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Soothsayer05
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Hello everyone. 

When using RTK, I have always setup my base over a known State Survey mark or our known grid control coordinates.

I have a job coming Thursday and it is slightly tricky to do what I've always done. I do have a Trig station 3km away from my site so I've got the option of using repeater. 

However, I have heard of the method of setting up base over a unknown mark. It is something I could do and drive around and check onto few known marks around the site. I checked Trimble's website and apparently I can do a 'here' function and have a rough global coordinate. 

I haven't really grasped on how this system works. My understanding of the principle is that in RTK, each measurement is corrected based on the base station as its a vector observation from the known mark of a particular reference system.

Since in this case, the vectors are corrected from a rough global coordinates, my observation quality wouldn't be as good as proper control marks right?

So is the solution to use my observation onto known marks to apply swing/translate later on?

Ps: we usually do a P E N Z only export in csv format and use civilcad software. I do have tbc where I usually only reduce static obs. 

Any help would be greatly appreciated.

Thank you

 

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oldpacer
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I typically have an unknown base and observe multiple knowns. I thinks my "observation quality" is better that way. But I am not normal and post process everything. I would do simultaneous static before setting up a repeater, but then again, I am not normal.

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Soothsayer05
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@oldpacer fair enough. I like doing static tbh. just haven't done post processing of RTK data while running static simultaneously. I will look into it further to have that tool in my belt. thanks for writing back!

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350RocketMike
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@oldpacer 

This is the way our office always does it. 

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Rover83
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Posted by: @soothsayer05

Since in this case, the vectors are corrected from a rough global coordinates, my observation quality wouldn't be as good as proper control marks right?

Your observation quality will be as good as site conditions and your observation methods allow.

Vectors will be flowing out from your base station location. No matter where it is set up, or whether you use a repeater or not, the vector precisions will remain the same as when you observed them.

How you manipulate that base position afterwards is up to you. Use of a "here position" almost always indicates that the user will be logging static data at the base and will be processing against published reference stations afterward.

And that really is the question - is there a local coordinate system that you need to tie into and use for your project? Is that system related to a geodetic coordinate system? Will you be able to position your base using published reference stations, downloading their static data and processing in TBC?

As @oldpacer mentioned, you can indeed "back in" your base position if you tie into enough existing control with your rover. It can really speed up your workflow, but it does require knowledge of how that local system relates to the global system the GNSS gear is in.

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Soothsayer05
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@rover83 I should've been clearer on my questioning I suppose. When I mentioned using repeater, that would have the case when I would set up on a known mark. (At the moment my office is in a COVID hotspot so I can't get the repeater which is why I was looking for alternatives).

and to answer your question: Yes, there is a local coordinate system to tie my project to and it is related to a geodetic system. And, the question of processing in TBC, I've only used it to process static data in the network in previous projects but haven't done so for RTK data. I found some tutorials on it. I will dive into it. 

Appreciate your response mate!

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Williwaw
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Done all the time. If you’re using Trimble Access you will ‘key in’ your base point number and description. I add a ‘_here’ to point number to indicate an autonomous position. Several options available to you. You can either download the base file and convert to Rinex format and submit to OPUS and later translate everything to the OPUS solution (if you’re working on a standard projection, or if you have a good control point within radio range you can do a single point localization. Still a good idea to check into something else or get an OPUS solution as a check. Last option is to download CORS directly and post process all your observations. Matching a local system is a little trickier, particularly with elevations.

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oldpacer
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@williwaw   I add a ‘_here’ to point number to indicate an autonomous position

Yep, that really helps. I add a '_start' to mine.

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Soothsayer05
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@williwaw I see. that's really helpful to know the options available. my project got delayed till next week so I will play with it over the weekend to get a feel for it. To be honest, my nearest CORS is roughly 13 km away. Here is Australia, we've got AUSPOS which is slightly similar to OPUS from what I can gather. 

AUSPOS is not as great to heights as much as we would like to most likely I will use the State Survey Marks around me to get the coordinates for my base station. 

Appreciate your response!

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Chris87
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I do this on a daily basis.

Setup base wherever works good, use here to get autonomous coordinates for base, we will call this point 1.

Shoot a point on an existing monument that we know the coordinates of, this will be point 2.

Key in said known cordinates of existing monument, this will be point 3.

Translate points 1 and 2, from point 2 to point 3.

