# Presentation: 479686

## GPS Derived Heights:A Height Modernization PrimerMay 8 and May 10, 2007National Geodetic Survey

- Renee Shields
- National Geodetic Survey
- National Oceanic and Atmospheric Administration

## What types of heights are involved?
Orthometric heights
Ellipsoid heights
Geoid heights

How are these heights defined and related?

How accurately can these heights be determined?

- Orthometric heights
- Ellipsoid heights
- Geoid heights
- How are these heights defined and related?
- How accurately can these heights be determined?

**How to achieve accurate GPS heights**

**Notes:**
Need to understand the three different heights to fully understand GPS derived orthometric heights.

Heights & Datums  - traditionally orthometric heights meant above sea level.  

You need to be aware of the limitations of each height system to understand what you get when using GPS to determine heights.

## What is a GEODETIC DATUM?

- A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating coordinates of points on the Earth*
- [above] together with the coordinate system and the set of all points and lines whose coordinates, lengths, and directions have been determined by measurement or calculation.*
-                      *Definitions from the Geodetic Glossary, September 1986

**Notes:**
Understanding what a datum is will help in understanding where the heights come from and the differences between the heights.
At NGS we use the second definition – note that the 2nd definition does not include the actual computed coordinates.  If it did, then if the coordinates changed, the name of the datum would have to change.   For example, because of GPS technology and the ability to compute better and better positions, the original NAD83 (86) coordinates across the country have been superceded by coordinates recomputed using CORS and High Accuracy Reference Networks as control.  NGS devised a way to distinguish the original coordinates from the superceded coordinates with the use of a ‘datum tag,’ labeling the datum with the year the coordinates were computed.  But the datum remained NAD83.

## Not To Be Confused With:

- Ellipsoid
- A closed surface, whose planar sections are either ellipsoids or circles.*
- Mathematical figure which helps define a Reference Frame
- Clarke 1866, GRS80
- Reference Frame
- A coordinate system associated with a physical system.*
- NSRS, ITRF
-      *Definitions from the Geodetic Glossary, September 1986

**Notes:**
Ellipsoid isn’t the datum - it is part of the definition of a datum.
You need a mathematical figure on which to do computations.  The earth is not a uniform shape, so we need something to approximate the shape and size of the earth.  The ellipsoid helps define the scale needed in your definition of a datum.  
The reference frame gives you the means to define the orientation of the system.
Together, these can define the datum.  At NGS, we add the points and observations so they are included in the definition.

## Horizontal Control Datum?

- A Geodetic Datum specifying the coordinate system in which horizontal control points are located.*
- Defined by 8 Constants
- 3 define location of the origin of the coordinate system.
- 3 define orientation of the coordinate system.
- 2 define dimensions of the reference ellipsoid.
- NAD 27, NAD 83
-                                                                                                     *Definition from the Geodetic Glossary, September 1986

**Notes:**
Horizontal datum is defined as a 3-dimensional datum because of earth centered ellipsoid.

This came about when NAD83 was done because Doppler and VLBI observations were used to orient the ellipsoid.  Also, gravity has an effect on some observations, so curvature of the earth has to be taken into account.

## 
                                                       		    NAD 27                      NAD 83

ELLIPSOID          CLARKE 1866	    	      GRS80
       a = 6,378,206.4 M                    a = 6,378,137.0 M
       1/f = 294.9786982                   1/f = 298.257222101

DATUM POINT    Triangulation Station                 NONE
       MEADES RANCH, Kansas           EARTH MASS CENTER

ADJUSTMENT      25k STATIONS                         250k STATIONS
    Several Hundred Base Lines         Approx. 30k EDMI Base Lines
    Several Hundred Astro Azimuths   5k Astro Azimuths
                                                      Doppler Point Positions
                                                      VLBI Vectors

BEST FITTING      North America                         World-Wide

-                                                        		    _**NAD 27**_**                      **_**NAD 83**_
**ELLIPSOID**          CLARKE 1866	    	      GRS80
-        a = 6,378,206.4 M                    a = 6,378,137.0 M
-        1/f = 294.9786982                   1/f = 298.257222101
**DATUM POINT**    Triangulation Station                 NONE
-        MEADES RANCH, Kansas           EARTH MASS CENTER
**ADJUSTMENT**      25k STATIONS                         250k STATIONS
-     Several Hundred Base Lines         Approx. 30k EDMI Base Lines
-     Several Hundred Astro Azimuths   5k Astro Azimuths
-                                                       Doppler Point Positions
-                                                       VLBI Vectors
**BEST FITTING**      North America                         World-Wide

**Compare Horizontal Datum Elements**

**Notes:**
This shows the components of the NAD 27 and NAD 83 datums.

## NAD 27 and NAD 83

**NAD 27 and NAD 83**

**Notes:**
Up to 100 meter shift – in Alaska

## Vertical Control Datum?

- “A Geodetic Datum specifying the system in which vertical control points are located.” *
- A set of fundamental elevations to which other elevations are referred
- NGVD 29, NAVD 88 – Orthometric, “Sea Level”
- Others – IGLD, Cairo, Local Tidal
-           *Definitions from the Geodetic Glossary, September 1986

