Overview of Datums Commonly Used in Michigan and the National Spatial Reference System Michigan Society of Professional Surveyors 70 th Annual Meeting Frankenmuth, MI February 24 th 2011

DATUMS

GEODETIC DATUMS A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating coordinates of points on the Earth. Specific geodetic datums are usually given distinctive names. (e.g., North American Datum of 1983, European Datum 1950, National Geodetic Vertical Datum of 1929) Characterized by: A set of physical monuments, related by survey measurements and resulting coordinates (horizontal and/or vertical) for those monuments A set of physical monuments, related by survey measurements and resulting coordinates (horizontal and/or vertical) for those monuments

GEODETIC DATUMS HORIZONTAL 2-D (Latitude and Longitude) (e.g. NAD 27, NAD 83 (1986)) VERTICAL/GEOPOTENTIAL 1-D (Orthometric Height) (e.g. NGVD 29, NAVD 88, Local Tidal) GEOMETRIC 3-D (Latitude, Longitude and Ellipsoid Height) Fixed and Stable(?) - Coordinates seldom change (e.g. NAD 83 (1993), NAD 83 (2007)) also 4-D (Latitude, Longitude, Ellipsoid Height, Velocities) Coordinates change with time (e.g. ITRF00, ITRF08)

The IGLD is a dynamic height system. Dynamic heights, by definition, define a water level surface. Their use is not limited to the Great Lakes. They are useful for large hydraulic systems everywhere. IGLD 85 is the current version of the Great Lakes datum NAVD 88 heights are orthometric heights. Orthometric heights do not define a water level surface. IGLD heights are corrected dynamic heights. Hydraulic correctors (HC) are applied to dynamic heights to remove various observational and other inconsistencies in the NSRS. HC’s are based on water levels observed at the gauges. IGLD … what is it?

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 overall Great Lakes basin is tilting at a rate of about 60 cm (two) feet per century.

IGLD … Why is it useful? Because it is a dynamic height system, it 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 Each country has adopted other vertical datums for general use. The Canadian vertical datum is CGVD 28. The US vertical datum is NAVD 88.

IGLD … Why is there more than one? IGLD 85 is the current datum. It was preceded by IGLD 55. Efforts are underway now to compute IGLD 2015 The IGLD is revised about every 30 years to remove the effect of GIA from published heights.

IGLD … How are IGLD heights made available? Dynamic heights are published by NGS on all bench mark datasheets IGLD 85 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 85 heights. This software applies a hydraulic corrector.

IGLD Transformation Tools VDATUM – Performs transformations between NAVD 88 and IGLD 85 IGLD85 – Currently not available to users. Issues with both programs

NAVD 88 to IGLD 85 HC are not applied to sloping surfaces, use the Dynamic Height

IGLD 85 heights “ some important details” Hydraulic correctors are applied to dynamic heights at locations on or very near the lake surface, which is assumed to be flat. Hydraulic correctors are not applied on rivers and connecting channels, which are sloping water surfaces. On sloping water surfaces the IGLD 85 height and dynamic height are one and the same

ELLIPSOID - GEOID RELATIONSHIP H = Orthometric Height (NAVD 88) H = h – N h = Ellipsoidal Height [NAD 83 (199x), NAD 83 (2007), NAD 83 (CORS96)] N = Geoid Height (GEOID 09) H h Ellipsoid GRS80 N Geoid GEOID09

The National Geodetic Vertical Datum of 1929 is referenced to 26 tide gauges in the US and Canada

The North American Vertical Datum of 1988 is referenced to a single tide gauge in Canada

4 cm 125 cm 70 cm 85 cm 102 cm NGVD 29 Referenced to 26 Tide Gages NAVD 88 Referenced to 1 Tide Gage (Father’s Point) NAVD88 minus LMSL( ) -23 cm -11 cm

Problems using traditional leveling to define a NEW National Vertical Datum Leveling the country can not be done again –Too costly in time and money Leveling yields cross-country error build-up; problems in the mountains Leveling requires leaving behind passive marks –Bulldozers and crustal motion do their worst

Definitions: GEOIDS versus GEOID HEIGHTS “The equipotential surface of the Earth’s gravity field which best fits, in the least squares sense, (global) mean sea level.”* Can’t see the surface or measure it directly. Can be modeled from gravity data as they are mathematically related. Note that the geoid is a vertical datum surface. A geoid height is the separation between an ellipsoidal datum to the geoid. *Definition from the Geodetic Glossary, September 1986

Official NGS policy as of Nov 14, 2007 –$38.5M over 10 years Airborne Gravity Snapshot Absolute Gravity Tracking Re-define the Vertical Datum of the USA by 2018 Transition to the Future – GRAV-D Gravity for the Redefinition of the American Vertical Datum

Gravity Survey Plan National Scale Part 1 –Predominantly through airborne gravity –With Absolute Gravity for ties and checks –Relative Gravity for expanding local regions where airborne shows significant mismatch with existing terrestrial

Finished up work in Alaska for this year and will be working along California Coast during the winter.

