# Presentation: 552166

## GPS Derived Heights: A Height ModernizationPrimer

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

**Marquette, Michigan.......January 24, 2006**
**Flint, Michigan..............January 26, 2006**

**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.

## Height Modernization is...

**...**the establishment of accurate, reliable heights
-  using GPS technology in conjunction with
-  traditional leveling, gravity,
- and modern remote sensing information.

- ...the ability to compute
- accurate orthometric (“sea level”) heights
- using GPS technology

## 1) What types of heights are involved?
		•  Orthometric heights
		•  Ellipsoid heights
		•  Geoid heights

2) How are these heights defined and related?

3) How accurately can these heights be determined?

- 		•  Orthometric heights
- 		•  Ellipsoid heights
- 		•  Geoid heights
- 2) How are these heights defined and related?
- 3) 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?

-  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?

- Horizontal Control Datum
- “A Geodetic Datum specifying the coordinate system in which horizontal control points are located.”
- Defined by 8 Constants
- 3 – specify the **location of the origin** of the coordinate system.
- 3 – specify the **orientation** of the coordinate system.
- 2 – specify the 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. 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             Appox. 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. 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             Appox. 30k EDMI Base Lines
-        Several Hundred Astro Azimuths               5k Astro Azimuths
-                                                                      Doppler Point Positions
-                                                                             VLBI Vectors
- BEST FITTING                             North America                                      World-Wide

- Comparison of Horizontal Datum Elements

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

## NAD 27 and NAD 83

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

## Vertical Control Datum?

- 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 – Cairo, Local Tidal
- *Definitions from the Geodetic Glossary, September 1986

## 
                                                     NGVD 29                      NAVD 88

DATUM DEFINITION             26  TIDE GAUGES                    FATHER’S POINT/RIMOUSKI 
                                         IN THE U.S. & CANADA                       QUEBEC,  CANADA

BENCH MARKS       	              100,000                                                450,000 

LEVELING (Km)                                102,724                                              1,001,500 

GEOID FITTING                Distorted  to Fit MSL Gauges                  Best Continental Model                 

-                                                      _NGVD 29_                      _NAVD 88_
- DATUM DEFINITION             26  TIDE GAUGES                    FATHER’S POINT/RIMOUSKI
-                                          IN THE U.S. & CANADA                       QUEBEC,  CANADA
- BENCH MARKS       	              100,000                                                450,000
- LEVELING (Km)                                102,724                                              1,001,500
- GEOID FITTING                Distorted  to Fit MSL Gauges                  Best Continental Model

- Orthometric HeightsComparison of Vertical Datum Elements

**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.

## NGVD 29 and NAVD 88

**Notes:**
As much as 5 feet – or a meter and a half, even 2 meters in some areas, with most of it in the Rockies of Colorado.  You can see the zero shift line running from Father’s Point down to Mexico, taking a meandering route as it approaches the Gulf.

## Conventional Leveling

**BM**
**100’**

- ROD 1

- INSTR

**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.

## 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

## What is the GEOID?

- “The *equipotential surface* of the Earth’s gravity field which best fits, in the least squares sense, mean sea level.”*
- Can’t see the surface or measure it directly.
- Can be modeled from gravity data.
-    *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.  But we are dependant on leveling observations on the surface to create our datum.

## In Search of the Geoid...

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

## 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 ModelsG99SSS (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.
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.

## Slide 20

**Notes:**
-although GSFC00.1 created significant changes when looking at the difference between G99SSS and USGG2003, the magnitude of those differences is dwarfed by the total signal

## 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 **
**2.7**** 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.

2.7 cm RMS when comparing to bench mark data.

## Slide 23

## GPS on Bench

- Marks in Michigan

## Composite Geoids

- Gravity Geoid  systematic misfit with benchmarks
- Composite Geoid biased to fit local benchmarks

**Earth’s Surface**

**Ellipsoid**

**Composite Geoid**

**Gravity Geoid**

**0.271 M in **
**Traverse City – **
**1999 model**

**0.308 M in **
**Traverse City – **
**2003 model**

- 01-Jun-04

**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.

## 
        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

**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   iFT  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   iFT  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

##  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

- 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.  

