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RTK Base VS. RTK CORS network

BurnorMJ
(@burnormj)
10+ posts Member

Hello community, I am hoping that someone can clear some information up for me real quick. Our office use the Trimble R10 head unit and we run off our states cors network which works out for establishing control and getting on grid. However, if i go the the same control different times of the day or a different day of the week there are h & v inaccuracies. I was tasked to find out if we got another head and set up a base station would I be able to use this form if RTK, which would still be tied to the state plane coordinate system to remove these inaccuracies. I have no experience with stations so i cannot shed light of the subject. I use the RTK cors to get control and run with a robot, but we are looking to use our gps more heavily for topo. 

 

thank you

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Topic starter Posted : September 3, 2020 9:47 am
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jim.cox
(@jimcox)
1,000+ posts Supporter

> would I be able to use this form if RTK, which would still be tied to the state plane coordinate system, to remove these inaccuracies

In my experience, Yes you will see better numbers (but maybe not by much).

Using our local Trimble VRS network we were seeing unexplained jumps in height of about 75mm (that's 3 inches for metricly impaired).

We have abandoned VRS, and now only use RTK.

If you search this site you will some discussion of the topic in other posts

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Posted : September 3, 2020 10:12 am
rkinnie
(@rkinnie)
10+ posts Member

The use of GNSS is always a little confusing - the result of a measurement is affected by many things and needs to be validated - averaging measurements over time is a good way as most systems are designed to be -/+ 2cm or an inch - even though the stored coordinate has it to four decimal places. Taking two gps shots 100 feet apart can have a large error ( maybe a tenth or more) and is always affected by length of baseline, canopy, weather, solar storms, electric wires and other things that affect radio transmission. It is best to use the right tool for the project, if you only need data to the tenth of a foot and there is little canopy cover GNSS works well - otherwise use the GNSS to set control 1800+ feet apart and then run through it with a total station to refine the values and collect the data - you can expect it to be +/- 0.02 that way

Using a base helps with the length of baseline, but you need a license to broadcast from the FCC ( in the US ) for more than 1 watt and that might be good for 1000 feet

Having a clear view of the southern sky helps with measurements too, any hills, wires, trees will all affect the signals from the satellites and degrade measurements, but know that in the best cases you might get +/- 0.05 in the vertical since the signals are traveling 24 miles to get to your receiver  

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Posted : September 3, 2020 10:20 am

eastkysurveyor
(@eastkysurveyor)
10+ posts Member

new to the site, but will try to help...

 

The differing solutions at different times in the day is most easily explained by saying you are picking up signals from a differing constellation of satellites at different times of the day (there may be some other stuff going on but that is common). A base/rover setup will allow (in most cases) for your vector lengths to be shorter, which in turn should allow for less residuals in the measurements (which in most cases will help with the precision of the measurement). 

 

It sounds like you maybe a construction user, so as far as accuracy. You set your base up on a known project control point, this will reference you to the project design. Assuming all the control and design data have been verified by a qualified person.

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Posted : September 3, 2020 10:28 am
Totalsurv
(@totalsurv)
500+ posts Member

The best way to check the accuracy you are getting with VRS is to stakeout points you have shot at other times and note the differences. You are probably seeing +/- differences in vertical when comparing with total station i.e one control point could be +20mm(or more) and another -20mm(or more) resulting in a 40mm difference. Least squares can be your friend.

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Posted : September 3, 2020 10:35 am
MightyMoe
(@mightymoe)
5,000+ posts Supporter

I was just looking at some check-ins using base/rover and the R-10:

Checkin to an existing Section Corner first measured using 4700/5800 units: H0.044', V0.044'

Checking to a control point first using the same R-10's: H0.015', V0.021'.

Checking to a property corner first located by R-8 units: H0.020', V0.033".

These points were tied using the same base point that's been occupied over and over since the early 2000's.

I have heard that VRS is iffy for consistent accuracy.