**Notes:**
NGVD29 was originally called the Mean Sea Level Datum of 1929 
Changed to National Geodetic Vertical Datum of 1929
	- NGVD 29, July 2, 1973 Federal Register notice

## NAVD 88

**North American Vertical Datum of 1988**

**Federal Register / Vol. 58, No. 120**
**SUMMARY: This Notice announces a decision by the Federal Geodetic Control Subcommittee (FGCS) to affirm the North American Vertical Datum of 1988 (**_**NAVD 88**_**) as the **_**official civilian vertical datum for surveying and mapping**_** activities in the United States performed or financed by the Federal Government, and to the extent practicable, legally allowable, and feasible, require that **_**all Federal agencies using or producing vertical height information undertake an orderly transition to NAVD 88**_**.**

***	***- NAVD 88, June 24, 1993

## 
                                                                NGVD 29                                     NAVD 88

DATUM DEFINITION      26  Tide Gauges                    FATHER’S POINT/Rimouski 
                            in the U.S. & Canada                       Quebec,  Canada

GEOID FITTING      Distorted to Fit MSL Gauges                 Best Continental Model

CORRECTIONS	       Normal Gravity, Level, 	          Observed Gravity, Level, 
                                          Rod, Temp.		 Rod, Temp. Magnetic, Refraction 

BENCH MARKS       	 100,000                                          450,000 

LEVELING (Km)         106,724 (U.S. + Canada)                         1,001,500 

-                                                                 _**NGVD 29**_**                                     **_**NAVD 88**_
**DATUM DEFINITION**      26  Tide Gauges                    FATHER’S POINT/Rimouski
-                             in the U.S. & Canada                       Quebec,  Canada
**GEOID FITTING**      Distorted to Fit MSL Gauges                 Best Continental Model
**CORRECTIONS**	       Normal Gravity, Level, 	          Observed Gravity, Level,
-                                           Rod, Temp.		 Rod, Temp. Magnetic, Refraction
**BENCH MARKS**       	 100,000                                          450,000
**LEVELING (Km)**         106,724 (U.S. + Canada)                         1,001,500

**Vertical Datum Elements: NGVD29 vs NAVD88**

**Notes:**
This shows the differences in the components of the vertical datums.

The datum is not just the values, but the way the computations are done.  For the 1929, normal gravity was used to correct the orthometric height, for 1988 actual gravity was used and geopotential nos were computed.  For 1929 only level, Rod and temp corrections were applied, for 1988 we also used astronomic, magnetic, and refraction corrections.
Only one point was constrained in the NAVD 88 adjustment because it had been demonstrated that there were variations in sea surface topography, that mean sea level at different tide gauges is not the same equipotential surface.

## Differential Leveling – NGVD29

**Differential Leveling – NGVD29**

## Differential Leveling – NAVD88

**Differential Leveling – NAVD88**

## Height Difference - NGVD29 to NAVD88

**0**

**0**

**Height Difference - NGVD29 to NAVD88**

- NOAA/NGS

## Slide 15

**Notes:**
National color map - differences between NGVD29 and NAVD88 datums

Portrays general east - west tilt; rugged areas indicate major changes whereas smooth are minor changes. 2 & 3 cm level differences over steep gradients.

## Conventional Leveling

**Conventional Leveling**

**(NAVD88)**** H**

**H = Orthometric Height**  **(leveling)**

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

- TOPOGRAPHIC SURFACE

**h = Ellipsoidal Height (GPS)**

**N = Geoid Height (model)**

**H ****(NAD83)**

- Ellipsoid

**N**

-   Geoid

- Geoid *Height*
-   **(GEOID03)**

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

**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.

## 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

- Dr. Dan Roman

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

## High Resolution Geoid Models, basically -

**Start with gravity measurements**
**Add Digital Elevation Data**
**Add a Global Earth Gravity Model (EGM96)**
**Result – A Gravitational (purely scientific) geoid model**
**But is this what we need?**

