Is height conversion between ellipsoids necessary?

Question

I have measurement data that uses X/Y in different zones of JGD2011 and an ellipsoid height above GRS1980 (but it also includes a geoid height as well). My task is to transform this data to "WGS84 LLA".

For the horizontal part I used a transform from (for example) EPSG:6677 to EPSG:4326, and all seems well. However, I am really confused what to do with the height (and whether I even need to do something).

The WGS84 and GRS80 ellipsoids seem to differ, but I was unable to find out by how much, and how to convert the data.

I tried a conversion from EPSG:6667 to EPSG:4979 with adjusted input data, but the result is a noop, so maybe there is no difference after all.

So my question is: how should I handle the height? I am not even sure if I should output the ellipsoid height or geoid height, WGS84 also includes a geoid definition ...?

Answer 1

If you have an accuracy near centimeter-level and you are wondering if you need to convert from WGS84 to GRS80 ellipsoid height, there is no reason to

The difference between both ellipsoids is at a sub-millimeter level (US and European made a different rounding on the ellipsoid flattening)

WGS84 doesn't have any geoid definition. The use of ellipsoid or geoid height depends on your application. If you just want to have a geometric precise value, stay in ellipsoid height or if you are working on things like gravimetry or geology, you might prefer geoid height. Also, the geoid definition might be the one of your country (GEOID18 for the US, QGF96 in France, ...) or a global one like EGM2008. But the accuracy of the EGM2008 is at 5 centimeters on the sea and 50 centimeters on the grounds and it has a bad spatial resolution.

With more details, I can advise you on this height question