# Presentation: 751424

## The NGS Gravity Program                   and Geoid ModelingProfessional Land Surveyors of Colorado2006 Fall Technical Program and Annual Meeting

- Daniel R. Roman
- National Geodetic Survey
- National Oceanic and Atmospheric Administration

**Notes:**
How?  Why now?  What has changed that enables us to get quality heights from GPS when we weren’t able to before?  In this presentation I show how improvements in technology together with our understanding of the relationships of these heights has enabled us to develop procedures to attain accurate heights with the help of GPS.

## OUTLINE OF TALK

- Introduction
- Overview of current gravimetric geoid models
- Overview of current hybrid geoids
- Heights and the datasheet
- Plans for Geoid Modeling at NGS
- Ongoing research areas
- Of local interest
- Conclusions

## 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 ellipsoidal height from an ellipsoidal datum to a geoid.
- Hence, geoid height models are directly tied to the geoid and ellipsoid that define them (i.e., geoid height models are _not_ interchangeable).
-    *Definition from the Geodetic Glossary, September 1986

**Notes:**
-What does equipotential surface mean? 
-If we could see or measure the geoid, this could be our vertical datum.  
-It may be in the future, and we must plan for a transition.
-For now, we are dependant on leveling observations on the surface to create our datum. 
-The leveling datum may or may not be a true equipotential surface (i.e., NAVD 88 =/ true geoid).
-use the geoid height models to transform between the ellipsoidal and vertical datums.

## In Search of the Geoid...

**In Search of the Geoid...**

- Courtesy of Natural Resources Canada  www.geod.nrcan.gc.ca/index_e/geodesy_e/geoid03_e.html

**Notes:**
-The geoid extends everywhere and provides a common datum for all.

## Earth Gravity Model of 1996 (EGM96)
2.6 million terrestrial, ship-borne, and altimetric gravity measurements
30 arc second Digital Elevation Data
3 arc second DEM for the Northwest USA
Decimated from 1 arc second NGSDEM99
Computed on 1 x 1 arc minute grid spacing
GRS-80 ellipsoid centered at ITRF97 origin

**2.6 million terrestrial, ship-borne, and altimetric gravity measurements**
**30 arc second Digital Elevation Data**
**3 arc second DEM for the Northwest USA**
**Decimated from 1 arc second NGSDEM99**
**Computed on 1 x 1 arc minute grid spacing**
**GRS-80 ellipsoid centered at ITRF97 origin**

**High Resolution Geoid Models****G99SSS (Scientific Model)**

**Notes:**
- G99SSS (scientific model) - determined from 2.6 million gravity measurements taken over the last century and stored in NGS database.  Major improvements over previous models; more data.
- 30 arc second, 1 km x 1 km, grid of digital elevation data; with updated Canadian Rockies data from G96SSS.
- 3 arc second DEM - northwestern portion of U.S. which has history of poor geoid interpretation.
- EGM96 (Earth Gravity Model of 1996) developed by National Imaging and Mapping Agency (NIMA) - global model of gravity and geoid undulations; good in large areas but not as good for smaller areas.
- 1-D spherical Stokes’ FFT (Fast Fourier Transformation) - “remove-compute-restore” (fix and put back into model - iterative process).

Final outcome - 1 x 1 arc minute spacing interpolation of the geoid model; 2' x 2' in Alaska.  1 arc minute equals about 1.8 km on ground.
ITRF97 (1997.0) COM (Center Of Mass) based origin for model.

## High Resolution Geoid ModelsUSGG2003 (Scientific Model)

**2.6 million terrestrial, ship, and altimetric gravity measurements**
**offshore altimetry from GSFC.001 instead of KMS98**
**30 arc second Digital Elevation Data**
**3 arc second DEM for the Northwest USA**
**Decimated from 1 arc second NGSDEM99**
**Earth Gravity Model of 1996 (EGM96)**
**Computed on 1 x 1 arc minute grid spacing**
**GRS-80 ellipsoid centered at ****ITRF00**** origin**

**Notes:**
Offshore altimetry from a different source => better defines ocean topography.
ITRF00 used instead of ITRF97.

## Gravity Coverage for GEOID03

**Notes:**
This shows the extent of the gravity data that went info the USGG2003 geoid model.
Terrestrial data onshore; shipborne and altimetric data offshore.

## Slide 8

**Notes:**
-although GSFC00.1 created significant changes (50 cm or more in some regions when looking at the difference between G99SSS and USGG2003, the magnitude of those differences is dwarfed by the total signal

This model is tied to a GRS-80 ellipsoid centered at ITRF00 and the Wo value of EGM96 ... but that isn’t what we need to use it with level data.

**H**

**H = Orthometric Height**  **(NAVD 88)**

**H** ** = ****h**** - ****N**

- TOPOGRAPHIC SURFACE

**h = Ellipsoidal Height (NAD 83)**

**N = Geoid Height (GEOID 03)**

**Ellipsoid, Geoid, ****and Orthometric Heights**

- A

- B

**Notes:**
Orthometric height is the height on the surface above the geoid.  But we can’t measure from the geoid so we use leveling. The NAVD88 is defined from the control point, B, in Quebec.  Because the ortho ht at A is computed from leveling observations, where error is modeled or estimated, it probably isn’t right at the geoid, right at “sea level”.

