RTK, vs RTPK wide open areas, setting control.

Nate The Surveyor

Well-Known Member
Which one is generally considered more accurate?
And, how long should an observation be?
This is to set up control stations, for use on a construction site.
Has there been any studies performed?
Thanks,
Nate
 

John Evers

Well-Known Member
5PLS
My opinion is that PPK is more accurate, except when it is not.
I would think that 5, 20 second observations, then averaged would be really hard to beat in the open. Might even be overkill.
 

Matthew D. Sibole

Well-Known Member
5PLS
I have not done any studies on this. Only antidotal evidence. I am getting ready to extend some control for a roadway project done back about 15 years ago (only 3 original control points left). I plan on occupying each point 4 times. I plan on taking 3 minute observations each time and rotating the pole 90deg each shot to eliminate pole leveling errors. I will use RTPK observations on each one likely.

I will also likely do some testing with this extension as well. I will run static 2 hour sessions on each point and compare them to the averaged RTPK data.

I would have to respectfully disagree with John in that 5-20 second observations would not give me enough time to average heights that I would feel comfortable with. However, that is my personal opinion. Take it for what you will.
 

Nate The Surveyor

Well-Known Member
@Matt Sibole
What about orientation with the LS screen to the north?
Very technically, would you not orient the LS to point the screen to the north, AND rotate the pole only, to average out the bubble error?
Just running at the brain here.....
N
 

Matt Johnson

Well-Known Member
5PLS
RTPK with both the base and rover aligned to north is the most accurate. I have preformed a study to confirm this. The observation does not need to be long if the baseline is short. In my test the average duration was only 11.45 seconds.

DATA-ANALYSIS-SCREEN-STATISTIC_20200421-20.02.36.png
 

Wes Hand

Active Member
I will be sending a project to support this week as I just finished up today with gathering data. Inside 4 miles rtk to rtpk was consistently agreeing with no distinguishable difference. Outside of that I am seeing up to 0.14’ difference predominantly in the same direction from the base. If I went due north the difference in rtk vs rtpk was due north south. If I went predominantly to the east the difference was east west and not in the northing. Will present this better sometime this week as soon as possible. This difference was seen on observations ranging 5-12 miles from the base. At 5 miles I was seeing 0.07’ and and it grew to 0.14’ at 11 mile range. I rarely go outside of 3 miles but I ran that distance just to see how the difference grew. I’ll give more details later but I saw the post by Nate and thought I’d comment. Could this be simply a PPM baseline length issue??
 

Nate The Surveyor

Well-Known Member
@Wes Hand , so a simple scale factor, using the base as "scale base" it brings the RTPK on top of RTK? If so, then that could explain some things. Hmmmm
Thanks for adding this piece.
N
 

Wes Hand

Active Member
Nate,

Just to make sure we are on the same page. Im working on grid..no scaling to ground. What I am seeing is a "disagreement" between rtk and rtpk over 6 miles. I grows with distance from base. I will try my best to send the project to Alexey today.
 

Daniel Coleman

New Member
I will be sending a project to support this week as I just finished up today with gathering data. Inside 4 miles rtk to rtpk was consistently agreeing with no distinguishable difference. Outside of that I am seeing up to 0.14’ difference predominantly in the same direction from the base. If I went due north the difference in rtk vs rtpk was due north south. If I went predominantly to the east the difference was east west and not in the northing. Will present this better sometime this week as soon as possible. This difference was seen on observations ranging 5-12 miles from the base. At 5 miles I was seeing 0.07’ and and it grew to 0.14’ at 11 mile range. I rarely go outside of 3 miles but I ran that distance just to see how the difference grew. I’ll give more details later but I saw the post by Nate and thought I’d comment. Could this be simply a PPM baseline length issue??
I had the same exact problem today. In the AM my base was a little over 4 miles away with a constant 0.1’ difference in RTK and RTPK. Wide open sky’s too.

I went to another location in the afternoon with base only about 1/4 mile down the road with no issues.
 

Nate The Surveyor

Well-Known Member
Just to make sure we are on the same page. Im working on grid..no scaling to ground. What I am seeing is a "disagreement" between rtk and rtpk over 6 miles. I grows with distance from base. I will try my best to send the project to Alexey today.