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

## Slide 20

**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.

##  KK1393  DESIGNATION -  ALAMEDA
 KK1393  PID         -  KK1393
 KK1393  STATE/COUNTY-  CO/JEFFERSON
 KK1393  USGS QUAD   -  FORT LOGAN (1994)
 KK1393
 KK1393                         *CURRENT SURVEY CONTROL
 KK1393  ___________________________________________________________________
 KK1393* NAD 83(1992)-  39 42 39.39630(N)    105 03 57.01077(W)     ADJUSTED  
 KK1393* NAVD 88     -      1663.505  (meters)    5457.68   (feet)  ADJUSTED  
 KK1393  ___________________________________________________________________
 KK1393  X           -  -1,277,430.994 (meters)                     COMP
 KK1393  Y           -  -4,745,618.632 (meters)                     COMP
 KK1393  Z           -   4,054,400.023 (meters)                     COMP
 KK1393  LAPLACE CORR-         -12.19  (seconds)                    DEFLEC99
 KK1393  ELLIP HEIGHT-        1646.84  (meters)          (12/03/02) GPS OBS
 KK1393  GEOID HEIGHT-         -16.65  (meters)                     GEOID03
 KK1393  DYNAMIC HT  -        1661.883 (meters)    5452.36  (feet)  COMP
 KK1393  MODELED GRAV-     979,592.8   (mgal)                       NAVD 88
 KK1393
 KK1393  HORZ ORDER  -  FIRST
 KK1393  VERT ORDER  -  FIRST     CLASS II
 KK1393  ELLP ORDER  -  FOURTH    CLASS II

-  KK1393  PID         -  KK1393
-  KK1393  STATE/COUNTY-  CO/JEFFERSON
-  KK1393  USGS QUAD   -  FORT LOGAN (1994)
-  KK1393
-  KK1393                         *CURRENT SURVEY CONTROL
-  KK1393  ___________________________________________________________________
-  KK1393* NAD 83(1992)-  39 42 39.39630(N)    105 03 57.01077(W)     ADJUSTED
-  KK1393* NAVD 88     -      1663.505  (meters)    5457.68   (feet)  ADJUSTED
-  KK1393  ___________________________________________________________________
-  KK1393  X           -  -1,277,430.994 (meters)                     COMP
-  KK1393  Y           -  -4,745,618.632 (meters)                     COMP
-  KK1393  Z           -   4,054,400.023 (meters)                     COMP
-  KK1393  LAPLACE CORR-         -12.19  (seconds)                    DEFLEC99
-  KK1393  ELLIP HEIGHT-        1646.84  (meters)          (12/03/02) GPS OBS
-  KK1393  GEOID HEIGHT-         -16.65  (meters)                     GEOID03
-  KK1393  DYNAMIC HT  -        1661.883 (meters)    5452.36  (feet)  COMP
-  KK1393  MODELED GRAV-     979,592.8   (mgal)                       NAVD 88
-  KK1393
-  KK1393  HORZ ORDER  -  FIRST
-  KK1393  VERT ORDER  -  FIRST     CLASS II
-  KK1393  ELLP ORDER  -  FOURTH    CLASS II

**Sample Datasheet**

**N**

**H**

**h**

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

##  KK1393.The horizontal coordinates were established by GPS observations
 KK1393.and adjusted by the National Geodetic Survey in January 1998..
 KK1393
 KK1393 The orthometric height was determined by differential leveling
 KK1393 and adjusted by the National Geodetic Survey in June 1991.
 KK139
 KK1393.The X, Y, and Z were computed from the position and the ellipsoidal ht.
 KK1393
 KK1393.The Laplace correction was computed from DEFLEC99 derived deflections.
 KK1393
 KK1393 The ellipsoidal height was determined by GPS observations
 KK1393 and is referenced to NAD 83.
 KK1393
 KK1393.The geoid height was determined by GEOID03.
 KK1393
 KK1393.The dynamic height is computed by dividing the NAVD 88
 KK1393.geopotential number by the normal gravity value computed on the
 KK1393.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45
 KK1393.degrees latitude (g = 980.6199 gals.).
 KK1393
 KK1393.The modeled gravity was interpolated from observed gravity values.

-  KK1393.and adjusted by the National Geodetic Survey in January 1998..
-  KK1393
-  KK1393 The _**orthometric height was determined by differential leveling**_
-  KK1393 _**and adjusted by the National Geodetic Survey in June 1991**_.
-  KK139
-  KK1393.The X, Y, and Z were computed from the position and the ellipsoidal ht.
-  KK1393
-  KK1393.The Laplace correction was computed from DEFLEC99 derived deflections.
-  KK1393
-  KK1393 The _**ellipsoidal height was determined by GPS observations**_
-  KK1393 _**and is referenced to NAD 83**_.
-  KK1393
-  KK1393.The _**geoid height was determined by GEOID03**_.
-  KK1393
-  KK1393.The dynamic height is computed by dividing the NAVD 88
-  KK1393.geopotential number by the normal gravity value computed on the
-  KK1393.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45
-  KK1393.degrees latitude (g = 980.6199 gals.).
-  KK1393
-  KK1393.The modeled gravity was interpolated from observed gravity values.