Our vertical datum is defined.  We need a geoid that we can use that is relative to the vertical datum. 

Note that in this picture the geoid is shown above the ellipsoid.  In the continental United States, the geoid is actually below the ellipsoid, so the value of the geoid height is negative.

## Composite Geoids

**Composite Geoids**

- Gravity Geoid  systematic misfit with benchmarks
- Composite Geoid biased to fit local benchmarks
- e = h – H - N

**Earth’s Surface**

**Ellipsoid**

**Hybrid or Composite Geoid =~ NAVD 88**

**Gravity**** Geoid**

**0.055 M in **
**Lakewood, CO – **
**1999 model**

**0.015 M in **
**Lakewood, CO  – **
**2003 model**

**Notes:**
- Gravity geoid is from gravity measurements
- NAVD 88 bench marks are adjusted using a sea level height at Point au Pere
- There is going to be a slight difference between the 2.   If we want to use geoid to compute NAVD 88 heights, it must be consistent with the NAVD 88
Therefore we “bias” the geoid to be consistent with the NAVD 88 using high accuracy GPS on NAVD 88 bench marks.
Use Least Squares Collocation to determine the systematic components while allowing for random GPS observation errors (2-5 cm standard).
-Use the control points (GPSBM’s) to define a surface that can be interpolated to make internally consistent predictions (precision versus accuracy).
As you can easily see, the quality and distribution of the control, data will directly impact the quality of the predictions.

## Slide 11

**Notes:**
-These are the control data used to make GEOID03 (GPS on bench marks: GPSBM’s).
-Note the inequitable distribution.
-You could practically grid the GPSBM’s in South Carolina and get a good result.
-Other places are not so fortunate...
-Also note that this shows distribution but not quality of the points. 
-Some regions (e.g., Texas) have systematic problems that impact the GPSBM’s and the derived hybrid geoids.
-Current techniques rely on creating models of the systematic effects at multiple wavelengths. If the spatial density doesn’t support the shorter wavelength models, then the quality of predictions will commensurately be reduced.

CO: # PTS = 514   Average = 0.0 cm   STD = 3.3 cm
This average is much worse than the national average of 2.4 cm and implies more significant problems exist either in the gravity data, leveling, or GPS observations.

## High Resolution Geoid ModelsGEOID03 (vs. Geoid99)

**Begin with USGG2003 model **
**14,185**** NAD83 GPS heights on NAVD88 leveled benchmarks (vs. 6169)**
**Determine national bias and trend relative to GPS/BMs**
**Create grid to model local (state-wide) remaining differences**
**ITRF00****/NAD83 transformation (vs. ITRF97)**
**Compute and remove conversion surface from USGG2003**

**Notes:**
GEOID03 - best model for North America; not a true interpretation of the geoid but includes bias to establish best orthometric heights relative to NAVD88.

14,185 GPS/levels bench marks (NAD83/NAVD88); more to be included to further improve future models.  GPS/BM constrained to help model reflect NAVD88 orthometric heights then unconstrained for final model.

## High Resolution Geoid ModelsGEOID03 (vs. Geoid99)

**Relative to non-geocentric GRS-80 ellipsoid **
**2.4**** cm RMS nationally when compared to BM data (vs. 4.6 cm)**
**RMS ******** ****50%**** improvement over GEOID99 (Geoid96 to 99 was 16%)**

**Notes:**
-NAD83 non-COM - model warped to reflect NAD83 (86) non-COM origin.
-2.4 cm RMS when comparing to bench mark data – includes 2 cm random error in GPS observations.
-The improvement largely resulted from the improved technique (multi-matrix), which is why we created it. 
-Future models will adopt a similar modeling approach.

## GEOID03 Conversion Surface

**Notes:**
The differences shown here represent the systematic differences

## GEOID99 Conversion Surface

## 
        National Geodetic Survey,   Retrieval Date = DECEMBER 28, 2005
 PL0314 ***********************************************************************
 PL0314  DESIGNATION -  V 27
 PL0314  PID         -  PL0314
 PL0314  STATE/COUNTY-  MI/GRAND TRAVERSE
 PL0314  USGS QUAD   -  
 PL0314
 PL0314                         *CURRENT SURVEY CONTROL
 PL0314  ___________________________________________________________________
 PL0314* NAD 83(1994)-  44 39 02.41202(N)    085 46 04.27942(W)     ADJUSTED  
 PL0314* NAVD 88     -       257.838  (meters)     845.92   (feet)  ADJUSTED  
 PL0314  ___________________________________________________________________
 PL0314  X           -     335,419.145 (meters)                     COMP
 PL0314  Y           -  -4,532,722.532 (meters)                     COMP
 PL0314  Z           -   4,459,971.520 (meters)                     COMP
 PL0314  LAPLACE CORR-           5.18  (seconds)                    DEFLEC99
 PL0314  ELLIP HEIGHT-         223.17  (meters)          (07/17/02) GPS OBS
 PL0314  GEOID HEIGHT-         -34.68  (meters)                     GEOID03
 PL0314  DYNAMIC HT  -         257.812 (meters)     845.84  (feet)  COMP
 PL0314  MODELED GRAV-     980,508.8   (mgal)                       NAVD 88
 PL0314

