Could you please clarify what you mean by "applying the geoid to the elevations instead of the ellipsoid." I don't understand what you're trying to say.
The ellipsoid is a model of the earth. It's off from the real world, particularly in the z value. The geoid models are an attempt to correct the ellipsoid height to the leveled elevations. In my area the slope between the ellipsoid heights and elevations is pretty extreme and the geoid models are getting pretty good at modeling that slope. But if you take an opus or cors value and apply the geoid to it you will not match very close to the NAVD88 elevation on a first order bench. Each time there is new geoids released that reduced elevation changes. Thus there is no stability which is a huge problem. We are now working on a project controlled in the 90s for a new highway. You can't have elevations bouncing around cause of new geoids being released. So instead of using the constantly changing numbers from opus/cors, I hold the elevation values on the bench marks apply the geoid to them and let the ellipsoid height value float cause no one cares what it is anyway. This is important since so many things like county wide mapping, FEMA, highway projects, dam construction, subdivisions, sewers, water systems are all tied to the bench marks.
> I hold the elevation values on the bench marks apply the geoid to them and let the ellipsoid height value float cause no one cares what it is anyway. This is important since so many things like county wide mapping, FEMA, highway projects, dam construction, subdivisions, sewers, water systems are all tied to the bench marks.
I'm not sure I understand how you are applying the "Geoid" or why you even need a geoid in the case you are using
Ok
So what you're saying is you're using the slope of the geoid height differences not "the geoid." This makes sense. While the absolute accuracy of the geoid height in any area can easily be in the 4-7 cm range, the differences in the geoid heights is typically quite good and can achieve better orthometric heights if applied correctly.
Note that Z does not refer to any height, it is the distance from the plane of the equator north or south to a point on the Earth's surface in an Earth-Centered Earth-Fixed cartesian coordinate system. Orthometric heights are properly referenced as H.
Kinda, I'm actually surveying with the geoid applied to my file but I'm totally unconcerned what the ellipsoid height is. Yes I understand about the z value and ECEF. I'm just simply saying that in the classical xyz world the ellipsoid is "OFF" up and down. Which created the need for the geiod models to correct it to the real world measured by guys with levels tramping around the country.
Now if cors/opus can become stable in the t value with respect to elevations then we have something. But it sure ain't there yet:-(
I wonder if they are gonna make a new and improved Geiod2015 that will be even further away from the fema elevation maps not to mention the old Topographic maps with contours. Lol. It's crazy. I go out do a survey. Hit some ngs monuments with established elevations and the geoid "the man" messed with again is even further off. Not even close to the published elevations. And just to cover their behinds they say oh u can't use that until you do this this this and this and it's still gonna be different than actual elevation u need. In the mean time I will continue to tie into established ngs monuments because that's the only way I can feel confident in my elevations. I just wish someone, anyone would write a program where I can input lat/lon/ellips and get a "real" elevation. And yes I understand u have to specify which model u are inputting in order for the output to be correct. Thank you all for all of the replies but I guess there just isn't an easy answer to my question. Ugh
You will love the new vertical datum NGS plans to role out in 2022 - bench marks will be a thing of the past, they will not define the datum - the vertical datum will be the geoid model in your data collector.
Topconman, feel free to send me your contact info. My email is on my profile. I'd be more than happy to talk you through the ins and outs of what's going on, and how to approach your scenario.
"I wonder if they are gonna make a new and improved Geiod2015 that will be even further away from the fema elevation maps not to mention the old Topographic maps with contours."
Aren't most FEMA maps and old topo maps NGVD29???
"I just wish someone, anyone would write a program where I can input lat/lon/ellips and get a "real" elevation."
Why not just load a geoid model into your data collector or whatever software you are using for processing?
I can feel what you're saying. I was hard on you earlier because I have a high regard for our profession.
We are the professionals. If there was some simple solution to everything we did and no need for special knowledge, we'd be irrelevant. The reason you are the go to guy when it comes to elevations is because you study and understand what's going on and what all of this means and how it relates to your client's needs. I'll stand right with you and resist unnecessary complication, as it is frustrating. These geoids and reference frames and adjustments all exist for a reason, though. Keeping up with them and driving yourself to maintain a working knowledge of them is the hallmark of a professional. Looking for a simple, "don't bog me down with the details" solution is the sign of a hack.
You are so right!!!!! The new "geoid/ellipsoid" combination in my area is about 2.3' more or less above NGVD29. Sooooo.....now they are showing new numbers to further adjust NAVD88 and they are dropping NAVD88 about 2'. Think of the implications, for my neck of the woods we could have just stayed with the original numbers and been fine and those old guys tramping around on foot over 1000 miles from any ocean are the accurate ones.
I just wanna say thank you for everyone's input. You guys are great. This forum is the TheBomb.com! Gives me something to do when Im drinking my coffee. 🙂
I really like hunting for NGS monuments anyway. Kinda like hunting for Easter Eggs. At least I have a real check on my GPS elevations when I tie into published NGS monuments. Not an "emulated" check via OPUS. lol. I feel bad for the guys that roll with the OPUS elevations and dont check back into an actual monument. Never know when all that fancy pants OPUS software or Satellite geometry is gonna hickup. It does every now and then on a blue moon. Most of the time you can look at your gps residuals and spot it tho. Yall have an excellent day. 🙂
thank you!