**Sample Datasheet**

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

##  KK1393;                    North         East     Units Scale Factor Converg.
 KK1393;SPC CO C     -   513,324.546   951,632.174   MT  0.99999251   +0 16 25.8
 KK1393;SPC CO C     - 1,684,132.28  3,122,146.56   sFT  0.99999251   +0 16 25.8
 KK1393;UTM  13      - 4,395,677.639   494,356.607   MT  0.99960039   -0 02 31.4
 KK1393
 KK1393!             -  Elev Factor  x  Scale Factor =   Combined Factor
 KK1393!SPC CO C     -   0.99974170  x   0.99999251  =   0.99973422
 KK1393!UTM  13      -   0.99974170  x   0.99960039  =   0.99934220
 KK1393
 KK1393:                Primary Azimuth Mark                     Grid Az
 KK1393:SPC CO C     -  ALAMEDA AZ MK OFFSET                     088 03 11.6
 KK1393:UTM  13      -  ALAMEDA AZ MK OFFSET                     088 22 08.8
 KK1393                          SUPERSEDED SURVEY CONTROL
 KK1393
 KK1393  ELLIP H (01/12/98) 1646.88   (m)                       GP(       ) 4 1
 KK1393  NAD 83(1992)-  39 42 39.39665(N)    105 03 57.01291(W) AD(       ) 1
 KK1393  NAD 83(1986)-  39 42 39.39304(N)    105 03 57.01147(W) AD(       ) 1
 KK1393  NAD 27      -  39 42 39.43921(N)    105 03 55.06755(W) AD(       ) 1
 KK1393  NAVD 88 (01/12/98) 1663.51   (m)         5457.7    (f) LEVELING    3  
 KK1393  NGVD 29 (??/??/??) 1662.577  (m)         5454.64   (f) ADJUSTED    1 2
 KK1393
 KK1393.Superseded values are not recommended for survey control.
 KK1393.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.
 KK1393.See file dsdata.txt to determine how the superseded data were derived.

-  KK1393;SPC CO C     -   513,324.546   951,632.174   MT  0.99999251   +0 16 25.8
-  KK1393;SPC CO C     - 1,684,132.28  3,122,146.56   sFT  0.99999251   +0 16 25.8
-  KK1393;UTM  13      - 4,395,677.639   494,356.607   MT  0.99960039   -0 02 31.4
-  KK1393
-  KK1393!             -  Elev Factor  x  Scale Factor =   Combined Factor
-  KK1393!SPC CO C     -   0.99974170  x   0.99999251  =   0.99973422
-  KK1393!UTM  13      -   0.99974170  x   0.99960039  =   0.99934220
-  KK1393
-  KK1393:                Primary Azimuth Mark                     Grid Az
-  KK1393:SPC CO C     -  ALAMEDA AZ MK OFFSET                     088 03 11.6
-  KK1393:UTM  13      -  ALAMEDA AZ MK OFFSET                     088 22 08.8
-  KK1393                          SUPERSEDED SURVEY CONTROL
-  KK1393
-  KK1393  ELLIP H (01/12/98) 1646.88   (m)                       GP(       ) 4 1
-  KK1393  NAD 83(1992)-  39 42 39.39665(N)    105 03 57.01291(W) AD(       ) 1
-  KK1393  NAD 83(1986)-  39 42 39.39304(N)    105 03 57.01147(W) AD(       ) 1
-  KK1393  NAD 27      -  39 42 39.43921(N)    105 03 55.06755(W) AD(       ) 1
-  KK1393  NAVD 88 (01/12/98) 1663.51   (m)         5457.7    (f) LEVELING    3
-  KK1393  NGVD 29 (??/??/??) 1662.577  (m)         5454.64   (f) ADJUSTED    1 2
-  KK1393
-  KK1393.Superseded values are not recommended for survey control.
-  KK1393.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.
-  KK1393.See file dsdata.txt to determine how the superseded data were derived.

**Sample Datasheet**

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

##  KK1393  DESIGNATION -  ALAMEDA
 KK1393  PID         -  KK1393
 KK1393  STATE/COUNTY-  CO/JEFFERSON
 KK1393  USGS QUAD   -  FORT LOGAN (1994)
 KK1393
 KK1393                         *CURRENT SURVEY CONTROL
 KK1393  ___________________________________________________________________
 KK1393* NAD 83(1992)-  39 42 39.39630(N)    105 03 57.01077(W)     ADJUSTED  
 KK1393* NAVD 88     -      1663.505  (meters)    5457.68   (feet)  ADJUSTED  
 KK1393  ___________________________________________________________________
 KK1393  X           -  -1,277,430.994 (meters)                     COMP
 KK1393  Y           -  -4,745,618.632 (meters)                     COMP
 KK1393  Z           -   4,054,400.023 (meters)                     COMP
 KK1393  LAPLACE CORR-         -12.19  (seconds)                    DEFLEC99
 KK1393  ELLIP HEIGHT-        1646.84  (meters)          (12/03/02) GPS OBS
 KK1393  GEOID HEIGHT-         -16.65  (meters)                     GEOID03
 KK1393  DYNAMIC HT  -        1661.883 (meters)    5452.36  (feet)  COMP
 KK1393  MODELED GRAV-     979,592.8   (mgal)                       NAVD 88
 KK1393
 KK1393  HORZ ORDER  -  FIRST
 KK1393  VERT ORDER  -  FIRST     CLASS II
 KK1393  ELLP ORDER  -  FOURTH    CLASS II