-         National Geodetic Survey,   Retrieval Date = DECEMBER 28, 2005
-  PL0314 ***********************************************************************
-  PL0314  DESIGNATION -  V 27
-  PL0314  PID         -  PL0314
-  PL0314  STATE/COUNTY-  MI/GRAND TRAVERSE
-  PL0314  USGS QUAD   -
-  PL0314
-  PL0314                         *CURRENT SURVEY CONTROL
-  PL0314  ___________________________________________________________________
-  PL0314* NAD 83(1994)-  44 39 02.41202(N)    085 46 04.27942(W)     ADJUSTED
-  PL0314* **NAVD 88     -       257.838  (meters)     845.92   (feet)  ADJUSTED**
-  PL0314  ___________________________________________________________________
-  PL0314  X           -     335,419.145 (meters)                     COMP
-  PL0314  Y           -  -4,532,722.532 (meters)                     COMP
-  PL0314  Z           -   4,459,971.520 (meters)                     COMP
-  PL0314  LAPLACE CORR-           5.18  (seconds)                    DEFLEC99
-  PL0314  **ELLIP HEIGHT-         223.17  (meters)          (07/17/02) GPS OBS**
-  PL0314  **GEOID HEIGHT-         -34.68  (meters)                     GEOID03**
-  PL0314  DYNAMIC HT  -         257.812 (meters)     845.84  (feet)  COMP
-  PL0314  MODELED GRAV-     980,508.8   (mgal)                       NAVD 88
-  PL0314

**Sample Datasheet**

**N**

**H**

**h**

**Notes:**
NGS datasheets show all heights where we have them.

##  PL0314
 PL0314  HORZ ORDER  -  FIRST
 PL0314  VERT ORDER  -  FIRST     CLASS II
 PL0314  ELLP ORDER  -  FOURTH    CLASS I
 PL0314
 PL0314.The horizontal coordinates were established by GPS observations
 PL0314.and adjusted by the National Geodetic Survey in February 1997.
 PL0314
 PL0314.The orthometric height was determined by differential leveling
 PL0314.and adjusted by the National Geodetic Survey in June 1991.
 PL0314
 PL0314.The X, Y, and Z were computed from the position and the ellipsoidal ht.
 PL0314
 PL0314.The Laplace correction was computed from DEFLEC99 derived deflections.
 PL0314
 PL0314.The ellipsoidal height was determined by GPS observations
 PL0314.and is referenced to NAD 83.
 PL0314
 PL0314.The geoid height was determined by GEOID03.
 PL0314
 PL0314.The dynamic height is computed by dividing the NAVD 88
 PL0314.geopotential number by the normal gravity value computed on the
 PL0314.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45
 PL0314.degrees latitude (g = 980.6199 gals.).
 PL0314
 PL0314.The modeled gravity was interpolated from observed gravity values.
 PL0314

-  PL0314  HORZ ORDER  -  FIRST
-  PL0314  VERT ORDER  -  FIRST     CLASS II
-  PL0314  ELLP ORDER  -  FOURTH    CLASS I
-  PL0314
-  PL0314.The horizontal coordinates were established by GPS observations
-  PL0314.and adjusted by the National Geodetic Survey in February 1997.
-  PL0314
-  PL0314.The _**orthometric height**_ was determined by differential leveling
-  PL0314.and adjusted by the National Geodetic Survey in June 1991.
-  PL0314
-  PL0314.The X, Y, and Z were computed from the position and the ellipsoidal ht.
-  PL0314
-  PL0314.The Laplace correction was computed from DEFLEC99 derived deflections.
-  PL0314
-  PL0314.The _**ellipsoidal height**_ was determined by GPS observations
-  PL0314.and is referenced to NAD 83.
-  PL0314
-  PL0314.The _**geoid height**_ was determined by GEOID03.
-  PL0314
-  PL0314.The dynamic height is computed by dividing the NAVD 88
-  PL0314.geopotential number by the normal gravity value computed on the
-  PL0314.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45
-  PL0314.degrees latitude (g = 980.6199 gals.).
-  PL0314
-  PL0314.The modeled gravity was interpolated from observed gravity values.
-  PL0314

