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1. What is KSA-GEOID21?
KSA-GEOID21 is a hybrid geoid model for the Kingdom of Saudi Arabia determined by using land & ship-borne pointwise data together with GASGI GPS levelling data, airborne gravity and grid data together with satellite data (altimetric & gravity satellite missions data). The KSA-GEOID21 is a major component of KSA-VRF, which allows to compute orthometric heights for the points over KSA different from KSA-NVN be​nchmarks.

2. How accurate is KSA-GEOID21?
Accuracy of KSA-GEOID21 is more homogeneous through the Kingdom compared to KSA-GEOID17 due to the better distribution of input data. The average accuracy of derived geoid height from KSA-GEOID21 is better than 2 cm for the entire kingdom.
 
3. What will be the accuracy of orthometric heights if KSA-GEOID21 is used?
The accuracy of orthometric heights computed by using KSA-GEOID21 model will depend on the two following factors:
  • ​Accuracy of the KSA-GEOID21 geoid model (see FAQ 2).
  • Accuracy of the ellipsoidal height provided by the user.

4. Who is responsible for the maintenance and development of KSA-GEOID?
General Authority for Survey and Geospatial Information (GASGI, former GCS), KSA.

5. How can I access and use KSA-GEOID21?
There are several ways how to do it:
  • ​You can use KSA-CORS network services
  • You can transform your observed ellipsoidal height to orthometric height by using Online GRF-TT  tool on GASGI Portal.
  • You can transform your orthometric height from older KSA geoid models to KSA-GEOID21 model by using Online VRF-TT  tool on GASGI Portal.
  • If you need a grid for computation of geoid heights for some local area, please, contact us by e-mail address info@gasgi.gov.sa

6. What is the grid resolution of the KSA-GEOID21 model?
Original resolution of the KSA-GEOID21 model is 0.02o x 0.025o (Lat, Lon) or ≈ 2km x 2.5km in latitude and longitude correspondingly. 

7. What are the older geoid models available in KSA?
There are two older geoid models:
  • ​KSA-GEOID09
  • KSA-GEOID17
8. How can I transform my orthometric height from KSA-GEOID09/KSA-GEOID17 to KSA-GEOID21?
A specially designed online tool is ready for use by you. Please, follow this link Online VRF-TT  tool to transform geoid height of a single point (manual input) or geoid heights of multiple points (file input).

9. What should be the format of my orthometric heights to transform them from KSA-GEOID09/KSA-GEOID17 to KSA-GEOID21 with Online VRF-TT​ ?
The input data formats both for Single Point and MultiPoints modes are described in the tool’s Graphical User Interface (GUI) itself.

10. What should be the format of my data with ellipsoidal heights to transform them to orthometric heights in KSA-GEOID21 with Online GRF-TT ?
The input data formats for Single Point and MultiPoints modes are the same in Online VRF-TT and Online GRF-TT  tools. The input data formats for Single Point mode are described in the Online GRF-TT  tool’s GUI itself. The input data format description for MultiPoints mode can be found in the GUI of Online VRF-TT​

11. What are the differences between KSA-GEOID17 and KSA-GEOID21? 
Table. Differences between KSA-GEOID17 and KSA-GEOID21 geoid models and 
advantages of KSA-GEOID21
#
Item
KSA-GEOID17 KSA-GEOID21 Advantage of KSA-GEOID21
​1
​Area size
​Lat: 16° - 33° 
Lon: 34° - 56°
Lat: 10° - 35°
Lon: 30° - 62°
​wider coverage
​2
​Geoid Height Standard Accuracy
​≈2-3 cm close to GPS / Levelling BMS in East of KSA;
≈10-20 cm  in the rest of KSA
≈1.5 cm along the Levelling baselines;
≈2.5-3.5 cm in the rest of KSA, mostly, in South-West (mountains)
​More homogeneous accuracy over the KSA with average accuracy better than 2 cm 
​3
​Reference Earth global gravity model (GGM)
​EIGEN6C4
​XGM2019e
​Latest Earth GGM as the reference model
​4
​GGM in use to  fill-in gravity anomaly data outside KSA
​No
​Yes: EGM2008
(372793 points)
​Edge and aliasing effects are avoided
​5
​Shipborne data use
​139331
​final 245167 points
​More shipborne data
​6
​Airborne data use
​No
​Yes:
- Grid 1’ x 1’ 
(1800 x 1800 m);
- Line data 
​Additional data which cover large area; were used for land gravity data validation
​7
​Global marine altimetry gravity data use to fill-in data in marine areas where shipborne data are not available
​DTU15 (301101 offshore points)
​DTU18 (raw 147480 near-shore points up to 20 km);
SIO S&S29.1 (raw 626913 off-shore points >20 km);
Points in total: 
- raw 774393 
- final 771500
​2.5 times more data points 
​8
​Ultra-high Earth Residual Terrain Modelled (ERTM) gravity effects
​SRTM30
(the model has resolution 30” (900 m))



dv_ell_EARTH2014 
(up to spherical d/o 2160 (spherical  d/o 2190) for scales >10 km)

ERTM2160 (for spheroidal d/o 2160 to 90000 - latest model (released in 2014) taking into account effects at scales from 10 km to 250 m)
​Higher resolution in topography
​9
​Use of Digital Elevation Model (DEM) to compute rigorously quasi-geoid to geoid separation
​N/A
​SRTM3 (the model has resolution 3” (90 m)); 
GASGI DTM (resolution 0.1” (3 m))
​Rigorous computation of separation
​10
​Number of raw observation points
890975 (final)
2 383 559 (raw)
2 010 766 (final)
Higher number of  observation points
​11
​Observation data validation





Two more methods  of validation: 
1.Least Square Collocation (LSC);
2.Airborne data down warding to land surface.
​12
​GPS Levelling benchmarks number

​280 - GASGI
3522 – GASGI NVN BMs
(13206 – ARAMCO
801 – other institutions
17529 – in total with GASGI BMs)
More homogeneous fitting of geoid to BMs over the KSA
13
​Model of differences of  GRAV-GEOID21 geoid and GPS/Levelling BM geoid heights when fitting gravimetric geoid model to BMs.
​No
​Yes: 
combined deterministic and stochastic model
​The approach provides more homogeneous and accurate distribution of fitting errors


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