Now your coordinates for your base have changed. Don't forget to start survey again and setup base with new coordinates.

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jim.cox
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@chris87 

Easier to use 'site calibration' to associate points 2 and 3.

Then there is no need to restart the survey.

Don't forget to check against another known mark

 

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Chris87
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@jimcox I prefer to stay on the spcs. no site calibrations here.

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lukenz
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@chris87 

Think you'll find that Jim is using Trimble Access which does a single point "site calibration" which only translates XYZ and effectively is same as your method just less manual. Practically all he is doing is "updating" the base co-ordinate to be in terms of the control point surveyed so you are still on the grid coordinate

 

Javad has m-local which also can be used similarly and Lecia don't love random base setups and make you run through a long winded two step coordinate transformation. To get around this with Lecia we set the base up randomly with here position, shoot the control point, take the inverse between base and rover and then cogo traverse from control coordinate using calculated vector to get adjusted base coordinate then head back and restart the base on adjusted coordinate, a PIA and the only thing I think Lecia stuffed up on.

 

Your comment about site calibration taking you off the grid is valid where you shot multiple points and apply the rotation and share based on them all. Then you really have to know what your doing or it gets messy fast.

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Williwaw
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@chris87 One thing to be aware of when you do it that way, translating your base coordinates, the GNSS lat/long coordinates will get translated as well and it can really screw things up. Don’t ask how I know.

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Chris87
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@williwaw well yes, that's the whole point is to translate base coordinates.

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Williwaw
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@chris87 Including your autonomous lat/long that was off by 5-10’ and now is off by however far you just translated your grid coordinates? You want to move your grid coordinates but not your lat/long. That’s why a localization is much better.

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Chris87
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@williwaw yes the lat/long gets translates too, that's exactly what you want.

I have a feeling you do not understand how the equipment uses the lat long and grid coords. Gps receivers only know geodetic coords, they know nothing of grid coords.

Grid coords are only a product of your data collector calculating a northing and easting from the supplied latitude and longitude that your receiver is giving it.

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Williwaw
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@chris87 Never mind. You are clearly the expert.

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Chris87
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@williwaw localization just skews your lat/longs to make your grid coords fit.

we keep everything on spcs and just adjust our scale factor for the area we are working in. This way all of our data is on the spcs and it is relative to everything else. We can set our base up on cords from one job and go shoot in points on a different job and it doesn't matter cuz it's all tied together.

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jim.cox
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@chris87 

Good point.

But...

The huge majority of the New Zealand coordinate systems I use are based on GRS80 and ITRF96.

So WGS84 Lat/Long are of little use to me

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Andy J
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@chris87  Are you are letting your scale factor 'float' between jobs??

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lukenz
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@williwaw 

If the base lat/long is being adjusted by the difference between the rover shot (projection grid coordinate) and the projection grid coordinate for the control point then the base lat/long ends up being adjusted to true geodetic position (assuming control point is correct). What's wrong with that?

 

This is assuming grid coordinates on a known projection, not talking about site grid coordinates here.

 

Seems a bit of confusion in this thread due to different terminologies. Single point "site calibration"/translation using projection grid coordinates vs proper site calibration between geodetic and some local site control. And also how the different field software's manage the processes. Feels like each post needs to include the field software the person is using so you can understand the context of the comment.

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Williwaw
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@lukenz Good point. Moving the lat/long a short distance within a standard projection, 5-10’, no problem. Translating the base lat/long 5000’ in eastings using some random local plane projection? Big problem. Localizations or calibrations can be a finicky thing in my experience and it doesn’t take much to screw them up and if I forget that just in starting my base, I’m pairing my grid coordinates with a lat/long, I’ve effectively just done a single point localization. Screwing with it by translating it any significant distance can really mess things up, especially if working within a local plane projection. The entire point I was trying to make is that the OP recommended translating the base grid coordinates, which also btw translates the lat/long and potentially can bone up that pairing. 
Carry on.

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lukenz
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@williwaw 

We are lucky here in NZ that almost all work (boundary, topo, dirt works and even the odd building setout that shouldn't be) is done on local transverse mercator projections that sound similar to the LDPs I hear about on here so 99% of "calibrations" are just tweaking the here base coordinate to be in terms of the control points with published coordinates from our equivalent of your NGS

 

True site calibrations are really only for the odd legacy construction project where there are multiple control points you know are tight and even then if it's not a massive site you have to ask yourself if using the total station would be safer.

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