-  KK1393  PID         -  KK1393
-  KK1393  STATE/COUNTY-  CO/JEFFERSON
-  KK1393  USGS QUAD   -  FORT LOGAN (1994)
-  KK1393
-  KK1393                         *CURRENT SURVEY CONTROL
-  KK1393  ___________________________________________________________________
-  KK1393* NAD 83(1992)-  39 42 39.39630(N)    105 03 57.01077(W)     ADJUSTED
-  KK1393* NAVD 88     -      1663.505  (meters)    5457.68   (feet)  ADJUSTED
-  KK1393  ___________________________________________________________________
-  KK1393  X           -  -1,277,430.994 (meters)                     COMP
-  KK1393  Y           -  -4,745,618.632 (meters)                     COMP
-  KK1393  Z           -   4,054,400.023 (meters)                     COMP
-  KK1393  LAPLACE CORR-         -12.19  (seconds)                    DEFLEC99
-  KK1393  ELLIP HEIGHT-        1646.84  (meters)          (12/03/02) GPS OBS
-  KK1393  GEOID HEIGHT-         -16.65  (meters)                     GEOID03
-  KK1393  DYNAMIC HT  -        1661.883 (meters)    5452.36  (feet)  COMP
-  KK1393  MODELED GRAV-     979,592.8   (mgal)                       NAVD 88
-  KK1393
-  KK1393  HORZ ORDER  -  FIRST
-  KK1393  VERT ORDER  -  FIRST     CLASS II
-  KK1393  ELLP ORDER  -  FOURTH    CLASS II

**Sample Datasheet**

**N**

**H**

**h**

- NAVD88 – Ellipsoid Ht + Geoid Ht = ...
- 1663.505 – 1646.84 – 16.93 =  -0.265   USGG2003
- 1663.505 – 1646.84 – 16.65 =  +0.015   GEOID03

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

## The Missing Link

- Leveling-Derived Orthometric Heights
- Modeled Geoid Heights
_**What’s left?**_

**Notes:**
We know how to get 
  - Accurate orthometric heights at benchmarks by leveling – this is expensive and time consuming!  But these are still the best heights you can get.

We now are able to develop a hybrid geoid model from a gravimetric geoid model with
 - Accurate ellipsoid heights
 - More benchmarks with GPS measurements – the better the NAVD88, the better the geoid model

How do we get improved ellipsoid heights?
Better field procedures
Accurate ellipsoid heights at control stations

**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.

## enter GPS...

***enter GPS...***

***(******GNSS******)***

## Guidelines

**Guidelines**

- Available “On-Line” at
- the NGS Web Site:
- www.ngs.noaa.gov/PUBS_LIB/pub_index.html

**Notes:**
Guidelines - will eventually become routine;  not so much explanation of why its done but provides background information. Repeat baselines, station spacing, fixed height antenna setups, identify and control all error sources, tie local networks together, etc..

NOS NGS-58 GPS-Derived Ellipsoid Heights Guidelines will lay the foundation for GPS-Derived Orthometric Heights Guidelines.  Produce 2 cm ellipsoid heights to be able to obtain 2 cm orthometric heights. 

## Components of NGS-58

- Equipment requirements
- Field Procedures/Data Collection Parameters
- Basic Control Requirements
- Processing/Analysis Procedures

## Equipment Requirements

- Dual-frequency, full-wavelength GPS receiver
- Required - observations > 10 km
- Preferred - ALL observations regardless of length
- Geodetic quality antennas with ground planes
- Choke ring antennas; highly recommended
- Successfully modeled L1/L2 offsets and phase patterns
- Use identical antenna types if possible
- Corrections must be utilized by processing software when mixing antenna types

**Notes:**
Different receiver types and models - Trimble, Ashtech, Leica, Novatel, Allen Osborne, Topcon, Javad, etc.; full and half phase L1/L2 signal coding.

Dual frequency correction - two different frequencies; remove iono effects - iono free solution, L1/L2 solution, L3 combined L1/L2 solution.

Antenna phase center variation - there were error sources in conventional leveling - now there are problems in GPS.  Using same antenna types tends to cancel some of the influences by antenna on satellite signal.

Mixing antenna types could introduce up to 10 cm uncertainty in height component.  Signal coming down from satellite at varying elevations and azimuths; defined phase center moves up and down relative to direction of the incoming signal.  

2 to 10 km separation between stations produce similar reception angle of satellite signal received at antennas and effects tend to cancel; 20 to 50 km reception angle is different and doesn’t tend to cancel.  

