Been using GPS for a while now without too many issues that I couldn’t work though and am usually knowledgeable enough to know when things aren’t right in the field and in the office. With that said…the new GEOID12 has me scratching my head a little bit.
Scenario: Have most of a town tied out to 2-3 main GPS points which we have used over the last several years to establish conventional control, tie monuments and perform topographic surveys. Our main base point is an NGS monument commonly used in the area(it’s really the only one in the immediate vicinitiy) Said NGS monument has an established NAVD88 elevation, this elevation has not changed with the new GEOID12, yet when I switch the TGO project to GEOID12 all of my observations from said base point change by 0.10+/-. So…the question I have is this: How are the elevations changing and are they now more accurate? ( I think not, but again I’m not quite sure). Hopefully someone can explain this more clearly than some of the NGS and NOAA site do.
This repeats itself in other areas with similar setups.
Should I abandon all of my current projects using the now older GEOID09 and simple update all base points, NGS or not, to GEOID12?
First, until geoid models have no error, they are ALL best used in a relative sense.
Second, each new model is based on the new ellipsoid heights, so Geoid12 was designed around NAD2011 ellipsoid heights AND the same NAVD88 heights as they always have been.
If the ellipsoid height changed on your NGS point that your network is based from, then obviously unless you update ALL of your network ellipsoid heights to reflect that fact, the new model will NOT give the same ortho heights as the previous Geoid09 which was tied to an older realization of NAD83, at least not in an absolute sense.
Having said all that, I am guessing that the delta separation for any pair of points in your area is pretty close to the same between Geoid09 and Geoid12, my guess is that the 0.1 foot difference is mostly due to a 0.1 foot change in ellipsoid heights between NAD83(2007)/NAD83(CORS96) AND NAD83(2011).
SHG
I'm no expert here, but I would suggest that any new projects you do in Geoid 12, and you don't update the older projects. Make sure you publish your 'metadata' (if that's the right term) along with your particular project. If you have to add to an old project look at that project specifically. I would probably try to add on in the same geoid that the old project was done other. You don't want to change it after you have already put out drawings or maps or whatever your product is; as that could get too confusing (especially for clients).
Of course you probably know better than I do whether that's a viable solution for your particular situation with the different projects. (ie; are they all for one client, do they have to match within a tolerance you need? etc.)
Geoid 12 here in Louisiana has some SERIOUS ERRORS. It has made news.
