We have been asked to quote a photo control job, setting out targets and coordinating them. They are asking for undulation, elevation of the elipsoid and standard deviations @ 95% for all points in addition to xyz.
My question here is does topsurv on a fc2500 and hiper Ga's spit this data out some how...we have never had to supply this kind of data. About to break the books out but thought I might get a quicker answer here.
I know GNSS Solutions have Ellipsoidal Ht & 95% standard deviations. Never been asked to provide WGS84 Undulation values.
Undulation is the delta between the ellipsoid and orthometric heights for each point, or at least that is what I think they are asking for, this would be the geoid model heights if there isn't any adjustment to those to fit local BM's.
Most surveyors do something like this, fix orthometric heights, apply a geoid model and get what they then label as ellipsoid heights, they really aren't because you are making the assumption that there is no error in the geoid model when you do that, it is called a "model" for a reason, it isn't perfect.
The correct way to get ellipsoid heights is to fix those in your adjustment, then add the geoid model and then test your resulting orthometric heights against some known GOOD vertical marks, preferable very stable ones that are 1st order. Once that is done you can adjust your network again fixing orthometric heights. It is my understanding that the NGS follows this same general procedure when publishing points.
The difference between the two vertical adjustments would then be your undulation of the geoid.
The reason the vast majority of everyday in the dirt survey work doesn't go through all those steps is it is time consuming AND typically surveyors don't care about true ellipsoid heights.
I have seen entire mega City and County wide networks done where the published ellipsoid heights are bogus (they don't match published NGS ellipsoid heights), they really are no more than the difference between the orthometric heights and the geoid model flavor of the week used on the project. But it is so easy to push this button that says "ellipsoid heights".
SHG
> They are asking for .... standard deviations @ 95% for all points...
Probably the best place to get that is from a Least Squares Adjustment report. Which would give you the other things they are asking for, too.
In the good old days of TrimVec and TrimNet, you would create an undulation model by holding ellipsoid values and orthometric height values along with a given Geoid model. The resulting "undulation model" is a warp the geoid model to fit a localized area.
A couple of things have happened since those distant times. First, the geoid models have become much better. Second, the software manufactures have really dumm-ed down their products to the point that you can not create the undulation models. Now days I think the manufactures consider the Undulation by synonymous with Geoid Separation.
The day of the passive bench mark is not long lived. In the CORS age we usually hold the ellipsoid values of the CORS and derive orthometric heights by applying the geoid model.
That helps some...thanks guys..
I dont think we are going to be doing it anyway. Big project and over half the points are in the woods??? not sure why but thats where they want them.