## Equipment Requirements

**Tripod**

**Equipment Requirements**

**Notes:**
Example of fixed height pole with antenna.

## Data Collection Parameters

**VDOP < 6 for 90% or longer of 30 minute session**
**Shorter session lengths stay < 6 always**
**Schedule travel during periods of higher VDOP**
**Session lengths > 30 minutes collect 15 second data**
**Session lengths < 30 minutes collect 5 second data**
**Track satellites down to 10° elevation angle**
**Repeat Baselines**
**Different days**
**Different times of day**
**Detect, remove, reduce effects due to multipath and having almost the same satellite geometry**

**Notes:**
- Vertical element critical relative to Sat geometry – position/elevation derived by averaging of all signals - key to accurate heights is to average all affects.
- Outliers with low VDOP values- may have multipath at site. When bad multipath detected; raise mask but pay close attention to VDOP values.  Maybe required to record more data.
- Increased masks - time vs. mask - can’t observe above indicated line due to VDOP cutoff of  6.  Same data sets using different masks sometimes produce 5 cm differences - pay attention to VDOP.
- Session lengths to average affects.  Longer sessions to average affects of longer baselines whereas shorter sessions get by with shorter baselines as affects at each station are similar.
- 10 degree mask for data collection allows the receiver to “cleanly” lock on to low elevation satellites providing strong, continuous satellite lock when processing at 15 degrees.  Collecting data down to 10 degrees typically does not require much additional storage space.

## Two Days/Same Time

**Two Days/Same Time**

**Difference = 0.3 cm**

**“****Truth” =  -10.276**

**Difference = 2.3 cm**

**Two Days/**
**Different Times**

**-10.254**

**-10.295**

**> -10.275**

**Difference = 4.1 cm**

**“****Truth” =  -10.276**

**Difference = 0.1 cm**

**   ****-10.254**
**   ****-10.251**

**>  -10.253**

**Notes:**
Dh is computed for a series of 30 minute sessions over 2 days and the average dh is found
Taking the observation from the same time each day (when the satellite geometry is similar), the difference in the dh is only .3 cm, but the difference from the average is 2.3 cm.  
Now if you take the dh from different times on the 2 days (different geometry), the difference between the 2 vectors is 4.1 cm, but the difference from the average is only .1 cm

## Station pairs with large residuals, i.e., greater than 2.5 cm, also have large repeat base line

- differences.  _NGS guidelines for estimating GPS-derived ellipsoid heights require user to_
- _ __re-observe these base lines_.  Following NGS guidelines provides enough redundancy for
-  adjustment process to detect outliers and apply residual on appropriate observation, i.e., the
-  bad vector.

**Notes:**
Repeat baseline comparisons also indicate which baselines will show large residuals.

## Four Basic Control Requirements

**Occupy stations with known NAVD 88 orthometric heights**
**Stations should be evenly distributed throughout project**
**Project areas < 20 km on a side, surround project with NAVD 88 bench marks**
**i.e., minimum number of stations is four; one in each corner of project**
**Project areas > 20 km on a side, keep distances between GPS-occupied NAVD 88 bench marks to less than 20 km**
**Projects located in mountainous regions, occupy bench marks at base and summit of mountains, even if distance is less than 20 km**

**Notes:**
This same information is required by the GPS-derived Orthometric heights guidelines.  Producing ellipsoid heights at the 2 cm level of accuracy is essential to producing accurate GPS-derived orthometric heights.
Surrounding project areas will be difficult in many instances with existing NAVD88 bench marks.  Level ties may be necessary to provide this requirement.
Extra bench marks allow independent analysis through the adjustment process and may help identify potential bench marks with questionable stability or poor published elevations.
Bench marks at base and top of mountains may help identify/rectify issues that are not otherwise apparent in the geoid model.

## Sample Project

**Area: East San Francisco Bay Project**
** ****Latitude       37° 50” N  to   38° 10” N**
**Longitude   121° 45” W to 122° 25” W**
**Receivers Available: 5**
**Standards: 2 cm GPS-Derived Heights**

**Notes:**
Northeastern part of San Francisco Bay and continuation of the San Francisco Bay Demonstration Project.

All planning requires pertinent information like how many receivers are going to be used and to what standards is the survey being performed.

## Primary Base Stations

**Primary Base Stations**

**Notes:**
Primary base stations are observed for 3 sessions of 5 hour duration using the two HPGN and one CORS as control.

Baseline lengths indicate that the spacing between primary base stations is less than the required 40 km.

An adjustment is performed on these sessions to produce positions/ellipsoid heights for the two new primary base stations, MART and D191.

## GPS-Usable Stations

**GPS-Usable Stations**

**Spacing Station**

**Primary Base Station**

**Notes:**
Project planning- what stations are being created and which available GPS usable stations will be used for control.  Observation logistics – can I get there from here?
This project had several “gaps” where initial planed local control stations were not selected.  To maintain a consistent average 10 km spacing several “filler” stations were included in the project.  4 NAVD88 bench marks surround the project area.
Primary base stations were selected from the existing HPGN and two other stations that surround the project area, one a vertical control station.

