Rookie question...
When looking at NGS monument datasheets there is 'X' amount of difference between the two vertical datums near a project. Is it a bad idea to apply that X amount to elevations on the project to get from one datum to another?? A client need the elevations in 29 and the project is in 88.
USE CORPSCON TO GET THE DIFFERENCE IN ELEVATIONS.
> USE CORPSCON TO GET THE DIFFERENCE IN ELEVATIONS.
I agree on using corpscon. That being said, it will most likely be an identical conversion depending on how 'close' the BM is.
For some details about VERTCON see: http://geodesyattamucc.pbworks.com/f/VERTCON4ProfSurveyor.pdf
This is a preprint graphics not included in the archive version linked from the NGS site.
HTH,
DMM
CORPSCON uses VERTCON
VERTCON documentation stated that it is not appropriate for 1st, 2nd or 3rd order.
Most states require 3rd order: 0.05' per sq. rt. mile.
Therefore VERTCON may not provide the answer you seek.
In my Town, VERTCON returns a difference of 0.9'. Comparing the record monuments results in 1.1' to 1.2'. Using long GPS observations puts one closer to 1.4'. (Multiple 24-hour sets)
I would check it. Grab the coordinates, check the differences. Find some monuments with published elevations on both datums and see how close things line up.
Also look at the CORS stations for the velocities. Now look at the dates when the NGVD and NAVD elevations were established and see how close the difference to VERTCON matches the upward velocity.
> When looking at NGS monument datasheets there is 'X' amount of difference between the two vertical datums near a project. Is it a bad idea to apply that X amount to elevations on the project to get from one datum to another?? A client need the elevations in 29 and the project is in 88.
I think that is okay for typical development project work. I think I'd try to get a few datasheets that bracketed the project area and use an average. And make sure the elevations are products of high quality levelling. And then check with Corpscon (it's free, what the hey).
If this is control work purporting to establish NGVD29 elevations relative to the national datum to some high degree of precision - not so much.
For most of our work around here we subtract 1.08 feet. Which for most purposes works well.
> For most of our work around here we subtract 1.08 feet. Which for most purposes works well.
The difference in Portland area is about 3.3 feet. It varies from place to place.
Thanks for all the input!