**Sample Datasheet**

**Notes:**
The source for each height is explained below the coordinates

##  PL0314
 PL0314.The modeled gravity was interpolated from observed gravity values.
 PL0314
 PL0314;                    North         East     Units Scale Factor Converg.
 PL0314;SPC MI C     -   149,194.606 5,888,865.237   MT  0.99992569   -0 59 23.3
 PL0314;SPC MI C     -   489,483.62 19,320,424.01    FT  0.99992569   -0 59 23.3
 PL0314;UTM  16      - 4,944,883.803   597,700.224   MT  0.99971738   +0 51 57.6
 PL0314
 PL0314!             -  Elev Factor  x  Scale Factor =   Combined Factor
 PL0314!SPC MI C     -   0.99996501  x   0.99992569  =   0.99989070
 PL0314!UTM  16      -   0.99996501  x   0.99971738  =   0.99968240
 PL0314
 PL0314                          SUPERSEDED SURVEY CONTROL
 PL0314
 PL0314  ELLIP H (02/03/97)  223.19   (m)                       GP(       ) 4 1
 PL0314  NAD 83(1986)-  44 39 02.41257(N)    085 46 04.28315(W) AD(       ) 1
 PL0314  NAD 83(1986)-  44 39 02.38347(N)    085 46 04.27988(W) AD(       ) 3
 PL0314  NAVD 88 (09/30/91)  257.84   (m)          845.9    (f) LEVELING    3  
 PL0314  NGVD 29 (??/??/92)  257.915  (m)          846.18   (f) ADJ UNCH    1 2
 PL0314
 PL0314 Superseded values are not recommended for survey control.
 PL0314.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.
 PL0314

-  PL0314.The modeled gravity was interpolated from observed gravity values.
-  PL0314
-  PL0314;                    North         East     Units Scale Factor Converg.
-  PL0314;SPC MI C     -   149,194.606 5,888,865.237   MT  0.99992569   -0 59 23.3
-  PL0314;SPC MI C     -   489,483.62 19,320,424.01    FT  0.99992569   -0 59 23.3
-  PL0314;UTM  16      - 4,944,883.803   597,700.224   MT  0.99971738   +0 51 57.6
-  PL0314
-  PL0314!             -  Elev Factor  x  Scale Factor =   Combined Factor
-  PL0314!SPC MI C     -   0.99996501  x   0.99992569  =   0.99989070
-  PL0314!UTM  16      -   0.99996501  x   0.99971738  =   0.99968240
-  PL0314
-  PL0314                          SUPERSEDED SURVEY CONTROL
-  PL0314
-  PL0314  ELLIP H (02/03/97)  223.19   (m)                       GP(       ) 4 1
-  PL0314  NAD 83(1986)-  44 39 02.41257(N)    085 46 04.28315(W) AD(       ) 1
-  PL0314  NAD 83(1986)-  44 39 02.38347(N)    085 46 04.27988(W) AD(       ) 3
-  PL0314  NAVD 88 (09/30/91)  257.84   (m)          845.9    (f) LEVELING    3
-  PL0314  NGVD 29 (??/??/92)  257.915  (m)          846.18   (f) ADJ UNCH    1 2
-  PL0314
-  PL0314 _**Superseded values are not recommended for survey control.**_
-  PL0314.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.
-  PL0314

**Sample Datasheet**

**Notes:**
-Modeled gravity comes from the NAVD 88 gravity interpolation program NOT the Surface Gravity Interpolation tool
-The Surface tool draws from the existing database, while NAVD 88 tool draws from a database made static at the time of the national adjustment (1991) to make sure values are consistent.
-impact can be decimeter in high altitudes.

##  PL0314_U.S. NATIONAL GRID SPATIAL ADDRESS: 16TEQ9770044884(NAD 83)
 PL0314_MARKER: DB = BENCH MARK DISK
 PL0314_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT
 PL0314_SP_SET: CONCRETE POST
 PL0314_STAMPING: V 27 1930 846.176
 PL0314_MARK LOGO: CGS   
 PL0314_MAGNETIC: N = NO MAGNETIC MATERIAL
 PL0314_STABILITY: B = PROBABLY HOLD POSITION/ELEVATION WELL
 PL0314_SATELLITE: THE SITE LOCATION WAS REPORTED AS SUITABLE FOR
 PL0314+SATELLITE: SATELLITE OBSERVATIONS - October 24, 1992
 PL0314
 PL0314  HISTORY     - Date     Condition        Report By
 PL0314  HISTORY     - 1930     MONUMENTED       CGS
 PL0314  HISTORY     - 1951     GOOD             NGS
 PL0314  HISTORY     - 1984     GOOD             NGS
 PL0314  HISTORY     - 19890428 GOOD             NGS
 PL0314  HISTORY     - 1990     GOOD             USPSQD
 PL0314  HISTORY     - 19910701 GOOD             NGS
 PL0314  HISTORY     - 19920824 GOOD             MIDT
 PL0314  HISTORY     - 19921024 GOOD             MIDT
 PL0314  HISTORY     - 19971029 GOOD             USPSQD
 PL0314
 PL0314                          STATION DESCRIPTION
 PL0314
 PL0314'DESCRIBED BY NATIONAL GEODETIC SURVEY 1951
 PL0314'IN INTERLOCHEN.
 PL0314'AT INTERLOCHEN, 131 FEET EAST OF THE JUNCTION OF THE ABANDONED
 PL0314'BRANCH OF THE MANISTEE AND NORTHEASTERN RAILROAD AND THE C AND