## Observation Sessions

**Observation Sessions**

**Notes:**
Observation scenario planning – five receivers G number of sessions.  Each session observed twice under different satellite geometry.  Short baseline lengths, short observation sessions.
Observation progression provides minimal move times between sessions enabling the crews to complete all sessions, A through G, in one day.  Poor “windows” of satellite geometry planned during move times.
Individual sessions overlap previous session providing good network configuration and redundancy.
Session A started on known control station which allows coordinate seeding through the unknown stations.
There are many number of observation scenarios to provide similar results.

## Perform 3-D minimally constrained (free) adjustment 
Analyze adjustment results
Compute differences between GPS-derived orthometric heights from free adjustment and published NAVD88 BMs  
Evaluate differences to determine which BMs have valid NAVD88 height values 
Perform constrained adjustment with results from previous step

- Analyze adjustment results
- Compute differences between GPS-derived orthometric heights from free adjustment and published NAVD88 BMs
- Evaluate differences to determine which BMs have *valid* NAVD88 height values
- Perform constrained adjustment with results from previous step

**Processing: Five Basic Procedures**

**Notes:**
Compare free adjusted NAVD88 elevations against published elevations.  Determine trends from differences, such as slope, and which bench marks don’t fit.

Check monument type and setting information, reported station condition, and history data (i.e., mark set and leveled in 1950) to see if there is a reason for station elevation to not fit with the adjustment.

## GPS Ellipsoid Height Hierarchy

**GPS Ellipsoid Height Hierarchy**

**Notes:**
Appendix B.
Guidelines guarantee proper ties and provide acceptable relative accuracies.

HARN not better than 5 cm vertical accuracy absolute.
CORS getting down to 3 cm vertical accuracy absolute.

Layering of control to produce local network stations.  Shorter baseline lengths, shorter observation times.

## Start with CORS

**Table Mountain CORS**

**Platteville CORS**

**Aztec CORS**

**Notes:**
Top level control network – backbone of NSRS

## Next Level - Colorado HARN

**Notes:**
Further densify Control network – provide additional ellipsoid height control.  Include bench marks with published NAVD88 heights to get “GPS on bench marks” for the geoid model.

## Have good NAVD88 Control

**Notes:**
Observe leveling if the bench mark control network is inadequate.

## Height Modernization Project

**Height Modernization Project**

**HARN - Average 50km, max 75km**

**Primary – 20-25km, max 40km**

**Secondary - Average 12-15km, max 15km**
**            ****Local – Average 6-8 km, max 10km**

**Notes:**
Sample Monument Spacing:

HARN 		50 km	(75 km max for 2 cm)
Primary GPS		20-25 km	(40 km max for 2 cm)
Secondary GPS	12-15 km	(15 km max for 2 cm)
Local GPS		6-8 km	(10 km max for 2 cm)

Use results of leveling to primary stations, with GPS for Secondary and Local networks to get Ortho heights on all stations.

##  KK1393  DESIGNATION -  ALAMEDA
 KK1393  PID         -  KK1393
 KK1393  STATE/COUNTY-  CO/JEFFERSON
 KK1393  USGS QUAD   -  FORT LOGAN (1994)
 KK1393
 KK1393                         *CURRENT SURVEY CONTROL
 KK1393  ___________________________________________________________________
 KK1393* NAD 83(1992)-  39 42 39.39630(N)    105 03 57.01077(W)     ADJUSTED  
 KK1393* NAVD 88     -      1663.505  (meters)    5457.68   (feet)  ADJUSTED  
 KK1393  ___________________________________________________________________
 KK1393  X           -  -1,277,430.994 (meters)                     COMP
 KK1393  Y           -  -4,745,618.632 (meters)                     COMP
 KK1393  Z           -   4,054,400.023 (meters)                     COMP
 KK1393  LAPLACE CORR-         -12.19  (seconds)                    DEFLEC99

-  KK1393  PID         -  KK1393
-  KK1393  STATE/COUNTY-  CO/JEFFERSON
-  KK1393  USGS QUAD   -  FORT LOGAN (1994)
-  KK1393
-  KK1393                         *CURRENT SURVEY CONTROL
-  KK1393  ___________________________________________________________________
-  KK1393* NAD 83(1992)-  39 42 39.39630(N)    105 03 57.01077(W)     ADJUSTED
-  KK1393* NAVD 88     -      1663.505  (meters)    5457.68   (feet)  ADJUSTED
-  KK1393  ___________________________________________________________________
-  KK1393  X           -  -1,277,430.994 (meters)                     COMP
-  KK1393  Y           -  -4,745,618.632 (meters)                     COMP
-  KK1393  Z           -   4,054,400.023 (meters)                     COMP
-  KK1393  LAPLACE CORR-         -12.19  (seconds)                    DEFLEC99

**Sample Datasheet – Leveling**** **

-  KK1393 The _**orthometric height was determined by differential leveling**_
-  KK1393 _**and adjusted by the National Geodetic Survey in June 1991**_.
-  KK139