## What’s Left?

- Leveling-Derived Orthometric Heights
- Modeled Geoid Heights
- GPS-Derived Ellipsoid Heights

**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

## Guidelines

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

**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. 

## GPS Error Sources

- Orbit and clock error
- Broadcast v.s. Predicted Precise v.s. Post-processed
- Clock error corrections
- Atmospheric Effects
-  Ionosphere, Troposphere
- Multipath
- Height of phase center above mark

**Notes:**
You can choose which orbit to use – Post-processed is most accurate, but predicted is now also very accurate.
Clock error is negligible.
Ionosphere and Troposphere are modeled now, and can be corrected for.  Corrections are generally in the software.

## Atmosphere-based Ionospheric Delay

**Ionosphere**

**< 10 km**

**> 10 km**

**Notes:**
Long baselines are improved if atmospheric corrections are applied to correct for the effects of “different atmospheres” at each end of the line

## Multipath

**August 1987 -Ionospheric refraction and Multipath Effects  in GPS Carrier Phase Observations**
**Yola Georgiadou and Alfred Kleusberg                                                **
**IUGG XIX General Assembly Meeting, Vancouver, Canada****             **

**ø**

**ø**

**Figure 1**
**Multipath Description**

**h**

**Notes:**
Signal received from satellite by the most direct route, shortest path is the signal we want.  “Ghosting” is created by delayed signal bouncing from objects.

## Slide 36

**Notes:**
Referencing the antenna where satellite signals are acquired to the mark on the ground.  Height above the mark to the antenna reference point (ARP), typically the base of the antenna mount, provides the necessary height relationship of the antenna to the mark on the ground.  Processing software includes the constant antenna relationship information, i.e., ARP to bottom/top of ground plane, and to L1/L2 phase centers.

## Slide 37

**Notes:**
Additional information relating ARP to various antenna components.

## 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

- Fixed-height
- tripods *required*
- for 2 cm standard
- Fixed-height poles
*preferred* for 5 cm
- standard

**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.

## Gg

- gg

- 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.

## After performing minimum constraint adjustment, plot ellipsoid height

-  residuals (or dU residuals) and investigate all residuals greater than 2 cm.

**Notes:**
Plot of free adjustment ellipsoid height residuals by baseline length.

Identify and investigate residuals greater than 2 cm.  Reprocess if possible, re-observe if necessary.

## 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

## 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**

**CORS**
**HARN**
**NAVD’88 BM**
**New Station**

**121°40’W**

**122°35’W**

**37°50’N**

**38°20’N**

**LATITUDE**

**LONGITUDE**

**Primary Base Station**

**MOLA**

**MART**

**LAKE**

**10CC**

**D191**

**29.6km**

**25.8km**

**38.7km**

**19.0km**

**28.7km**

**25.7km**

**38.3km**

**31.6km**

**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

**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.

## Independent Base Lines

**Notes:**
Non-trivial baselines determined from the scheduled observations.  Each session was observed twice providing repeat vectors for height analysis.

Adjacent stations are connected, loops are fairly short in length and involve separate sessions providing a good analysis tool.

Additional sessions or different observation scenario could “strengthen” the network configuration in the west.

## 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.

## Appendix B: 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

- Mt. Pleasant

- Harbor Beach

- Bayer at Saginaw

**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.

## Next Level - Michigan HARN

**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.

## Have good NAVD88 Control

- L26235, phases 1 & 2, courtesy of students at Ferris State University

**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.

## 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.

## Basic Concept of Guidelines

- Stations in local 3-dimensional network connected to NSRS to at least 5 cm uncertainty
- Stations within a local 3-dimensional network connected to each other to at least 2 cm uncertainty
- Stations established following guidelines are published to centimeters by NGS

**Notes:**
Basic concepts - nutshell overview - if errors are not large they’re probably not visible; requires redundancy and a network.  5 cm accuracy - ties to the net at least to 5 cm (network) and internally to 2 cm (local).
Local & network accuracy - accuracies identified for project (local or relative to adjacent stations) and network (relative to national datum or closer approximation of absolute).
Local accuracy - uncertainty statement of coordinates/heights between adjacent stations.
Leveling - we see slight accumulation 4-5 cm; good closures over large loops but not so good over short distances.
Network accuracy - uncertainty statement of coordinates/heights to the geodetic datum; nationally the CORS network, known relatively to 2 to 3 cm.
Control stations - KNOWN control; things that fit between should fit the control stations; CORS approach 2 cm and the HARN 5 cm.

