Who Needs New Datums? NGS Says… Marti Ikehara California Geodetic Advisor, Sacramento ftp://ftp.ngs.noaa.gov/pub/marti

Outline New datums down the road GRAV-D Program New datum, epoch for CORS soon Visualizing Geodetic control Control for Imperial County

What IS a datum? It is not the same as a “projection”, which controls how maps are represented or “look” on paper or on a screen. It IS a reference, a basis, for coordinates, so a datum has an origin and/or ‘zero’. “Underlying” a horizontal datum system is an ELLIPSOID, that represents the global shape of the earth; a DATUM is the ‘continental’ shape as realized with physical marks (passive or active)

NGVD29 The National Geodetic Vertical Datum of 1929 was referenced to 26 tide gauges in the US and Canada Definition: NGVD29 – The National Geodetic Vertical Datum of 1929… The Sea Level Datum of 1929 was the vertical control datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. The datum was used to measure elevation or altitude above, or depth below, mean sea level (MSL). Mean sea level was measured at 26 tide gauges: 21 in the United States and 5 in Canada. The datum was defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. The adjustment utilized a total of 66,315 miles (106,724 km) of leveling with 246 closed circuits and 25 circuits at sea level. Since the Sea Level Datum of 1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National Geodetic Vertical Datum of 1929 (NGVD 29) in 1973. The NGVD 29 was subsequently replaced by the North American Vertical Datum of 1988 (NAVD 88) based upon the General Adjustment of the North American Datum of 1988.

NAVD88 The North American Vertical Datum of 1988 is referenced to a single tide gauge in Canada

EQUIPOTENTIAL SURFACES A leveling measurement, using a traditional spirit level, is affected or impacted by the gravitational force at the point on the earth’s surface. Equipotential contours are where the gravity potential is the same, equal, and a ‘surface’ along which water will not flow. It turns out that MSL is a close approximation to another surface, defined by gravity, called the geoid, which is the true zero surface for measuring elevations. Because we cannot directly see the geoid surface, we cannot actually measure the heights above or below the geoid surface. We must infer where this surface is by making gravity measurements and by modeling it mathematically. For practical purposes, we assume that at the coastline the geoid and the MSL surfaces are essentially the same. Nevertheless, as we move inland we measure heights relative to the zero height at the coast, which in effect means relative to MSL.

GPS data is referenced to the ellipsoidal surface, which does not have a constant offset—they are not parallel to the equipotential surfaces. It’s all about better determining N—geoid height, from better gravity data. What we have been doing for close to 20 years is collecting GPS data at leveled bench marks—the control for Ht Mod projects, and fitting the data in a least squares adjustment to the datum for leveling—NAVD88. We produce orthometric heights and input data to geoid models to try to account for the gravity term.

GEOID09 compared to GEOID03 09: teal, good; yellow: State advisor-rejected; red: analysis-rejected 03 (underlying dots): dk blue, kept; orange: rejected New points for 09 include data from Height Modernization projects: Leveling/GPS from BMs to CGPS in southern CA; GPS data on BMs in San Joaquin Valley and northern CA. Yellow dots represent those HPGN marks that were leveled to from NGVD29 but which were improperly coded as NAVD88 in the database. Their use in G03 probably contributed to the errors in the model in the central valley that were seen.

From Helmer’s article in Winter 09/10 issue of California Surveyor

Rationale for new Datums Terminology Horizontal becomes GEOMETRIC Vertical becomes GEOPOTENTIAL When? 2018, if GRAV-D program is complete Why? But why?! Primarily need a better vertical datum for “vertical” data(geoid, ortho ht) not obtained by traditional leveling

2009 National Research Council Report “…uniform national standards for FEMA flood maps cannot be met until an improved orthometric height datum and geoid model exist…” In this report from the NRC that was published last year, it clearly states that issues such as disparate vertical datums between areas separated by water, and other geodetic inconsistencies, are preventing FEMA from meeting uniform national map standards. When GRAV-D is complete, there will be one single, connected vertical datum for all areas of the United States except Guam, CNMI and American Samoa.

