After getting an OPUS static (> 2hrs) report there is listed 2 ortho heights.
Typically there is a .4 meter variance between the 2 - at least in my local. Granted the second ortho is a "precursor to the completion of GRAV-D". But I'm a little concerned about the magnitude of this difference as of right now.
When we do shift to GRAV-D, do we epoch the prior orthos? What is the alternative?
Looking to get some ideas of "best practice" to handle this may be interesting. Thanks.
GRAV-D is the process by which NGS is collecting the gravity data required for the new geopotential (vertical) datum to replace NAVD 88 which is currently scheduled for 2022. Surveyors should become familiar with the rational for the change and the magnitude of differences that will occur in their areas so they will be in a position to support their various clients needs. The height shifts across the country will range from at or near 0 in parts of Florida to slightly over 1 meter in the northwest (parts of ID,MT,OR & WA).
> GRAV-D is the process by which NGS is collecting the gravity data required for the new geopotential (vertical) datum to replace NAVD 88 which is currently scheduled for 2022. Surveyors should become familiar with the rational for the change and the magnitude of differences that will occur in their areas so they will be in a position to support their various clients needs. The height shifts across the country will range from at or near 0 in parts of Florida to slightly over 1 meter in the northwest (parts of ID,MT,OR & WA).
So, we are a long way from implementation of GRAV-D.
This makes the est. ortho a reference only in my book.
Thanks.
You are correct, it's just a reference. As I mentioned previously it gives the surveyor some pretty good info as to what's going to happen and allows them to help plan the impact on their community and/or other clients.
I suppose once GRAV-D becomes the current datum the change over will go much as the change between NGVD29 and NAVD88 has gone. We will all just have to be careful to specify which datum our survey is on.
I sure need to study up on this. I am seeing a rough average difference of 0.434 meters (1.424 us survey feet) in my area.
I am sure I will be swamped with calls once this goes into effect. Heck, in 2022, I may be one of the few guys still surveying in my county.
I think NGS has some training material available on their website.
Don't forget that the change in the vertical will be accompanied by a new horizontal (geometric) datum more closely related to ITRF. On any OPUS report just compare the NAD 83 (2011) lat/long/EHt with IGS08 values right next to them and you will get a sense of magnitude of the change in the horizontal as well.
