switching from geoid 12b to geoid 18
Hello all. I recently worked with a surveyor who let me know that geoid 12b is being phased out for geoid 18. I am beggining to start this process but have a few general questions on repercussions. My workflow is essentailly, Do a static survey that is corrected with opus in with a 12b geoid, design a plan based on those coordinates in autocad (i dont think geoid is relevant in cad for this purpose but i could be wrong), put my design points in my controller and stake it in the field.
1.) what do i do with all my old data that is in geoid 12b? If I have a design elevation at 800.3 feet that was based on 12b, and I update my controller job settings to 18, wont that through off my elevations?
2.) Can I get a general overview of how NAVD and geoid 12b/18 interact with regards to elevation? I work in napa county and use their contours as a reference for projects where I do not need to be very precise (I topo a site myself for more exact jobs). their meta data is here . http://gis.napa.ca.gov/giscatalog/viewXML.asp?Name=maingis.GIS.cont_02_200&meta_style=fgdc Their altitude datum is the north american vertical datum of 1988, how much difference is that elevation relative to geoid 12b and 18?
3.) How do I stay up to date on these changes? I also heard that the US survey feet may be going away and a new coord system is coming out in 2022.
The differences in geoid heights between 12B and 18 vary by location, primarily because of the densification of available data used for GEOID18 resulting from the GPS on Bench Marks campaign(s). You can use this tool to compute a GEOID18 geoid height (N) for your project location, then compare that to what the geoid height was for 12B. The difference will be the amount your derived orthometric height will change depending on which geoid model you apply to the ellipsoid height(s).
The area where you work has almost no NAVD88 bench marks, so it is unlikely any GPS on BM observations were done that would have changed the geoid heights in your area between 12B and 18.
The best place to stay informed on these issues is right here:
N = a negative distance in most of California
When I apply Geoid 18 in my area to NAD83 (2011) ellipsoid heights I tend to be closer to NAVD88 elevations than using Geoid 12b.
I normally correct the ellipsoid height to NAVD88 elevations with which ever flavor of Geoid I'm using.
In other words I fix my elevation and let the Geoid model move the ellipsoid height up or down.
Most people use the ellipsoid height and shift the elevation which causes you to have different elevations for each Geoid model, 90, 96, 99, 03, 09, 12, 18. So there would be at least seven different elevations for the same point through the last 28 years of GPS usage, or a change every four years or so. Of course each Epoch also changes the ellipsoid height and that adds to the number of elevation changes, 86, 93, 01, 07, 11. So mixing the seven Geoid model heights with the five ellipsoid heights can produce a large number of different elevations for the same point.
Basically, you need to stay on top of the changes, if you have an ongoing design like many engineering designs are, keep good records of how you derive elevations and let the next guy know where it all came from.