Frankly it's up to you to decide if what you are seeing is acceptable.

But you should be running checks all the time to be sure.

If measurements slide out of range of acceptable then it's time to change. 

The R-10's (in my experience) as base/rover units are extremely accurate when used properly. 

 

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Posted : September 3, 2020 10:56 am

BurnorMJ
(@burnormj)
10+ posts Member

Thank you for the information. I have talked to our rep several times and he states the same, that state VRS networks are iffy. I'm leaning towards renting a separate R10 base head to tried out and see. I would like to be able to reliably use our gps for 90% of our open topo, and use the gun for the boundary and pining of building and such.  

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Topic starter Posted : September 3, 2020 11:31 am
John Hamilton
(@john-hamilton)
1,000+ posts Member

I can consistently get sub-centimeter vertical repeatability with a local base using multi constellations in the open. Not so much for VRS. If not in the open, results are slightly degraded (1 to 3 cm typically) but still better than VRS. 

That said, I use VRS for a lot of projects, especially large area photo control, Lidar, etc. I use a local base for highway projects where higher accuracy is required and the area covered is smaller, I try to keep baselines under 10 km. We do everything over cellular. Which is not always available 100% of the time but is definitely better than using a low power radio. 

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Posted : September 3, 2020 11:50 am
dmyhill
(@dmyhill)
1,000+ posts Supporter

A stand along base will almost always provide for better precision if the baselines are very short and the base has good sky. The combination of short baselines with matched receivers on your local control creates the best conditions for precise RTK.

That said, if someone bumps your base, if you have long baselines, etc, then your network solution will provide more accurate solutions. (Notice I did not say more precise.)

Additionally, network does not equal VRS. You can use single base solutions from a network. Depending on where I am working, I find that this may provide more consistent results.

I am, of course, spoiled by the best network available, overseen by a geodetic conscience and with embarrassingly dense CORS. In most areas of my state where I work, I could use a single base from the network and still have very short baselines.

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Posted : September 3, 2020 12:59 pm

Norman Oklahoma
(@norman-oklahoma)
5,000+ posts Member

With a local base station paired with a rover, the assumption is that the atmospheric conditions at the 2 is identical. If the base is nearby the rover, that assumption is very likely to be valid. The further between them, the less so.

A VRS models the corrections based on a network of (sometimes) widely separated bases. If you are working a long way from the nearest base, the modelling may not be perfectly valid for the rover location. So I think that there will be a slight advantage to the base/rover pairing in terms of precision. But in practice it may be a distinction without a difference.

When I was recommending a purchase of some GPS a couple years ago I convinced the boss to go with the paired units on the grounds that:

  1. We may, at some point, want to work in an area with poor, or no, cell service.
  2. We could hook the two units up with VRS at a future time and operate them separately if the need arose.
  3. Until then, we wouldn't have to pay any subscription fee to the VRS.
  4. We wouldn't have to maintain a data plan on the cell phones (which we later did, anyway).       

Accuracy, although very important to me, really wasn't a consideration.

The cell service issue can be a thing even within the big city. There are always spots where the cell coverage isn't so good, and times when network demand is high.

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Posted : September 3, 2020 8:26 pm
BushAxe and Bill93 liked
RobertUSA
(@robertusa)
200+ posts Member

Does your network RTK service use all 4 constellations or just GPS and Glonass? If it’s not all 4 and you have some trees in the area, your  position observed will change some throughout the day as you have more and fewer satellites and their geometry improves and also gets worse.

This post was modified 2 years ago by RobertUSA
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Posted : September 4, 2020 6:19 am
BurnorMJ
(@burnormj)
10+ posts Member

@robertusa Just GPS and Glonass unfortunately

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Topic starter Posted : September 4, 2020 9:26 am

BushAxe
(@bushaxe)
500+ posts Member

I almost always use a local base and rover. When I don’t, I almost always wish I had. 

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Posted : September 6, 2020 5:24 pm
Jaccen and Rover83 liked
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