-  PL0314_MARKER: DB = BENCH MARK DISK
-  PL0314_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT
-  PL0314_SP_SET: CONCRETE POST
-  PL0314_STAMPING: V 27 1930 846.176
-  PL0314_MARK LOGO: CGS
-  PL0314_MAGNETIC: N = NO MAGNETIC MATERIAL
-  PL0314_STABILITY: B = PROBABLY HOLD POSITION/ELEVATION WELL
-  PL0314_SATELLITE: THE SITE LOCATION WAS REPORTED AS SUITABLE FOR
-  PL0314+SATELLITE: SATELLITE OBSERVATIONS - October 24, 1992
-  PL0314
-  PL0314  HISTORY     - Date     Condition        Report By
-  PL0314  HISTORY     - 1930     MONUMENTED       CGS
-  PL0314  HISTORY     - 1951     GOOD             NGS
-  PL0314  HISTORY     - 1984     GOOD             NGS
-  PL0314  HISTORY     - 19890428 GOOD             NGS
-  PL0314  HISTORY     - 1990     GOOD             USPSQD
-  PL0314  HISTORY     - 19910701 GOOD             NGS
-  PL0314  HISTORY     - 19920824 GOOD             MIDT
-  PL0314  HISTORY     - 19921024 GOOD             MIDT
-  PL0314  HISTORY     - 19971029 GOOD             USPSQD
-  PL0314
-  PL0314                          STATION DESCRIPTION
-  PL0314
-  PL0314'DESCRIBED BY NATIONAL GEODETIC SURVEY 1951
-  PL0314'IN INTERLOCHEN.
-  PL0314'AT INTERLOCHEN, 131 FEET EAST OF THE JUNCTION OF THE ABANDONED
-  PL0314'BRANCH OF THE MANISTEE AND NORTHEASTERN RAILROAD AND THE C AND

**Sample Datasheet**

## 
        National Geodetic Survey,   Retrieval Date = DECEMBER 28, 2005
 PL0314 ***********************************************************************
 PL0314  DESIGNATION -  V 27
 PL0314  PID         -  PL0314
 PL0314  STATE/COUNTY-  MI/GRAND TRAVERSE
 PL0314  USGS QUAD   -  
 PL0314
 PL0314                         *CURRENT SURVEY CONTROL
 PL0314  ___________________________________________________________________
 PL0314* NAD 83(1994)-  44 39 02.41202(N)    085 46 04.27942(W)     ADJUSTED  
 PL0314* NAVD 88     -       257.838  (meters)     845.92   (feet)  ADJUSTED  
 PL0314  ___________________________________________________________________
 PL0314  X           -     335,419.145 (meters)                     COMP
 PL0314  Y           -  -4,532,722.532 (meters)                     COMP
 PL0314  Z           -   4,459,971.520 (meters)                     COMP
 PL0314  LAPLACE CORR-           5.18  (seconds)                    DEFLEC99
 PL0314  ELLIP HEIGHT-         223.17  (meters)          (07/17/02) GPS OBS
 PL0314  GEOID HEIGHT-         -34.68  (meters)                     GEOID03
 PL0314  DYNAMIC HT  -         257.812 (meters)     845.84  (feet)  COMP
 PL0314  MODELED GRAV-     980,508.8   (mgal)                       NAVD 88
 PL0314

-         National Geodetic Survey,   Retrieval Date = DECEMBER 28, 2005
-  PL0314 ***********************************************************************
-  PL0314  DESIGNATION -  V 27
-  PL0314  PID         -  PL0314
-  PL0314  STATE/COUNTY-  MI/GRAND TRAVERSE
-  PL0314  USGS QUAD   -
-  PL0314
-  PL0314                         *CURRENT SURVEY CONTROL
-  PL0314  ___________________________________________________________________
-  PL0314* NAD 83(1994)-  44 39 02.41202(N)    085 46 04.27942(W)     ADJUSTED
-  PL0314* **NAVD 88     -       257.838  (meters)     845.92   (feet)  ADJUSTED**
-  PL0314  ___________________________________________________________________
-  PL0314  X           -     335,419.145 (meters)                     COMP
-  PL0314  Y           -  -4,532,722.532 (meters)                     COMP
-  PL0314  Z           -   4,459,971.520 (meters)                     COMP
-  PL0314  LAPLACE CORR-           5.18  (seconds)                    DEFLEC99
-  PL0314  **ELLIP HEIGHT-         223.17  (meters)          (07/17/02) GPS OBS**
-  PL0314  **GEOID HEIGHT-         -34.68  (meters)                     GEOID03**
-  PL0314  DYNAMIC HT  -         257.812 (meters)     845.84  (feet)  COMP
-  PL0314  MODELED GRAV-     980,508.8   (mgal)                       NAVD 88
-  PL0314

**Sample Datasheet**

**N**

**H**

**h**

- NAVD88 – Ellip Ht + Geoid Ht = ...
- 257.838 – 223.17 – 34.953 = -0.285   USGG2003
- 257.838 – 223.17 – 34.68  =  -0.012   GEOID03

**Notes:**
-In a perfect world these heights would add up mathematically.  But every height is derived in a way that includes some measure of error, whether it is from an observation and adjustment process or simply because it is derived from a model.  The purpose of creating a version of the geoid model that is biased to fit the NAVD88 is to provide a means to compute that NAVD88 height from GPS and the model alone.  You can see here how the change from the scientific model to the hybrid model provides a better fit between the 3 heights.  And as the geoid model improves, along with our ability to measure and compute better ellipsoid heights, these differences will get smaller and smaller.  