I'm thinking that RTPK is using a very slightly different scale factor, such that:
It adds, Or subtracts 0.015' (or some similar amount) per mile, so that at one mile from the base, it's real close.
But, at 10 miles, it begins accumulating.
It could be coming from one factor, being rounded too much, or such like.
An accumulating radial difference between base and rover seems to have that characteristic.
N
 

Matthew D. Sibole

Well-Known Member
5PLS
There are no scale factors with gps observations and that is all RTK and RTPK looks at. Scale factors are applied after the solutions are computed.
 

Nate The Surveyor

Well-Known Member
I once did a job where all the RTPK were a certain brg and dist from RTK.
About 0.15’ or o.20’ n50e or so.
To find the RTPK diverging radially from base. Makes me try to imagine
 

Jim Campi

Active Member
I have not done any studies on this. Only antidotal evidence. I am getting ready to extend some control for a roadway project done back about 15 years ago (only 3 original control points left). I plan on occupying each point 4 times. I plan on taking 3 minute observations each time and rotating the pole 90deg each shot to eliminate pole leveling errors. I will use RTPK observations on each one likely.

I will also likely do some testing with this extension as well. I will run static 2 hour sessions on each point and compare them to the averaged RTPK data.

I would have to respectfully disagree with John in that 5-20 second observations would not give me enough time to average heights that I would feel comfortable with. However, that is my personal opinion. Take it for what you will.
Hi Matt,

I have brought this up before but not sure I understand what is happening. You mentioned rotating the rover to eliminate leveling error. My license is in engineering so in CA, I can't do boundary work. I primarily do topos and various surveys to support my engineering projects. What I have found, with all of the topo work, is that there are areas of a parcel that will "fix" much faster if the LS is set to a specific bearing. It can make a difference by as much as 20 seconds or more. If we multiply 20 seconds by 100 and divide by 60 that can mean as much 16 extra minutes per 100 points just waiting for a fix. Is this just a function of the operation of the antenna? Is rotating the rover to achieve a faster fix considered something other than best practice. I suppose the most accurate solutions should converge on areas of the parcel where the rover had a bearing of N; however, I have not noted that accuracy follows a pattern related to bearing.

One more point. On nearly all of my projects I import an aerial image. It would be very easy to see a pattern emerge with point accuracy. What I typically see is high uniform precision. Shifting the aerial a few ft in the x and y direction and nearly all points align with the appropriate surface features on the aerial. I presume this is associated with the aerial and survey on different coordinate systems.

I am getting off into the woods here...one more comment/question. I believe that google uses WGS84. If I matched their coordinate system, would the differential disappear?

Thanks for any thoughts/comments anyone may wish to contribute. I know this post rambled a bit and swerved way off topic.
 

John Thompson

Well-Known Member
Is this just a function of the operation of the antenna?
I would think so. Waiting 20 seconds for a fix on each point is a long time. Is this in the open or with multipath? How far away is your base? Are you using UHF? A network?

If you're doing hundreds of topo shots, you probably don't need to worry about pole leveling errors. Point the rover whichever direction works and keep moving. If you're setting control and trying to eliminate small errors, averaging shots with different pole rotations makes sense.

Shifting the aerial a few ft in the x and y direction and nearly all points align with the appropriate surface features on the aerial. I presume this is associated with the aerial and survey on different coordinate systems.
I believe that google uses WGS84. If I matched their coordinate system, would the differential disappear?
Probably. If you export kml from the T-LS do the points match the Google Earth image?
 

Mark Wheeler

Active Member
Did the question of RTPK differing from RTK radially from the base ever get resolved. I notice the same thing when using the state RTN. The difference between the two coordinates appears to be in the direction of the nearest reported State CORS. This is just an observation since it was brought up on this thread.
Mark
 

nusouthsc

Active Member
Mark,
I believe it has been acknowledged but not resolved. I haven’t seen anything “official”, only in conversation was I told anything. I was working at 5 miles yesterday and was seeing N 0.10 E 0.10 very consistently whether I was in the open or woods. However, I believe it has been determined that RTK is the culprit which was my feeling as well. I have tested RTPK at over 20 miles several times with excellent results to known SPC positions.

If there is more info I would love to know where we are on this. I realize there are not a lot of us running long baselines on a regular basis but for those of us who do, this is a major problem that I hope is getting some priority.
 
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