Rationale for new Datums Terminology Horizontal becomes GEOMETRIC Vertical becomes GEOPOTENTIAL When? 2018, if GRAV-D program is complete Why? But why?! Primarily need a better vertical datum for “vertical” data(geoid, ortho ht) not obtained by traditional leveling Geodetic Data collection relies upon navigation (GNSS) satellites orbiting around the mass center of the Earth NAD83 reference frame/datum is not geocentric

Earth-Centered Earth-Fixed Z Earth-Centered Earth-Fixed (ECEF) Coordinate System XA, YA,ZA (not NAD83) -Y -X Greenwich Meridian Earth Mass Center X, Y, Z = 0 Y X GRS 80 Ellipsoid -Z 13

Simplified Concept of NAD 83 vs. ITRF00 (or WGS84) h83 h00 Earth’s Surface ITRF (International Terrestrial Reference Frame) Ellipsoid heights NAD83 vs. ITRF00/WGS84 - Defined origins were best estimate—at the time-- of the center of mass; NAD83 is not geocentric but ITRF and WGS84 are. ITRF 00 Origin 2.2 meters Identically shaped ellipsoids (GRS80) a = 6,378,137.000 meters (semi-major axis) 1/f = 298.25722210088 (flattening) NAD 83 Origin 14

Rationale for new Datums Terminology Horizontal becomes GEOMETRIC Vertical becomes GEOPOTENTIAL When? 2018, if GRAV-D program is complete Why? But why?! Primarily need a better vertical datum for “vertical” data(geoid, ortho ht) not obtained by traditional leveling Geodetic Data collection relies upon navigation (GNSS) satellites orbiting around the mass center of the Earth NAD83 reference frame/datum is not geocentric Earth is not stable in all directions; reference frame must account for this to be as geodetically accurate as possible NAD83 is not defined to include vertical velocities

Horizontal Velocities, relative to ‘stable’ NA tectonic plate, are reflected in HTDP model CA/western NV to some extent is now used to dealing with horizontal velocities

Worldwide Tectonic Horizontal Motions Velocities horizontally 18

CORS Data allows for Monitoring Vertical Crustal Motion We can and do see vertical velocities, as shown by this graph illustrating vertical velocities related to post-glacial crustal motion Vertical velocities associated with Glacial Isostatic Adjustment (rebound) 19

Evolution of Geodetic Datums: from NAD27/NGVD29 to NAD83/NAVD88 to ?/ ? GPS H + V 2 + 1 27, 29 H + V 2 + 1 83(86), 88 H + VE + VO 2 + 1 + 1 83(92), 88 +VELOCITIES (time) H + Ht + VE + VO 2 + 2 + 1+ 1 83(07)+HTDP, 88 H + Ht +VE+ VEt 2 + 2 + 1+ 1 GEOMETRIC (VE+G)[+VEt] + Gt 1 + 1 GEOPOTENTIAL Conceptual: Generally, Orange represents ‘horizontal’ or NAD83 at present; yellow represents orthometric vertical. 1)moved fr non-geocentric to 2)geocentric as well as we knew at the time from early satellite technology (DOPPLER). And leveling efforts resulted in a national vertical datum with only one constraint, rather than 26 (tide gauges). 3)Static GPS exploded on the scene which 4) led to 5)the HARN networks and state-specific adjustment. With GPS surveying, we got V(E), vertical ellipsoid and V(O), vertical orthometric. 6)With the results of repeat static surveys and the advent of CGPS (Continuous CGPS), such as NGS’ National CORS, 7)the velocity of the earth’s crust relative to a reference frame, could be determined over time. 8)Using the crustal motion model HTDP for horizontal velocities on the West Coast, eg, the Pacific-North American Plates interface, NGS produced 9) a National Re-adjustment in the NAD83 (NSRS2007) datum. (No change to ortho.) However, 10)underlying the NAD83 adjustment are data and analysis referenced to ITRF00, epoch 2002.00, which does take into account both horizontal AND vertical ellipsoid velocities. (velocity=time related terms shown as italicized.) This is what we have now. 11) With better global gravity models and airborne gravity data collected by NGS, we will 12) move forward with 13) new datums BOTH referenced to an ECEF, earth-centered, earth-fixed datum which we are calling 14) Geometric, for the horizontal and ellipsoid height values, and Geopotential for the orthometric height values which will be based on a a geoid model and changes in gravity over time (G(t). H + Ht +VE+ VEt 2 + 2 + 1+ 1 ITRF00 (2002.00) + GRAVITY (geoid model)

