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

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

3. 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)

4. 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 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 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.

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

6. 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.

7. 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.

8. 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.

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

10. 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

11. 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.

12. 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

13. Earth-Centered Earth-FixedZ
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

14. 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, meters (semi-major axis)
1/f = (flattening)
NAD 83
Origin 14

16. 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

17. 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

18. Worldwide Tectonic Horizontal Motions
Velocities horizontally 18

19. 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

20. 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
83(92), 88
+VELOCITIES (time)
H + Ht + VE + VO
83(07)+HTDP, 88
H + Ht +VE+ VEt
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 , 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
ITRF00 ( )
+ GRAVITY
(geoid model)

21. 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
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, 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.

22. 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

23. 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.
USACE funded

24. 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.

25. New datum, epoch for CORS
Current: ITRF2000,
Near end of year: ITRF2008,
and NAD83 (“CORS96A”), for CORS only
No change to passive monumentation, eg, NAD83 (NSRS2007),
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

26. Visualization of NGS Geodetic Control
Geodetic advisor website, located at Caltrans Office of Land Surveys webpages
Google Earth kmls
Google Maps online
Shapefiles
DSWorld, NGS partner software

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

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

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

31. 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

32. Using NGS DSWORLD software and Google Earth to get any vintage geodetic data from NGSIDB
Start by downloading “Google earth” at:
Click here

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

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

35. 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

36. 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

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