## 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.

## 1        National Geodetic Survey,   Retrieval Date = NOVEMBER 25, 2003
 AI6151 ***********************************************************************
 AI6151  DESIGNATION -  2K76
 AI6151  PID         -  AI6151
 AI6151  STATE/COUNTY-  WI/WASHINGTON
 AI6151  USGS QUAD   -  ALLENTON (1971)
 AI6151
 AI6151                         *CURRENT SURVEY CONTROL
 AI6151  ___________________________________________________________________
 AI6151* NAD 83(1997)-  43 25 39.39446(N)    088 18 24.15369(W)     ADJUSTED  
 AI6151* NAVD 88     -       343.002  (meters)    1125.33   (feet)  ADJUSTED  
 AI6151  ___________________________________________________________________
 AI6151  X           -     137,097.884 (meters)                     COMP
 AI6151  Y           -  -4,637,622.691 (meters)                     COMP
 AI6151  Z           -   4,362,336.158 (meters)                     COMP
 AI6151

-  AI6151 ***********************************************************************
-  AI6151  DESIGNATION -  2K76
-  AI6151  PID         -  AI6151
-  AI6151  STATE/COUNTY-  WI/WASHINGTON
-  AI6151  USGS QUAD   -  ALLENTON (1971)
-  AI6151
-  AI6151                         *CURRENT SURVEY CONTROL
-  AI6151  ___________________________________________________________________
-  AI6151* NAD 83(1997)-  43 25 39.39446(N)    088 18 24.15369(W)     ADJUSTED
-  AI6151* **NAVD 88     -       343.002  (meters)    1125.33   (feet)  ADJUSTED**
-  AI6151  ___________________________________________________________________
-  AI6151  X           -     137,097.884 (meters)                     COMP
-  AI6151  Y           -  -4,637,622.691 (meters)                     COMP
-  AI6151  Z           -   4,362,336.158 (meters)                     COMP
-  AI6151

- Sample Datasheet – Leveling

**AI6151 . . .**
**AI6151 The orthometric height was determined by **_**differential leveling**_
**AI6151 **_**and adjusted by the National Geodetic Survey in October 2000**_**.**
**AI6151**

## 1        National Geodetic Survey,   Retrieval Date = JUNE  7, 2004
 HL0673 ***********************************************************************
 HL0673  DESIGNATION -  FAA 5CO0 B
 HL0673  PID         -  HL0673
 HL0673  STATE/COUNTY-  CO/LA PLATA
 HL0673  USGS QUAD   -  LOMA LINDA (1968)
 HL0673
 HL0673                         *CURRENT SURVEY CONTROL
 HL0673  ___________________________________________________________________
 HL0673* NAD 83(1992)-  37 12 34.23430(N)    107 51 59.34354(W)     ADJUSTED  
 HL0673* NAVD 88     -      2038.7    (meters)    6689.     (feet)  GPS OBS   
 HL0673  ___________________________________________________________________
 HL0673  X           -  -1,560,867.458 (meters)                     COMP
 HL0673  Y           -  -4,842,221.299 (meters)                     COMP
 HL0673  Z           -   3,837,158.007 (meters)                     COMP

-  HL0673 ***********************************************************************
-  HL0673  DESIGNATION -  FAA 5CO0 B
-  HL0673  PID         -  HL0673
-  HL0673  STATE/COUNTY-  CO/LA PLATA
-  HL0673  USGS QUAD   -  LOMA LINDA (1968)
-  HL0673
-  HL0673                         *CURRENT SURVEY CONTROL
-  HL0673  ___________________________________________________________________
-  HL0673* NAD 83(1992)-  37 12 34.23430(N)    107 51 59.34354(W)     ADJUSTED
-  HL0673* **NAVD 88     -      2038.7    (meters)    6689.     (feet)  GPS OBS**
-  HL0673  ___________________________________________________________________
-  HL0673  X           -  -1,560,867.458 (meters)                     COMP
-  HL0673  Y           -  -4,842,221.299 (meters)                     COMP
-  HL0673  Z           -   3,837,158.007 (meters)                     COMP

- Sample Datasheet - GPS

-  **HL0673. . .**
** ****HL0673 The orthometric height was determined by **_**GPS observations and a**_
** ****HL0673 **_**high-resolution geoid model**_**.**
** ****HL0673**

##  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.**_

## 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**
**Grants Manager**
- N/NGS1, SSMC3
- 1315 East-West Highway
- Silver Spring, MD  20910
- 301-713-3231, x116
- www.ngs.noaa.gov
- Renee.Shields@noaa.gov

***http://www.ngs.noaa.gov/heightmod/MichiganMarquetteAgenda.pdf***

***http://www.ngs.noaa.gov/heightmod/MichiganFlintAgenda.pdf***