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GEOID18: Hybrid Geoid
Kevin Ahlgren, Ph.D.Geodesist and Geoid Modeler
GEOID18 Technical Lead
MSS Spring Technical ConferenceLinthicum Heights, MD
5 April 2019
Outline• Motivation for GEOID18• Geoid model basics• Differences from GEOID12B in Mid-
Atlantic• Model misfits
KeyTake-Aways:
• NAVD 88 ≠ Geoid• GEOID18 ≠ xGEOID18• NAVD 88 exists only on
‘stable’ bench marks
GPS Data on NAVD 88 leveled marks will support development of GEOID18 and the transformation tools to NAPGD2022
Change in Height from NAVD 88 to NAPGD2022
h is ellipsoid height measured using GPS
N is from a geoidmodel
Theoretically, the difference between these three values should be zero. In practice, using actual observations gives a residual, or measure of the misfit between the three. We use the residual to evaluate the observations.
Extensive Residual Analysis used to create a Prioritized ListResidual = h - H - N
H is an NAVD 88 Orthometric Height from Leveling
- -
Evolution of Recent NGS Geoid ModelsEGM08
GRACE
GOCE
USGG2012 GEOID12A/B
2012
2008
2014… 2018
xGEOID14B
xGEOID18B
+
GRAV-D
GOCO03sGOCO05S
GEOID18
NAPGD2022
EGM2008• N is from EGM2008
• Global gravimetric geoid model
• Produced by NGA• Only to 10 km
Min -166.3
Max 136.5
Mean 6.4
StdDev 56.4Based on 32,149 GPS on Bench marks used in GEOID18
[cm]
EGM2008• Bias and Tilt Removed
Min -39.8
Max 53.7
Mean -0.1
StdDev 6.4Based on 32,149 GPS on Bench marks used in GEOID18
[cm]
xGEOID18B• Bias and Tilt Removed• Higher resolution (2 km)
• Gravity data• Terrain data
• GRAV-D Airborne Gravity
Min -43.6
Max 48.4
Mean -0.2
StdDev 6.2Based on 32,149 GPS on Bench marks used in GEOID18
[cm]
GEOID18• BETA hybrid geoid model
Min -20.9
Max 20.5
Mean -0.2
StdDev 1.5Based on 32,149 GPS on Bench marks used in GEOID18
[cm]
Beta GEOID18 Statistics GPS on BM Available Flagged as bad
fitUsed in Model
Numbersince
GEOID12B
Used since GEOID12B
NGS IDB: 30,128 1,987 (6.6%) 28,141 6,610 6,324
OPUSShare: 2+ Obs.
3,313 288 (8.7%) 3,025 3,009 2,748
OPUSShare: 1 Obs.
2,350 - 212 2,141 186
Canada 579 14 565 0 0
Mexico 247 41 206 0 0
Total: 36,617 2,330 32,149 11,760 9,258
• IDB Data through 5 Dec. 2018
• OPUS Share Data through 10 Dec. 2018
This map shows all new GPSonBM data since 2012, both OPUS Share and Bluebooked data, that were evaluated for inclusion in GEOID18
Hybrid geoid models need GPSonBM data at 30km spacing to properly constrain to NAVD88 published heights.
This maps shows data gaps filled by new GPSonBM data
Impact of GRAV-D
ArcGIS Online GEOID18 Demo
JV1232• Two solutions
from 2015 and 2018 in OPUS Share
• Ellip. Heights: 107.980 & 108.031
JV1232
Location Specific Misfits• Cross Validation
(“Leave-one-out”)• 32,149 Geoid Model
Runs
For JV1232 (it is removed for the calculation):• Estimated residual (h –
H – N) = -15.5 cm• The ‘observed’ residual
(with mark included) (h -H - NBETA) = -19.8 cm
JV1232
GEOID12B GeoPDF
Public facing ArcGIS Online Webmap will be released to enable evaluation of GPSonBM data used and the improvements in the model resulting from new data
Data Sharing Tools
DEFLEC18DEFLEC18 is a companion product derived from the 2018 hybrid geoid model (GEOID18), and represents the deflections of the vertical (DOV's) at the surface of the Earth. These DOV's are also in the NAD 83 (2011) reference frame. These quantities are typically a few arc seconds, but can reach an arc minute of departure.
Beta GEOID18 released for public comment (Beta OPUS and online computation pages) 3/4/2019
Incorporate feedback & make necessary fixesIntegrate new geoid into other NGS products (OPUS, datasheets)
Final Product Release* Summer 2019
GEOID 18 Project Time Line
*Final release of GEOID18 is pending progress on xGEOID19 for the added GRAV-D blocks.
