 2022
Intent is to co-define IGLD 2020 with the new datum
Vertical velocity model will be partly based on IGLD GNSS campaign surveys … 1997, 2005, 2010, 2015 and Planning for a 2015 GNSS campaign has begun.

16. Further Information
NOAA, NOS, National Geodetic Survey
NOAA, NOS, Center for Operational Oceanographic Products and Services
Data requests including IGLD 1955 and USLS 1935 contact:
User Services: (301)