## Plans for Geoid Modeling at NGS

- Near term plans are to define gravimetric geoids and hybrid geoids for all U.S. territories (USGG2006 & GEOID06).
- Gravimetric geoids would all have a common Wo value (geoid datum) and be based on GRACE-based global gravity models such as the forthcoming EGM06 from NGA
- Gravimetric geoids will be tested against tide gauges and lidar-observed sea surface heights to confirm choice of Wo.
- Hybrid geoids would be tied to NAD 83 & local vertical datums
- NAVD 88 for Alaska and CONUS
- PRVD02 for Puerto Rico
- Etc.
- The quality of VDatum will be improved as the ties between the oceanic and terrestrial datums are better understood.
- Likewise, it would be very useful in providing decimeter or better *accurate *heights to estimate flooding potential.

## Long term goals are to define a cm-level accurate geoid height model valid for all of North America
Work is ongoing with the Canadians
Other nations joining in (Mexico/INEGI, etc.)
We likely will also adopt a vertical datum based on a refined geoid height model – the ultimate in Height Mod!
Conversion surface will provide means of transforming between this new datum and NAVD 88 – much as VERTCON does now between NGVD 29 and NAVD 88.
This maintains compatibility with archival data.
To do this, several major areas need work:
Gravity database cleansing/analysis/standardization
Acquisition of additional data sets 
Refinement of geoid theory

- Work is ongoing with the Canadians
- Other nations joining in (Mexico/INEGI, etc.)
- We likely will also adopt a vertical datum based on a refined geoid height model – the ultimate in Height Mod!
- Conversion surface will provide means of transforming between this new datum and NAVD 88 – much as VERTCON does now between NGVD 29 and NAVD 88.
- This maintains compatibility with archival data.
- To do this, several major areas need work:
- Gravity database cleansing/analysis/standardization
- Acquisition of additional data sets
- Refinement of geoid theory

**Plans for Geoid Modeling at NGS (cont.)**

**Notes:**
-Need seamless gravity data to reduce errors in gravity to geoid modeling
-Need additional gravity outside of U.S. areas – altimetric, neighboring countries
-Also need other data such as density anomalies and terrain data
-Current approach uses many simplifications – a more rigorous approach will reduce errors

-aerogravity fills in gaps and identifies systematic problems in gravity data (shipborne and terrestrial)

## We must have a consistent and seamless gravity field at least along the shorelines if not across all the U.S.
Use GRACE data to test long wavelength accuracy.
Use aerogravity to locate and possibly clean systematic problems in terrestrial or shipborne surveys (biases, etc.).
Determine and remove any detected temporal trends in the nearly 60 years of gravity data held by NGS. Ensure consistency of datums, corrections and tide systems.
This solves problems of current remove-compute-restore approach, which honors terrestrial data over EGM’s.
Exploration of utility of coastal/littoral aerogravity
Need a consistent gravity field from onshore to offshore.
Aids in database cleansing; also fills in coastal gaps.
Ties to altimetric anomalies in deeper water.
In conjunction with tide gauges & dynamic ocean topography models, this will aid in determining the optimal geopotential surface for the U.S. (Wo).

- Use GRACE data to test long wavelength accuracy.
- Use aerogravity to locate and possibly clean systematic problems in terrestrial or shipborne surveys (biases, etc.).
- Determine and remove any detected temporal trends in the nearly 60 years of gravity data held by NGS. Ensure consistency of datums, corrections and tide systems.
- This solves problems of current remove-compute-restore approach, which honors terrestrial data over EGM’s.
- Exploration of utility of coastal/littoral aerogravity
- Need a consistent gravity field from onshore to offshore.
- Aids in database cleansing; also fills in coastal gaps.
- Ties to altimetric anomalies in deeper water.
- In conjunction with tide gauges & dynamic ocean topography models, this will aid in determining the optimal geopotential surface for the U.S. (Wo).

**Ongoing research areas**

**Notes:**
-Need seamless gravity data to reduce errors in gravity to geoid modeling
-Need additional gravity outside of U.S. areas – altimetric, neighboring countries
-Also need other data such as density anomalies and terrain data
-Current approach uses many simplifications – a more rigorous approach will reduce errors

-aerogravity fills in gaps and identifies systematic problems in gravity data (shipborne and terrestrial)

## Must acquire data and models for outlying regions.
Definitely need surface gravity (terrestrial and shipborne) and terrain models for Guam, CNMI, American Somoa.
Desire to get such for nearest neighbors including Mexico, Caribbean nations, Central American nations, etc.
Also need to get any available forward geophysical models for all regions (such as ICE-5G for modeling the Glacial Isostatic Adjustment).
GPS/INS evaluation of the gravity field.
GPS & IMU information were also collected on flights.
This data can be used to derive gravity disturbances and to estimate gravity anomalies.
It may be useful in benign areas for determining the gravity field. Possibly cheaper and more cost-effective than aerogravity (run with other missions?).

