Activity Feed › Discussion Forums › GNSS & Geodesy › Correlation between PDOP and precision
You are seeing real world results. Typically H precision is 1/2 of V precision, I.E. the 33/66 ration you mentioned
Problem is, higher correlation coefficients imply better precision.
- Posted by: @mathteacher
Problem is, higher correlation coefficients imply better precision.
Higher precision in estimating one variable from another.
But not in estimating that variable relative to its true value (or long term average as a standin for true).
. Agreed, to an extent. But the poster was implying that a high correlation value for a typically lower precision value and a lower correlation value for a typically higher precision value is the normal state. It isn’t.
The water in this problem is muddied by the comparison of a unit-defined quantity, precision, to a unitless index, PDOP. High precision might be expected when the index value is low. If high precision is defined as the difference between measured position and actual position, or as some variance, then a low number will represent high precision, and the correlation will be positive. If precision is defined so that a high number means high precision, then the correlation will be negative.
In any event, complicated or not, there should be a measurable relationship between PDOP, horizontal position, and vertical position. But it doesn’t have to be linear, so the Pearson correlation coefficient might not be the best measure.
Last time I was observing, the least birds I had was 30+. PDOP was < 2 on everything. With multi-constellations we might be spinning our wheels analyzing PDOP stats. In the 90’s with 4 or 5 birds a different story. In my experience the two or three major horiz and vert error sources are pole errors. Out of plumb or bad phase center height above the mark. And those locations where it takes longer than normal to fix for whatever reason.
I’m with you there.
It’s gotten to be very rare when the unit won’t fix, either deep woods or some type of communication glitch.
My 4-wheeler sitting next to me running will interfere with the radio, turn it off and I fix instantly.
But tracking down the “error” in the R-10 unit locations would be exhausting.
Set up control points on a site with the R-10, occupy them with the S6 and there is .02′ of error, mostly vertical, over .04′ and we start looking for a problem.
This is repeated over and over.
Rod plumb, tribrach, GPS “errors”, PDOP, distance meter, scale factors.
A big bucket of mini teeny-tiny errors that add up to nothing. Interesting in an academic sense, but working in the field pfttt!!!
I recently finished a boundary locating a 1/4 corner that was surveyed by the state in 1996. On the state plat the surveyor noted that the monument is a recovered set stone and there is a northing, easting in state plane shown on a table on the plat. In 2001 the state re-monumented the stone with an Aluminum cap, no coordinates for that survey. My new survey is .07′ N, .04′ E. of the 1996 coordinate. Good times we live in.
- Posted by: @norm
Last time I was observing, the least birds I had was 30+. PDOP was < 2 on everything. With multi-constellations we might be spinning our wheels analyzing PDOP stats. In the 90’s with 4 or 5 birds a different story. In my experience the two or three major horiz and vert error sources are pole errors. Out of plumb or bad phase center height above the mark. And those locations where it takes longer than normal to fix for whatever reason.
In the wide open spaces, sure. I’m using RTK in an urban environment where
- real estate is priced by the square foot, and
- there is a lot of sky obstruction
I’m lucky to get 12 or 13 satellites at the rover and often am down to 8 or 9. And it matters which ones I’m getting. In these conditions the PDOP numbers are important. Granted, without all these extra constellations RTK is just not an option at all.
@norman-oklahoma
Will preface this with a disclaimer that I expect you do technology independent checks and are working within necessary error budget for your projects; the following is more a caution to those who treat GNSS as a magic stick.
If the sky visibly is that bad then is rtk the tool for job? Once I’m down to those satellite levels (with all four constellations) I’m looking at CQ readouts of 7-10mm HZ and 15-20mm V which you at least need to double to 95% CI. For urban control I’m wanting <=10mm in HZ repeatiblity so RTK isn’t there. Plus once it’s not wide open you get the problem of biased GNSS when multiple control shots at different times agree but when you shoot it with total station you find it’s ~20mm out. The more I use RTK GNSS the less I trust it (or the more I try to avoid pushing it’s limits).
Govt. auditing surveyor here tells a story of following a surveyor who called the middle of three stable control points that were placed on a line disturbed based on GNSS alone. Auditing surveyor set up his GNSS (different brand to first guy) and got pretty much the same result. He then set up total station at one end, shot the other, and checked the middle; what do you know all three marks were still in terms. The middle mark only had a mid sized tree to one side, not a poor site for GNSS at all but enough to bias the solution. I’ve had a similar experience next to a standard street powerpole, double tied with 10mm HZ repeatiblity by GNSS but was 20mm out when shot with total station.
Used Lecia, Trimble and even the fabled Javad but find the total station hasn’t been superseded for ~10mm HZ positioning in urban environment yet. Sure at ~20mm HZ the GNSS ups the production but thats typically not urban spec for control/boundary where you are signing on the plan where I work.
- Posted by: @lukenz
If the sky visibly is that bad then is RTK the tool for job?
We RTK what we can but traverse (almost) everything. In rare cases we double RTK tie things – different bases – if traversing is impractical. StarNet adjust everything. But, yes. RTK is not 100%.
The RTK’ing serves the purpose of putting the control on the grid, and also provides blunder detection.
@norman-oklahoma
That’s what I’d figured was your quality of practice given the other useful comments on here.
- Posted by: @lukenz
@norman-oklahoma
Govt. auditing surveyor here tells a story of following a surveyor who called the middle of three stable control points that were placed on a line disturbed based on GNSS alone. Auditing surveyor set up his GNSS (different brand to first guy) and got pretty much the same result. He then set up total station at one end, shot the other, and checked the middle; what do you know all three marks were still in terms. The middle mark only had a mid sized tree to one side, not a poor site for GNSS at all but enough to bias the solution. I’ve had a similar experience next to a standard street powerpole, double tied with 10mm HZ repeatiblity by GNSS but was 20mm out when shot with total station.
Used Lecia, Trimble and even the fabled Javad but find the total station hasn’t been superseded for ~10mm HZ positioning in urban environment yet. Sure at ~20mm HZ the GNSS ups the production but thats typically not urban spec for control/boundary where you are signing on the plan where I work.
Isn’t this because a GNSS unit measures the geodetic distance, i.e. curved distance on the surface of the globe (which isn’t flat btw) while a total station measures the horizontal distance. There is an old article I read about setting out a straight line over a long distance and the mistakes that lesser surveyors can make by not understanding geodesy.
Nope, we use ellipsoidal distances for boundary here (GNSS easy enough and apply sea level correction to get within mm’s for TS) so not a scale/geodesy problem. Simply a GNSS button pusher user in the wrong environment.
- Posted by: @minbarwinkle
Isn’t this because a GNSS unit measures the geodetic distance, i.e. curved distance on the surface of the globe (which isn’t flat btw) while a total station measures the horizontal distance. There is an old article I read about setting out a straight line over a long distance and the mistakes that lesser surveyors can make by not understanding geodesy.
You need to break down the process to understand what is actually measured as opposed to what you choose to display.
A GNSS solution is found in XYZ world coordinates and converted to any other form like lat-lon-ellip ht. When comparing two solutions the software can find ellipsoidal distance or mark-to-mark distance or whatever. Ellipsoidal distance can be converted to a ground distance using the El ht, and El height to ortometric using a geoid model.
A TS measures instrument-to-prism slant distance and angles. From those the software can calculate horizontal distance at the elevation where it sits.
.
Log in to reply.