Query regarding WGS84.img file

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06-06-2017 11:45 AM
MeghanKulkarni1
New Contributor III

Hello ESRI team,

I have a small query regarding WGS84.img file as part of ArcGIS installation.

I have gone through very useful article. URL below:

Converting from orthometric to ellipsoidal heights—Help | ArcGIS Desktop 

As part of standard equation:

h = H + N

It is little confusing as it mentions:

h = H + N

Where

h = ellipsoidal height
H = orthometric (geoid) height
N = geoidal separation

What I understand is orthometric and geoid heights are different.

Which part WGS84.img is in above equation?

My Best,
Meghan S Kulkarni
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8 Replies
CodyBenkelman
Esri Regular Contributor

The document you reference says:

" Choose the WGS84.img and click Add.    This represents N in the equation above."

Note that there is a more recent and more accurate geoid, EGM2008.  the difference between EGM96 and EGM2008 varies from about -13 to +15 meters worldwide (only about 2-3 meters in USA).  If you are using this for orthorectification of high resolution imagery, you may want to get the EGM 2008 geoid from NGA: (U) WGS 84 Earth Gravitational Model  

MeghanKulkarni1
New Contributor III

Hi Cody,

Thank you for your reply.

But still it doesn't answer my question:

h = H + N

 

Where

h = ellipsoidal height
H = orthometric (geoid) height
N = geoidal separation

Which part WGS84.img is in above equation?

I'm really sorry if I missed something in your reply.
My Best,
Meghan S Kulkarni
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CodyBenkelman
Esri Regular Contributor

Meghan

What data do you have, and what are you seeking to do?  

Are you starting with Orthometric height (measured relative to sea level) and you want to convert to ellipsoidal height (which is the vertical reference that satellites use for altitude values)?


Assuming yes, your DEM is "H" and the WGS84.img file represents "N", so if you are seeking ellipsoidal height, you add the height values in the WGS84.img file to your DEM.  If you are curious about how much difference this will be, you can load the WGS84.img file directly into the map to see the worldwide variation of the EGM96 Geoid.


This is typically done if you want to orthorectify satellite imagery onto your own DEM.  Note however that ArcGIS will do this for you behind the scenes; in the orthorectification process, if you select the "Geoid" checkbox, ArcGIS will apply this equation to adjust your input DEM using the built-in file (WGS84.img) that you are referencing.

Finding this Geoid checkbox depends on which interface you're using:

From ArcMap Image Analysis Window:

From ArcGIS Pro geometric function:

If that doesn't answer, please give us more detail on what data you have, which version you are using, and what you are seeking to do.  

Cody B

MeghanKulkarni1
New Contributor III

Hi Cody,

This information helped a lot. My goal is to get local mean sea level elevation for a particular GPS location.

I got geoid height(N) from USGS dataset. I have ellipsoid height(h) in addition with GPS co-ordinates.

Based on this I assume I will get Orthometric Height(H) which then can be converted from NAVD88 to local mean sea level.

I wanted to test this hypothesis in ArcMap. I maybe wrong here and if indeed please correct me. I got confused with equation H = orthometric (geoid) height. Because orthometric and geoid height I guess are different, right?

My Best,

Meghan S Kulkarni

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CodyBenkelman
Esri Regular Contributor

Meghan

If you need to convert between different vertical datums, I don't feel I am qualified to give you answers.  You may want to study other reference materials e.g. NOAA/NOS's VDatum: A tutorial on datums  to give you a deeper background on this subject.

If you need further assistance with functionality in ArcGIS for doing coordinate system and datum conversions, let us know as much detail as possible and I will forward your questions to our experts.

Best wishes

Cody B.

MelitaKennedy
Esri Notable Contributor

You're correct that orthometric heights are not exactly geoidal heights. It depends on the geoid model being used and the gravity-related (-based) vertical coordinate system. In the USA, if you use the geoid12B model, it generally gives you orthometric heights in the NAVD88 VCS. A worldwide geoid model like EGMxx will usually be less accurate than a local version. Local MSL may have a different value than the geoid model being used, just as it may be different than the local vertical coordinate system. Many VCS are onshore, and will have some offset to a local MSL. We don't have the NAVD88 - LMSL offset information if your in the US, nor that information for any other country yet.

I looked up a USA control point that's also a federal tidal station at Los Angeles, 9410660. I also looked up its data sheet in the NGS database (DY1083): 

DY1083* NAD 83(2011) POSITION- 33 43 11.44371(N) 118 16 22.21953(W) ADJUSTED
DY1083* NAD 83(2011) ELLIP HT- -31.725 (meters) (06/27/12) ADJUSTED
DY1083* NAD 83(2011) EPOCH - 2010.00
DY1083* NAVD 88 ORTHO HEIGHT - 4.131 (meters) 13.55 (feet) ADJUSTED

So the control point has an NAVD88 orthometric height of 4.131 meters. The tidal station website lists 4.03 m (maybe a mistake?) and a local mean sea level of 6.65 m

NGS control point data sheets can be looked up at National Geodetic Survey - Survey Marks and Datasheets 

Melita

MeghanKulkarni1
New Contributor III

Thank you Melita for your help. This clarification helped me sort out things.

My Best,

Meghan S Kulkarni

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JayantaPoddar
MVP Esteemed Contributor

MKennedy-esristaff‌: Maybe you could help.

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