Wondering if there is a utility that quickly converts NGS Geoid files (ascii data) into Trimble GGF. I'm at a new job, and have had to go from Leica to Trimble equipment. Leica had a simple utility that did this (asc. to .gem), but I can only find a subgrid function in TBC. Or if anyone already has USGG2012 in GGF format that would be great. Thank you.
Trimble Grid Factory will read in an ASCII file but there is not much documentation on the input format.
Who in the world would write software that wouldn't read NGS geoid files in their native format?!
Oh wait, I see... it's Trimble!
These are the formats it will read in.
Maybe one of these is more use?
I've only ever used ASCII files, but not with this program.
I'm on a recent version of TBC and it's in there automatically without me having to add it. Mine is in C:Program Files (x86)Common FilesTrimbleGeoData. If you don't spot it there, search your hard drive using *.ggf as the search string and look for a file named G12AUS.ggf.
Thinking that you want to use Geoid12A, it looks like Trimble has the ggf file available for download, no conversion needed:
http://www.trimble.com/globalTRLTAB.asp?Nav=Collection-89296
Is that the same geoid as USGG2012 then?
I must admit, all of the different systems go quite over my head.
It's relatively simple here.
> These are the formats it will read in.
I'm not familiar with that particular program, but as the native Geoid files are .bin format I would try importing using the Geoid99 template, and then specify the appropriate Geoid12a bin file to convert.
Geoid99 was a (now obsolete) iteration of the Geoid model. They get updated every 3 years or so.
EDIT: OK, that was for importing Geoid12a. Apparantly USGG2012 is something else. Thanks, Dave.
USGG2012 is NOT for converting between NAD83 and NAVD88.
USGG2012 is suitable for use with WGS84 and scientific applications.
This page explains it:
http://www.ngs.noaa.gov/GEOID/USGG2012/
Also See:
http://www.ngs.noaa.gov/GEOID/GEOID12/faq_2012.shtml
So what's the difference between a gravimetric geoid (USGG2012) and a hybrid geoid (GEOID12)?
> So what's the difference between a gravimetric geoid (USGG2012) and a hybrid geoid (GEOID12)?
As I understand it, a gravimetric geoid is based solely on gravity models and uses no level data. A hybrid geoid incorporates levelling data. I believe that a certain amount of warping of the 2 sets of data is required to make them match up.
Makes sense. I read a document about the calculation of "our" current geoid OSGM02 and it used all kinds of terrain and gravity data. It was then "warped" to match long standing primary datums I think. I would have to reread the whole thing. Probably a few times.
A geoid model based on levelling works best when you are using it close to where the levelling was done - typically along the roads through the valleys. When you interpolate a geoid height for a point on the mountain using data points collected only in the valley it doesn't work so well. Gravity data collected from airplane covers the area uniformly. Plus it is cheaper to cover wide areas that way.
This is the first time I've posted to this website. I didn't expect to get so much feedback. This is a great resource! I will give that Trimble Grid Factory a look-thanks.
I'm by no means a geodesist, but had to do some research for local geoid modeling. My understanding is that USGG2012 is purely a gravimetric geoid, and is the best approximation of mean sea level. It is not intended for use with NAD83 ellipsoid heights to achieve NAVD88 elevations. Geoid12A is a hybrid geoid. someone described it well by saying it is warped to fit datums. It is constrained to fit passive benchmarks that realize the NAVD88 datum. The reality is pipes in the ground move, and so this really is degrading the equipotential surface.
NGS' height modernization program is moving away from passive control, and I imagine hybrid geoid models will stop being used. Just like the foot is no longer defined by a physical object, mean sea level really is no longer practical to be used as a national reference surface. I would guess it will be an equipotential surface of a certain amount. But it is abstract and difficult to get people to think in terms of force, instead of geometric distance, when considering height.
Interesting that folks seem to feel no need to understand the differences between the hybrid and gravimetric models. I highly recommend taking a look at the writings of Professor Will Featherstone at: http://www.cage.curtin.edu.au/~will/publications.html
An early, very clearly written and accessible article I recommend is: http://www.cage.curtin.edu.au/~will/Featherstone_TAS_43_4.pdf
Professor Featherstone has written a good deal on the geoid in the UK see a recent article: Featherstone, W.E. and J.G. Olliver (2013) Assessment of EGM2008 over Britain using vertical deflections, and problems with historical data, Survey Review 45(322): 319-324, doi: 10.1179/1752270613Y.0000000048. PDF (180kb) at his publications link above. Not too uncomplicated ...
In addition to the creation of a fitting surface created from the differences between GPS-derived ellipsoid heights and NAVD88 heights, there are datum transformations. From the NGS site linked above:
The gravimetric geoid models (USGG2012) all are based on a GRS-80 ellipsoid shell in the IGS08 reference frame and a geopotential surface (W0) of 62,636,856.00 m**2/s**2. These models collectively define the same geopotential surface (geoid) determined from the underlying reference global earth gravity model (EGM2008). Hence, comparisons of these heights between different regions provide consistent values.
However, most people are interested in the hybrid geoid height models (GEOID12). A hybrid geoid model can transform between a NAD 83 ellipsoid height and the relevant vertical datum for each region. For CONUS and Alaska, a hybrid geoid model can transform from a NAD 83 ellipsoid height to a NAVD 88 orthometric height.
HTH,
DMM
This is a video created by a university colleague, I hope someone else serves.
