The horizontal and vertical datums are different surveys done at different times. In the early days, the horizontal was done with theodolites and the vertical was done with levels.
Your vertical benchmarks are referenced to mean sea-level at that place, at that time.
So you go to your vertical benchmark and look up the elevation of that benchmark, and that’s where you are. I don’t think there are models for vertical ‘drift’ like there are for horizontal, but I could be wrong…
edit: I shouldn’t say there are no models. I meant that I don’t think there are adjustments related to time. You just state this is the elevation in reference to this vertical datum ( from this year ).
So for the most part, seems MSL is the reference or basis of ZERO. Seems geoid is the similar but newer (NAV88) until the new 2022 datum is put in place… this is where i am confused.
Yeah, so you can think that a vertical benchmark doesn’t usually have an accurate horizontal location. Some of the published coordinates just get you to the general area. It can be pretty approximate when the coordinates are rounded off to the nearest minute. So you have to go by the text description of the location (on the the side of the bridge abutment, or on the south wall of the bank a the corner of this and that street).
Exactly. Very disclaimer-like. That’s why we are bound to the benchmarks as noted and therefore why I always perform a preconstruction topographic survey based upon those benchmarks. And guess what happens if one of those benchmarks is missing or disturbed? We just sit on our laurels until the engineer provides another one… Then, what do you think the chances are that they are going to find another original benchmark? Or they more likely to just set a new one by level looping the remaining? Nasty process.
