PDOP question

Stacy Carroll, post: 325412, member: 150 wrote: I've been performing static GPS surveys since ~2000 and have always been taught to work with a PDOP of less than 4 for any kind of control work. At the company I work for now, they use RTK when visibility allows. Someone had the setting for PDOP at a maximum of 3.0. We had excellent results yesterday with PDOP in the range of 4.1 to 4.8. We located six control points that we had run through conventionally and the worst difference was under 0.10'. The whole time we were getting warnings about the PDOP exceeding 3.0. What is a good rule of thumb for RTK?

I'm not sure there is a "good rule of thumb" for PDOP dependent field work. A loose field topo and control work for a boundary can both be performed with RTK, with the quality of acceptable results up to the surveyor. I have some older publications that indicate a PDOP of 2-4 is excellent and 4-6 is good..with 6-8 as fair..and so on. PDOPs (HDOP + VDOP) I guess are generally dependent of the geometry of the locations vectors with the SV constellation. I've had great results on high PDOP days, and lousy results on low PDOP days.

If the boss wants the PDOP the be under 3, make him happy. And as always, check, check, and then recheck from the other end.

One reason I'm asking is that *I* am the boss (over the surveying department anyway). I just read the same stats you quoted and I don't think I'd want to hang my hat on them. I was wondering if there was a difference when using RTK opposed to static.

Stacy Carroll, post: 325412, member: 150 wrote: ... Someone had the setting for PDOP at a maximum of 3.0. ...

That's not outrageous for doing control. And commonly achieved with GLONASS capable receivers.

Stacy Carroll, post: 325412, member: 150 wrote: ... We had excellent results yesterday with PDOP in the range of 4.1 to 4.8. We located six control points that we had run through conventionally and the worst difference was under 0.10'. ....

I'd be more interested in how your second, time separated, ties agreed with the first under those conditions. And +/-0.1' is fine for a lot of work but it isn't really all that great. If that precision satisfies the needs of your project then obviously you can observe with higher PDOP. But I'd have to do better than that to satisfy Oregon's minimum standard for boundary location.

Stacy Carroll, post: 325412, member: 150 wrote: ... What is a good rule of thumb for RTK?

http://www.dot.ca.gov/hq/row/landsurveys/SurveysManual/06_Surveys.pdf&apos ;">Check out this CALTRANS document. They go to 5, sometimes 6. But I, like Kent, put a much higher premium on LS analysis of redundant measurements than I do on numbers like DOP.

@Norman, I agree that it isn'r great for any kind of control. We were basically experimenting so i can get comfortable with the limitations of RTK. We ran through the control with a 1 second total station and then with RTK. The elevations were basically junk but the Horizontal was ok for blunder detection.

FWIW, Trimble defaults the max PDOP for RTK at 6. In the early to mid 90's I was always taught that anything under 8 was acceptable, but back then you had to take what you could get. I think 3 for an RTK survey is total overkill, I'd go at least 5.

With respect to DOP, bad DOP will generally cause bad results, but good DOP doesn't ensure good results. http://webapp.geod.nrcan.gc.ca/geod/publications/pdf/Canada%20RTK_UserGuide_v1%201_EN.pdf&apos ;">Natural Resources Canada recommends a DOP max of 2 - 3.

For example, it is possible to have a PDOP below 2.0, with only 5 satellites properly spread in the sky. http://latitude51.ca/dop.html&apos ;">Here is a tool where you can visualize DOPs by placing satellites on a skyplot and then moving them around to see how it affects DOP.

PDOP for me is rarely over 1.5 using R8/3 with GNSS. I just spot checked a couple dozen shots from this morning and it was hovering all around 1.2 with HDOP in the 0.5 area and VDOP in the 1.1 area. The formula HDOP+VDOP=PDOP does not seem to be correct.

For what it's worth, I don't recall PDOP ever being over 2.0 with this gear in the past three years.

vern, post: 325432, member: 3436 wrote: The formula HDOP+VDOP=PDOP does not seem to be correct.

I think it's more like PDOP = the square root of (HDOP squared + VDOP squared)

You want to know why the PDOP is 4. If it is because you have fewer satellites but they have good sky geometry, and your canopy is clear, carry on. It is when the PDOP is 3 because of many satellites but the sky geopmetry is not that good, nor the canopy, that is when you worry,

As in all good things redundancy helps more than anything else.

Paul in PA

Here is CT standards for GPS.

http://ctsurveyors.org/wp-content/uploads/2010/12/GNSS_20080626_online.pdf&apos ;">GNSS Standards