Hello all, trying my hand at the first post in this forum.?ÿ
Small disclaimer is that I am not a surveyor but rather a hydrologist that find myself neck-deep in hydrographic surveying all of the sudden.?ÿ
Basically I??ve got a trimble r8-mod 2 which I need to connect to an "acoustic doppler current profiler" via NMEA. I use the configuration toobox software. I have set the serial port to output NMEA messages GGA, VTG and GSV.
We figured out that when programming the r8 using the TSC2 and Survey controller to use our local datums and projections, this does not carry over to the serial NMEA output. It continues to use WGS84 (I found this out by adding the NMEA PJT message). My question is: What is the geoid used for the height coordinate? Is it GRS80 or does it relate to MSL?
I??ve also tried to upload a geoid by making a subgrid of the SWEN17 geoid with Grid Factory and use it in the "Coordinate system - Import from DC" box in configuration toolbox after finding some information online. I only get error messages in return (no LRecord returned).
I??m thinking that I??m better off just using the default coordinate system and geoid and convert in post processing, but then I need to be absolutely sure about the geoid.
I hope my questions makes sense, and that I haven??t misunderstood the concepts.?ÿ
I can upload sample NMEA files etc if needed!
By default Trimble receivers estimate Mean Sea Level using so called DMA WGS-84 geoid model prior to 4.70 firmware.
Copy paste from a search on the interweb:
"SWEN17_RH70 is a Swedish height correction model that has been computed by adapting the Nordic/Baltic gravimetric quasigeoid model NKG2015 to the Swedish three-dimensional reference system SWEREF 99 (ETRS89 realization) and to the old Swedish height system RH 70."
If this is the result you wish you need to use the geoid model SWEN17 with SWEREF 99.?ÿ
I'm simply looking at the search results, I didn't even access the article just copied the blurb under the search result.?ÿ
One thing I do know is that you are not on WGS84, regardless of what the Trimble DC might note. WGS84 is controlled by the US government and precise locations on it are not available for public use, at least many people more knowledgeable about GPS have told me this.?ÿ
To get on the local Swedish system you need to tie into some type of local control point, in the US there are broadcasting points and monumented available points to occupy. I suggest using those local points to put your survey on the correct Swedish system then apply the geoid model which is proper for that system. SWEREF 99 indicates to me a 1999 system which seems dated.
?ÿ
If you were using Access, it would be as simple as configuring the NMEA settings in your survey style to output job (local) coordinates. But I don't think Survey Controller has that functionality.
If I remember correctly, the GGA message includes records for both ortho height and geoid separation. Are you getting those values? If there is no geoid, I believe the separation record would be zero.
Post some of the strings and we can take a look.
One thing I do know is that you are not on WGS84, regardless of what the Trimble DC might note. WGS84 is controlled by the US government and precise locations on it are not available for public use, at least many people more knowledgeable about GPS have told me this.
Raw observables in the receiver are technically WGS84, as they are ranges to the satellites which are the physical manifestation of the WGS84 reference frame. It is up to the user to decide how to use/transform that data.
Nowadays WGS84 is comparable to ITRF at the sub-centimeter level, if you are comparing the same epoch/realization. They are effectively the same for all but the most precise work. If the user is using relative positioning from a station with ITRF(2014) coordinate values, they are effectively measuring in WGS84(G1762).
?ÿ
Wow! Thank you so much for the input! Seems like I struck gold 🙂 I??ve had contact with a local trimble retailer and because the age of the gear he was a bit unsure. I would love to get Trimble Access etc, but working for the government means I have to put it in next years budget and jump through a lot of hoops to do it.
The SWEREF99 is based on UTM with 12 local zones in Sweden, there is also a national cover system called SWEREF99 TM based on UTM 33N, and it is the standard coordinate system for government work in Sweden. I can swap coordinate system output via configuration toolbox without problems with the "coordinate system" part of the config file, its more of an issue with the ellipsoid/geoid.
I think I have come to the conclusion with the help of you guys that I??ll just use WGS84 through it all and do the conversions afterwards. The end results shouldnt be different if I do the conversion in the R8 or in HYPACK/QGis etc?
So, for the question re: orthometric heights and geoids, this is the GGA and VTG strings that are output:
$GPGGA,143856.80,5648.14936021,N,01434.81090931,E,4,13,0.8,140.685,M,35.423,M,1.8,1770*41
$GPVTG,186.40,T,,M,1.16,N,2.15,K,D*33
I guess this means that there is a geoid separation?
/Daniel