The Future of Height Mod: GRAV-D Gravity for the Redefinition of the American Vertical Datum Airborne gravity surveys (taking 10 years) Coastal areas surveyed first All USA states and territories www.ngs.noaa.gov/GRAV-D GOAL: Orthometric heights-”elevations” – to be good to 2 cm anywhere, anytime with an excellent ellipsoid height from GNSS technology. Currently *this* takes 3-4 hours at 15-second recording and dual-frequency GPS Height changes much more easily monitored than by leveling, using new technology – GNSS heights and gravity values Spot-checking/monitoring gravity changes with portable field units GRAV-D is the future height modernization. With only current funding, the entire country will likely be flown by 2022 or later, and a geoid available for the country sometime afterward. In 2008, we projected a completion date of 2018 with a budget of nearly $40 million over 10 years. Currently GRAV-D uses internal resources and partnerships, but if funding were to be increased, NOAA capabilities would be maxed, and part of the data collection would be outsourced. Sent letters to potential partners to share costs and resources. NOAA’s National Geodetic Survey (NGS) conducted an airborne survey in western interior Alaska in July, 2009. The National Geospatial-Intelligence Agency (NGA) was a partner in the survey. The operation was conducted aboard a Naval Research Laboratory (NRL) C-12 King Air aircraft with a Navy aircrew.

Planned GRAV-D Coverage Alaska “CONUS” Guam / Northern Marianas Hawaii Puerto Rico / Virgin Islands American Samoa GRAV-D will cover CONUS, Alaska, Hawaii, and the U.S. territories

Status of GRAV-D in early 2010 As of Jan 2010: 6.7% of planned areas have been flown NGA funded NRL plane So far: Gulf coast from Fla to Mexican border ; Puerto Rico and the Virgin Islands ; Anchorage ; Fairbanks. NRL=Naval Research Lab. http://www.ngs.noaa.gov/GRAV-D/news.shtml USACE funded

Preliminary GRAV-D Results A snapshot of gravity coming out of GRAV-D. While this plot shows gravity variations, these are highly correlated with geoid variations. As such, this plot gives an idea of the variation in the gravity field that represent the difference between ellipsoid heights (from GPS) and orthometric heights.

New datum, epoch for CORS Current: ITRF2000, 2002.00 Near end of year: ITRF2008, 2008.00 and NAD83 (“CORS96A”), 2010.00 for CORS only No change to passive monumentation, eg, NAD83 (NSRS2007), 2007.00 About 1000 CORS nationwide Those with >2 years of data Changes on the order of mm, but some outliers are showing closer to decimeters List will be prepared, showing amount of change

Visualization of NGS Geodetic Control Geodetic advisor website, located at Caltrans Office of Land Surveys webpages http://www.dot.ca.gov/hq/row/landsurveys/geodetic Google Earth kmls Google Maps online Shapefiles DSWorld, NGS partner software

http://www. dot. ca. gov/hq/row/landsurveys/geodetic/geodetic_control http://www.dot.ca.gov/hq/row/landsurveys/geodetic/geodetic_control.html Shown on the state are the 12 Caltrans districts; the counties are arranged by which CT District they are in.

Kml’s and shapefiles downloadable No “old” stuff: NAD83 (NSRS2007) only and non-VERTCON’s NAVD88

http://www. dot. ca. gov/hq/row/landsurveys/geodetic/geodetic_control http://www.dot.ca.gov/hq/row/landsurveys/geodetic/geodetic_control.html

To better spread out and make easier accesss to vertical control, NGS started an initiative in the mid-1990’s called Height Modernization, whereby GPS data was observed in networks often at the scale of a county. This shows Sac County before the Sac Valley HM project was loaded in mid- 2010.

Using NGS DSWORLD software and Google Earth to get any vintage geodetic data from NGSIDB Start by downloading “Google earth” at: http://earth.google.com/download-earth.html Click here

Open the NGS main page at www.ngs.noaa.gov Mouse-over the “TOOLS” button Click on “DOWNLOAD PC SOFTWARE”

Click on the link: “User Contributed Software is also available…..” Click here

Next, click on the DSWORLD (Version. ) link Next, click on the DSWORLD (Version *.**) link. A “file download” window will appear. Choose to save the file where you can easily find it. It will be a zip file, so you will need to unzip it and then double click on the setup.exe file. That will install the program for you. For additional information or assistance, please contact your NGS State Geodetic Advisor

Does not distinguish between VERTCON and other NAVD88 Does not distinguish between VERTCON and other NAVD88. Blue-first order, Red-second order, Black-other, eg, third or readjusted or posted

Anything Else?! Marti.ikehara@noaa.gov 916-227-7325 ftp://ftp.ngs. noaa.gov/pub/marti