- Definitely need surface gravity (terrestrial and shipborne) and terrain models for Guam, CNMI, American Somoa.
- Desire to get such for nearest neighbors including Mexico, Caribbean nations, Central American nations, etc.
- Also need to get any available forward geophysical models for all regions (such as ICE-5G for modeling the Glacial Isostatic Adjustment).
- GPS/INS evaluation of the gravity field.
- GPS & IMU information were also collected on flights.
- This data can be used to derive gravity disturbances and to estimate gravity anomalies.
- It may be useful in benign areas for determining the gravity field. Possibly cheaper and more cost-effective than aerogravity (run with other missions?).

**Ongoing research areas (cont.)**

**Notes:**
-Need seamless gravity data to reduce errors in gravity to geoid modeling
-Need additional gravity outside of U.S. areas – altimetric, neighboring countries
-Also need other data such as density anomalies and terrain data
-Current approach uses many simplifications – a more rigorous approach will reduce errors

-aerogravity fills in gaps and identifies systematic problems in gravity data (shipborne and terrestrial)

## 
Geodetic theory improvements.
Downward continuation of high altitude gravity observations.
Merging of gravity field components.
Current approach is remove-compute-restore.
Spectral merging of EGM, gravity and terrain data.
Would honor long wavelength (GRACE).
Retain character of the terrain and observed data.
Determination of geoid height using ellipsoidal coordinates instead of the spherical approximation.
Resolution of inner and outer zone effects from terrain on gravity observations.

- Geodetic theory improvements.
- Downward continuation of high altitude gravity observations.
- Merging of gravity field components.
- Current approach is remove-compute-restore.
- Spectral merging of EGM, gravity and terrain data.
- Would honor long wavelength (GRACE).
- Retain character of the terrain and observed data.
- Determination of geoid height using ellipsoidal coordinates instead of the spherical approximation.
- Resolution of inner and outer zone effects from terrain on gravity observations.

**Ongoing research areas (cont.)**

**Notes:**
-Need seamless gravity data to reduce errors in gravity to geoid modeling
-Need additional gravity outside of U.S. areas – altimetric, neighboring countries
-Also need other data such as density anomalies and terrain data
-Current approach uses many simplifications – a more rigorous approach will reduce errors

-aerogravity fills in gaps and identifies systematic problems in gravity data (shipborne and terrestrial)

## Gravity measurements help answer two big questions...

- Geoid

- Ellipsoid

- Earth’s
- Surface

- Coast

- Ellipsoid Ht
- From GPS

_***How “high above sea***_
_***level” am I?***_*** (FEMA, USACE, Surveying and Mapping)***

- Ocean
- Surface

- From
- Satellite Altimetry

_***How large are near-shore***_
_***hydrodynamic processes?***_
***(Coast Survey, CSC,***
***CZM)***

_**Gravity measurements help answer two big questions...**_

- Geoid Height
- From Gravity

- Orthometric Ht
- From Leveling

**Notes:**
This slide is an idealized situation demonstrating the usefulness of near-shore gravity, specifically the use of such gravity to accurately compute a gravimetric geoid model.  (The geoid is the one unique equipotential surface of the Earth’s gravity field which best fits, in a least-squares sense, global mean sea level).  

On land, the use of the Global Positioning System (GPS) for fast, accurate surveying is well documented.  However the height determined by GPS refers to a highly idealized, mathematically simple surface known as the ellipsoid.  Thus GPS heights are “ellipsoid heights”.  Unfortunately, ellipsoid heights are non-intuitive in many ways.  First, they can be dozens of meters different from heights above the geoid (known as orthometric heights, or more colloquially, but less correctly as “heights above sea level”).  Secondly, ellipsoid heights do not give an accurate portrayal of how water will flow, due to their complete disconnect from the gravity field of the Earth.  If one has an accurate model of geoid undulations (the separation between the geoid and ellipsoid), then one can take the GPS-derived ellipsoid heights, remove the geoid undulation, and arrive at your orthometric height (a.k.a. “height above sea level”).  This is a significantly faster method of determining orthometric heights compared to traditional surveying methods of spirit leveling.  

On the ocean, satellite altimetry is used to monitor sea level.  However, because the satellite orbits are known relative to the ellipsoid, and the altimetric measurement is from satellite to sea surface, the resulting information is the height of the sea surface above the ellipsoid, often called the “sea surface height” or SSH.  If one is interested, however, in the impact of winds, heating, currents and other phenomena on the ocean surface, then one must remove the gravitational impact on the ocean surface.  That is, one must know the separation between the ellipsoid and the geoid over the ocean in order to remove this signal, and arrive at the residual value.  This residual, the distance from the geoid up to the sea surface, is known by many names, the most common being “sea surface topography” (SST) or “dynamic ocean topography”.  

