I am a Civil Designer and have been learning about datums, coordinate systems, and transformations. It appears, based on my reading so far, that EGM2008 is used with WGS84 geoid to convert to WGS84 ellipsoid heights. Similarly, Geoid12b is used with Nad83 ellipsoid heights to convert to NAVD88. As a novice, I am never certain which geoid or EGM I should be using in changing certain geoids. Is there a known handbook or NGS site that helps determine the proper pairing of datums to get the best transformation results? Thanks in advance for any help with this topic.
I have used this paper to help people understand the relationship between specific hybrid geoid models and realizations of NAD83. One thing to note is that both GEOID12B and 18 were derived from NAD83 2010.00 ellipsoid heights, but they don't always yield the same orthometric heights, depending on location. In the areas out west that I have investigated, I have found as much as 4 cm of difference. Therefore, my advice is that if you started a project using 12B, do not switch to 18 in the middle of the project (without making checks), simply because it is the "newest" and potentially more accurate. You could induce a geoid bias by doing so.
You can use this tool to compute the geoid height (N) at a specific location using 12B:
Then do the same for 18 at the same location to see what the variation is, if any.
If you're just getting started with datums a note about WGS 84. First of all be very cautious when someone says they have positional information better than about 2-3 m in WGS 84. The reference frame is defined and maintained by the National Geospatial-Intelligence Agency (NGA) and they DO NOT provide any datum realization (passive or active control stations) that would provide high accuracy coordinates to those of us outside the Defense Department. If somehow you do think you have relatively high accuracy coordinates then they should be referenced by the specific WGS 84 iteration they were define in (e.g., WGS 84 (original), WGS 84 (G730), WGS 84 (G873), WGS 84 (G1150), WGS 84 (1674) or the current WGS 84 (G1762). I would highly recommend that you download the official NGS publication "WGS 84 It's Definition and Relationship with Local Geodetic Systems". Look at Table 2.1 on page 2-4 for their outline of these datum iterations. In addition, if you quote WGS 84 positions you should also provide the epoch of those values (e.g. Jan 26, 2021 = 2021.071). Any attempt to accurately transform WGS 84 to any other datum would require that metadata. If have a legitimate requirement for WGS 84 for a client you can use the ITRF values provided by the NGS CORS network and then just label the data as the appropriate WGS 84 as noted above. My long experience with many GPS users is that they often relabel NAD 83 as WGS 84 because they have been told they are the same -- THEY ARE NOT.
Wow. These two just dropped some good stuff for you. Read up.