Interesting map.
The preliminary heads-up is that our area will see a drop in the ortho elevations of 2'.
This will place the new elevations close to the NVGD 29 numbers.
To get to that number with the small Geoid shift will mean that the ellipsoid heights will also be lowered substantially.
2' of course being about 60cm
?ÿ
EDIT: NGVD29
In my region (Mountains of NC), NAVD 88 elevations were lower than NGVD 29 by about .3 to .4 feet. From the map, we're going to be even lower.
When does this go into effect or is this just a test bridge to the next datum?
As I think I understand it, Geoid18 will be released in mid-2019 (it's only a beta yet). It won't change the NAVD88 elevation on any NGS data sheets, but will be a (we hope) more accurate model for converting from ellipsoidal height in GPS sessions at locations between benchmarks.