Why I preach multiple observations

Clay Davidson

Clay Davidson

Active Member
Sometimes I do static surveys when I don't have radio. I always get multiple observations and gope they agree. Usually in thick canopy I like to get 3 10 min or longer sgots and see how they compare. Usually I get a couple that agree. If not I go back and check.
 
Sdrake14

Sdrake14

Active Member
I can relate to northern CA, further up here....and it is challenging. Right on Matthew repeatability is the mantra always. Now if you are a new JFielder I'll share something that seems to work for me in my redneck savvy sort of way. (My son said after observing my methods for a time "well you just redneck it till your right don't you?" :cool:) And how. Two things I have used a lot is having my screen user buttons configured to show at least these 2 functions: distance-to-last (thanks Shawn) and PDOP. I watch these all the time to sort of get a feel for the pulse of that particular shot. Along with this now we also have the raw data reports exportable in HTML or PDF which take screen shots along with the data listing and I can review PDOP on a shot after the processing as well. Where I am in 200'+ tall trees daily I will get bad shots no matter what but I can tell you that with JField I at least know on the spot what kinda quality I am getting real time and can punt accordingly and I rarely go home empty handed. I will attach examples of some of my post shot report reviews....
 

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Duane Frymire

Active Member
Another thing you can do is run the cogo, tools, accuracy report. This will not find a bad fix, but it can show which is better, ppk or rtk. Had a similar one recently where jfield picked the ppk solution. Accuracy report showed failed connection (at 0.07 95%), so changed to rtk solution and passed. About 0.2 between them. Also note that ppk will get better the longer the occupation; the one you had that disagreed was much shorter than the others. rtk is usually the correct one on shorter occupations.
Tyler said:
I have a situation where I took three measurements on the same point and after post processing, one of the points was 0.78' off of the other two solutions. The RTK solution was close to the other two measurements. I have posted the screenshots below (also not sure what the proper way to post images is since this takes up so much space):

These first two shots are the two showing a discrepancy from PPK and RTK.
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View attachment 8269

These other four shots seem to be reasonably close.
View attachment 8270

View attachment 8271

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View attachment 8273

Why does it choose to hold the ppk values in the first point when there is 0.7' between the rtk shot? The 3 RTK shots also seem to match somewhat reasonably so is that what I should be holding in this situation instead of ppk?

Also, what are the most important factors to look at when determining if a shot is accurate? Can we trust a single measurement with this equipment if there is enough time and epochs taken?

Is the base position the most important factor for gathering accurate measurements? I live in a difficult area for GPS with many steep hills and tall trees in Marin County, CA.

That is a lot of questions! Any help in figuring out what would cause these errors and what the best workflow for ensuring these issues are avoided is appreciated.
Thank you,
Tyler Brown
Click to expand...
 
Shawn Billings

Shawn Billings

Shawn Billings
5PLS
Duane, unless you can show that one solution was better than the other you are only relying on statistics. That doesn't really demonstrate that one is better than the other. Having said that, I usually go with the RTK over PPK because it does seem to tend to be a bit more precise, but I would not rely on the statistical report prove it's true.
 
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Duane Frymire

Active Member
Shawn Billings said:
Duane, unless you can show that one solution was better than the other you are only relying on statistics. That doesn't really demonstrate that one is better than the other. Having said that, I usually go with the RTK over PPK because it does seem to tend to be a bit more precise, but I would not rely on the statistical report prove it's true.
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Shawn, not sure I understand. I know if I have one solution on a point the statistical report could show it's a good solution, when in fact it's a bad fix and a foot off. But in the case of 2 solutions on the point, one of which is better than the other in relation to other known good solutions, I think that is evidence which is better. In other words, if I have already decided they are both good because they are in relatively close proximity, will the report not tell me which is better? If not, it seems the report is not what it claims to be, I'm not sure what use it would be. I have checked the reports via total station and seems to be a good indicator of expected error between positions.
 
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