How do I convert Ellipsoidal height to Orthometric after the fact?

dronemapper23 · October 12, 2023, 6:23pm

I created a survey project using Ellipsoidal height, when I should have downloaded and selected Orthometric height. Is there a way to convert Ellipsoidal height to Orthometric after I have already recorded the locations of several GCP’s? Can I do this in Emlid Studio? Thanks in advance…

timd1971 · October 12, 2023, 10:12pm

Multiple ways, but this site may help:

https://www.ngs.noaa.gov/NCAT/

I havent used Emlid Studio yet… so maybe someone can help there.

deraciao · October 12, 2023, 10:48pm

Does that site apply only in the US?

dronemapper23 · October 12, 2023, 10:48pm

Yes, I think so

dronemapper23 · October 12, 2023, 10:50pm

I was reading the NOAA website and found this:

dronemapper23 · October 12, 2023, 10:54pm

I am currently researching a software program called Corpscon, hopefully it will do what I need…

timd1971 · October 12, 2023, 10:56pm

Yeah, not sure then… I thought I used it at one point to do that… but maybe I used FieldGenius instead? Yeah, double check things ; )

timd1971 · October 12, 2023, 11:01pm

Not sure, as I use FieldGenius and not really Emlid Flow much… but wonder if you can export your ellipsoidal height collected points, create a new file with ortho height file, then import points into it???

I know in FieldGenius, you can try different ortho files and see the effect it has on ellipsoid height of the points.

dronemapper23 · October 13, 2023, 12:21am

Thanks, I’ll give FieldGenius a look and a try.

jbonde002 · October 13, 2023, 12:26am

In the US go here:
https://geodesy.noaa.gov/GEOID/GEOID18/computation.html

If your points are close - within a few km - the offset between ellipsoid and orthometric is essentially the same. Just shift them by xx.xxx meters. Whatever value the tool above gives you. If they are more spread out the tool will give you the correct offset at each site.

dronemapper23 · October 13, 2023, 1:21am

Thank You!

EBE111057 · October 13, 2023, 1:25am

As jbonde002 said, use can use a single geoid height for the entire project if it’s not very large, however sometimes I use the batch command selection for all points if the site is large.

Using the batch command is easy and it can give good geoid heights just as the single input does. If you have a large number of points you can even see on a medium site (1000 ac) the actual resolution of the geoid18 model which is very cool.

timd1971 · October 13, 2023, 3:55am

Here is another awesome tool fyi (only does GEOID12B):
GEOID12B for Google Earth (earthsurvey.us)

Earth Survey

dronemapper23 · October 13, 2023, 3:58am

Thanks, someone else showed me how to use the free level of Emlid Flow 360 to do the conversion, so that’s what I’m going to use for now. It’s good to know there are a few (free) tools to do this.

ruth.bongon · October 13, 2023, 4:00pm

It seems like you’ve found a workaround for converting heights in Emlid Flow, @dronemapper23. But let me share here the workaround so others can benefit from it too.

If Emlid Flow supports the geoid, you can convert it from Ellipsoid Heights by following the steps below:

Download the CSV file from your project.
Create a new project in Emlid Flow with the same coordinate system as your old project + geoid
Import the CSV into the new project.

Once the CSV is successfully imported, the heights will be automatically converted to orthometric heights. This workflow is also applicable if you have geographic (global) coordinates that you wish to apply projection and geoid to.

Zaz5400 · October 15, 2023, 7:03pm

This is a good one but I would recommend using the NGS one. It’s a simply ESRI ArcGIS Online map and you can get the geoid separation value between the ellipsoid and Geoid 18: https://noaa.maps.arcgis.com/home/webmap/viewer.html?webmap=344859eb43804bff9a17b0943bf50e8d&extent=-139.0839,3.0255,-55.1484,59.6872.

. There is also one for Geoide 12. Here you can input a lat/long and you’ll get the Geoid 18 and Geoid 12B orthometric heights, respectively: GEOID18 Interactive Computation | GEOID | Data & Imagery | National Geodetic Survey & GEOID12B - GEOID - Data XXXXXTITLEXXXXXamp; Imagery - National Geodetic Survey.

The best way is probably just to import into emlid flow like you already mentioned @dronemapper23 .

shohag.hesperit · October 16, 2023, 9:18am

To convert ellipsoidal height to orthometric height after the fact, you will need to use a geoid model. A geoid model is a mathematical representation of the Earth’s geoid, which is an equipotential surface that closely approximates mean sea level. The geoid model is used to calculate geoid heights, which are the differences between ellipsoidal heights and orthometric heights.

There are a number of different geoid models available, and the best model to use will depend on the accuracy required and the region of interest. Some popular geoid models include:

Earth Gravitational Model (EGM96)
Global Geopotential Model (GGM02c)
North American Geoid Height Model (GEOID96)
European Geoid Model (EGM96)

Once you have selected a geoid model, you can use it to calculate geoid heights using a variety of different software tools. Some popular software tools for geoid height calculation include:

NOAA’s Geoid Height Calculator
ESRI’s ArcGIS Pro
NASA’s GeographicLib

To calculate the orthometric height of a point, you will need to know the ellipsoidal height of the point and the geoid height at the point. You can then use the following equation:

Orthometric height = Ellipsoidal height - Geoid height

For example, if the ellipsoidal height of a point is 100 meters and the geoid height at the point is 10 meters, then the orthometric height of the point is 90 meters.

It is important to note that geoid height models are not perfect, and they may not be accurate in all regions. Therefore, it is important to consult with a geodetic expert if you need to convert ellipsoidal heights to orthometric heights with high accuracy.