##  KK1779  DESIGNATION -  BARBARA
 KK1779  PID         -  KK1779
 KK1779  STATE/COUNTY-  CO/JEFFERSON
 KK1779  USGS QUAD   -  GOLDEN (1994)
 KK1779
 KK1779                         *CURRENT SURVEY CONTROL
 KK1779  ___________________________________________________________________
 KK1779* NAD 83(1992)-  39 51 05.58504(N)    105 10 43.39354(W)     ADJUSTED  
 KK1779* NAVD 88     -      1773.1    (meters)    5817.     (feet)  GPS OBS   
 KK1779  ___________________________________________________________________
 KK1779  X           -  -1,284,184.274 (meters)                     COMP
 KK1779  Y           -  -4,733,530.907 (meters)                     COMP
 KK1779  Z           -   4,066,471.229 (meters)                     COMP

-  KK1779  PID         -  KK1779
-  KK1779  STATE/COUNTY-  CO/JEFFERSON
-  KK1779  USGS QUAD   -  GOLDEN (1994)
-  KK1779
-  KK1779                         *CURRENT SURVEY CONTROL
-  KK1779  ___________________________________________________________________
-  KK1779* NAD 83(1992)-  39 51 05.58504(N)    105 10 43.39354(W)     ADJUSTED
-  KK1779* **NAVD 88     -      1773.1    (meters)    5817.     (feet)  GPS OBS**
-  KK1779  ___________________________________________________________________
-  KK1779  X           -  -1,284,184.274 (meters)                     COMP
-  KK1779  Y           -  -4,733,530.907 (meters)                     COMP
-  KK1779  Z           -   4,066,471.229 (meters)                     COMP

**Sample Datasheet - GPS**

-  KK1779. . .
-  KK1779 The orthometric height was determined by _GPS observations and a_
-  KK1779 _high-resolution geoid model_.
-  KK1779

##  OM1256  CBN         -  This is a Cooperative Base Network Control Station.
 OM1256  DESIGNATION -  CAMBRIA GPS
 OM1256  PID         -  OM1256
 OM1256  STATE/COUNTY-  WI/COLUMBIA
 OM1256  USGS QUAD   -  RANDOLPH (1980)
 OM1256
 OM1256                         *CURRENT SURVEY CONTROL
 OM1256  ___________________________________________________________________
 OM1256* NAD 83(1997)-  43 34 10.47581(N)    089 06 09.35995(W)     ADJUSTED  
 OM1256* NAVD 88     -       303.84   (meters)     996.8    (feet)  GPS OBS   
 OM1256  ___________________________________________________________________
 OM1256  X           -      72,495.418 (meters)                     COMP
 OM1256  Y           -  -4,628,194.515 (meters)                     COMP
 OM1256  Z           -   4,373,750.772 (meters)                     COMP

-  OM1256  DESIGNATION -  CAMBRIA GPS
-  OM1256  PID         -  OM1256
-  OM1256  STATE/COUNTY-  WI/COLUMBIA
-  OM1256  USGS QUAD   -  RANDOLPH (1980)
-  OM1256
-  OM1256                         *CURRENT SURVEY CONTROL
-  OM1256  ___________________________________________________________________
-  OM1256* NAD 83(1997)-  43 34 10.47581(N)    089 06 09.35995(W)     ADJUSTED
-  OM1256* **NAVD 88     -       303.84   (meters)     996.8    (feet)  GPS OBS**
-  OM1256  ___________________________________________________________________
-  OM1256  X           -      72,495.418 (meters)                     COMP
-  OM1256  Y           -  -4,628,194.515 (meters)                     COMP
-  OM1256  Z           -   4,373,750.772 (meters)                     COMP

**Sample Datasheet – GPS Using NGS58**

** **OM1256  . . .
-  OM1256.The orthometric height was _determined by GPS observations and a_
-  OM1256._high-resolution geoid model using precise GPS observation and_
-  OM1256._processing techniques._

## Network / Local Accuracy

**Network / Local Accuracy**

**Notes:**
- A project performed in southern South Carolina has local accuracy of 2 cm relative to the points in that project.
- A project performed in northern South Carolina and southern North Carolina has local accuracy of 2 cm relative to the points in that project.
- The project in southern South Carolina is relative to the project in northern South Carolina and southern North Carolina at the 5 cm network accuracy.

## Goal Achieved?

- With the means to get more accurate ellipsoid heights, we can now use these ellipsoid heights on benchmarks to further improve the geoid model.
- With the improved geoid model we can use GPS to compute accurate orthometric heights.

## Questions?

**Renee Shields**
**Height Modernization Manager**
- N/NGS1, SSMC3, Room 9357
- 1315 East-West Highway
- Silver Spring, MD  20910
- 301-713-3231, x116
- Renee.Shields@noaa.gov

**Contact information**