Coastal areas prone to flooding would benefit both by knowing accurate land elevations (to aid in construction planning, floodplain mapping, etc) as well as knowing accurately how the near shore ocean processes work (to better model responses to hurricanes and other dynamic ocean phenomena).  It is an unfortunate fact that the largest areas missing accurate gravity measurements are those areas immediately offshore.  This is because ships with gravimeters can not get into these shallows, altimetrically derived gravity models are unreliable due to the inability to accurately compute near-shore tides, and terrestrial gravimeters can not measure in the water.  The only practical option to collecting the near-shore ocean gravity data is through airborne based technologies.

Dr. Dru A. Smith
Chief Geodesist
NOAA, National Geodetic Survey
Dru.Smith@noaa.gov
1/11/2006

## Extent of Gravity and Data Collection Flights

**Notes:**
-Surface gravity data are from NGS database: terrestrial, shiptrack and altimetry farther offshore (500 m depths)
-Flights extend from 50-100 km onshore (to stable regions) to well offshore (trustworthy ocean models & altimetry)
-Note lack of crossovers and actual data in Northeast corner => this will affect results later on

## Implied Geoid Changes

**Notes:**
-Unresolved gravity signal differences between airborne and terrestrial data at elevation (10 km)
-Note the stripes – residual biases in aircraft lines
-Also note the features that have significant lateral extents between the lines – particularly in the NW corner.
-This implies a significant problem in our terrestrial data. 
-Additional collections further North and West will be needed to verify and analyze this.
-These differences must be accounted for because they imply significant errors in a geoid model as can be seen here. 
-Note the decimeter level feature in the NW corner 9in the eastern part of the Louisiana subsidence area.
-Note the range of values from +18 cm to -2 cm along the shoreline.
-The aerogravity are consistent (agreement was at the +/- 2 mgal level with no systematic effects accept biases).
-Hence, the differences seem valid and imply a significant error in the terrestrial data.
-These must be resolved before proceeding with geoid generation.

## GPS/INS-Derived Aerogravity vs. Surface Point Gravity in CA

**GPS/INS-Derived Aerogravity vs. Surface Point Gravity in CA**

**Notes:**
-Note the agreement  between tracks at crossovers (internally consistency)
-Note the larger number of surface points in the flat plains and lack of points in mountains
-Note the increased disagreement of surface points in mountains WRT the aerogravity
-Given the reduced amount of data in the mountains and greater likelihood of error in it, the result is that geoid errors will be greater in and near the edges of the mountains – where flooding can more readily occur from runoff.

## GEOID03 Conversion Surface

**Notes:**
-This slide was shown earlier
-We’ve come back to it to look more closely at a possible source of the misclosure through the Eisenhower tunnel.
-Note the there are a lot of bubbly features in the mid-section of the state
-There are decimeter level changes over relatively short distances (for geoid undulations)
-These changes coupled with the larger than normal residual signals (3.3 vs. 2.4 cm) show that there are significant systematic features present in the leveling and/or GPS that occur in a relatively short data spacing.
-Such errors may result be the root cause of the misclosure when adjustment is attempted
-Another possible source of error may be in the level loop
-Remnant magnetism in the surrounding rock may affect the instrument
-the instruments are usually calibrated for the Earth’s geodynamo – but travel in a tunnel may alter the expected field and the resulting leveled heights.

## A Consistent vertical datum between all U.S. states and territories  as well as our neighbors in the region. 
Reduce confusion between neighboring jurisdictions.
Local accuracy but national consistency.
This provides a consistent datum for disaster management.
Storm surge, tsunamis, & coastal storms.
Disasters aren’t bound by political borders.
Heights that can be directly related to oceanic and hydrologic models (coastal and inland flooding problems). 
The resulting improvements to flood maps will better enable decision making for who does & doesn’t need flood insurance.
Updates to the model can be made more easily, if needed, to reflect any temporal changes in the geoid/gravity.
Finally, offshore models of ocean topography will be improved and validated. These models will provide better determination of offshore water flow (useful for evaluating the movement of an oil slick).

- Reduce confusion between neighboring jurisdictions.
- Local accuracy but national consistency.
- This provides a consistent datum for disaster management.
- Storm surge, tsunamis, & coastal storms.
- Disasters aren’t bound by political borders.
- Heights that can be directly related to oceanic and hydrologic models (coastal and inland flooding problems).
- The resulting improvements to flood maps will better enable decision making for who does & doesn’t need flood insurance.
- Updates to the model can be made more easily, if needed, to reflect any temporal changes in the geoid/gravity.
- Finally, offshore models of ocean topography will be improved and validated. These models will provide better determination of offshore water flow (useful for evaluating the movement of an oil slick).

**Expected Results**

## QUESTIONS?

**Geoid Research Team:**
**Dr. Daniel R. Roman, research geodesist **
**dan.roman@noaa.gov**
**Dr. Yan Ming Wang, research geodesist**
**yan.wang@noaa.gov**
**Jarir Saleh, ERT contractor, gravity database analysis**
**William Waickman, programming & database access**
**Website: http://www.ngs.noaa.gov/GEOID/**
**Phone: 301-713-3202**