CONTINUOUSLY OPERATING REFERENCE STATIONS (CORS) Installed and operated by more than 200 Federal-State-Local Govt and private partners NOAA/National Geodetic Survey NOAA/OAR Global Systems Division U.S. Coast Guard - DGPS/NDGPS Corps of Engineers - DGPS FAA - WAAS/LAAS State Department Of Transportations County and City Academia Private Companies

CONTINUOUSLY OPERATING REFERENCE STATIONS (CORS) NGS PROVIDES Horizontal and Vertical NSRS Connections NAD 83 and ITRF00 Coordinates Network Data Collection - Hourly & Daily Daily 3D Network Integrity Adjustment Public Data Distribution - Internet 16 Year On-Line Data Holding

As of 12/09/10

OPUS On-Line Positioning User Service Processes GPS data Global availability (masked) 3 goals: –Simplicity –Consistency –Reliability

You’ve got mail! OPUS solution

CORS solution 1 CORS solution 2 How does OPUS compute your position? 3 single baselines computed 3 positions averaged — simple mean (equal weights) CORS solution 3

OPUS Tool Box OPUS-Projects $$ Receivers 2-4 Hours of data Multiple Receivers Network Solution Results shared or not OPUS-RS $$ Receivers 15 Minutes of data Results not shared OPUS-DB $$ Receivers 4 Hours of data Results shared OPUS-S $$ Receivers 2 Hours of data Results not shared OPUS-Mapper ¢¢ Receivers Minutes of data Results not shared LOCUS Digital Bar-Code Leveling Integration with GPS? Results shared or not FLAVORS OF OPUS

GPS dataMark Information Local Referencesphoto(s) NGS website: OPUS-DB DATASHEET OPUS-DB OBSERVATION DATA STREAM NGS magic

OPUS DB

OPUS – DB Simple Shared Data NGS Archived

Problems with NAD 83 and NAVD 88  NAD 83 is not as geocentric as it could be (approx 1-2 m).  Surveyors don’t see this - Yet  NAD 83 is not well defined with positional velocities.  NAVD 88 is realized by passive control (bench marks) most of which have not been re-leveled in 40 years or longer.  NAVD 88 does not account for local vertical velocities (subsidence and uplift)  Post glacial isostatic readjustment  Subsurface fluid withdrawal  Sediment loading  Sea level rise

The National Geodetic Survey 10 year plan Mission, Vision and Strategy 2008 – 2018 Official NGS policy as of Jan 9, 2008 –Modernized agency –Attention to accuracy –Attention to time-changes –Improved products and services –Integration with other fed missions –Vetted through NSPS/AAGS 2018 Targets: –NAD 83 and NAVD 88 re-defined –Cm-accuracy access to all coordinates –Customer-focused agency –Global scientific leadership

Ten-Year Milestones (2018) 1) NGS will compute a pole-to-equator, Alaska-to- Newfoundland geoid model, preferably in conjunction with Mexico and Canada as well as other interested governments, with an accuracy of 1 cm in as many locations as possible 2) NGS redefines the vertical datum based on GNSS and a gravimetric geoid 3) NGS redefines the national horizontal datum to remove disagreements with the ITRF

Approximate height differences from NAVD 88 to new gravimetric datum (~ 2018).

PREDICTED POSITIONAL and HEIGHT CHANGES with NEW DATUMS (Geometric and Geopotential) in ~ 2018 computed for (pid DL4964) HORIZONTAL = 1.14 m (3.7 ft) ELLIPSOID HEIGHT = m (- 3.6 ft) Predicted with HTDP ORTHOMETRIC HEIGHT = m (- 2.0 ft) Predicted with HTDP and USGG2003

Simplified Concept of NAD 83 vs. ITRF meters NAD 83Origin ITRF 00 Origin Earth’s Surface h 83 h 00 Identically shaped ellipsoids (GRS-80) a = 6,378, meters (semi-major axis) 1/f = (flattening)

Reasons for replacing NAD 83 with a geocentric reference system. Elllipsoid hts can be converted to orthometric hts in the proposed U.S. geopotential datum via the equation H=h-N. Where N is based on a true gravimetric geoid. Geocentric coordinates would closely approximate WGS 84 coordinates. Geocentric coordinates would closely approximate ITRF coordinates which is popular within the scientific community. Geocentric coordinates would closely approximate the official coordinates used in Mexico. Europe and South America have each adopted a geocentric reference system. Wide Area Augmentation System (WAAS) uses ITRF.

Reasons for staying with NAD 83 State Plane Coordinates are based on NAD 83. Legislation in 48 states require the use of NAD 83. The NGSIDB contains only NAD 83 coordinates for more than 300,000 passive reference stations. Perhaps more that a million passive stations that are not in the NGSIDB have only NAD 83 coordinates. Many GIS, Imagery and other products are tied to or based on NAD 83. The Nationwide Differential GPS service (NDGPS) uses NAD 83. Canada uses NAD 83.

Preparing for the Future 1 – Keep your data holdings. 2 – Attend webinars offered by NOAA/NGS Corbin Training facility. 3 – Participate in the “County Scorecard” effort. NOAA NGS Corbin Training Center

Questions? Dave Rigney NGS’s Mission Statement