Jim Frame
Well-Known Member
I'm still pretty confused about the confidence and consistency numbers, and how to use them in recognizing a bad fix.
I recently hit a number of control points in a project with RTK using single-base RTN. The RTN base station is close by, less than 4,000 feet from the site. I use the Stop button rather than timed shots, with a minimum of 2 fixed engines. I've been playing around with observation times, and haven't really settled on a scheme. I started off trying for 180 epochs for control points like this, but lately I've reduced that to 60 when I see that the spreads are small. However, when conditions are dicey I often lose patience before I get to 60, especially when I know I'll have other means of verifying the position.
After the RTK observations I went through the points with a total station, using my normal procedures (D&R, careful setups and a pole-mounted miniprism forsight). I was quite surprised by what I found in a couple of instances. I don't like those kinds of surprises, so I'm hoping someone will be able to let me know if I'm dealing with anything unusual, or if this is just the way things are with RTK.
I'll use 3 of the points as examples. All dimensions are in feet.
The first point, 59, is pretty much wide open, and is representative of what I find with clear sites and good SV coverage. I clicked off 60 epochs in 65 seconds, with 5 or 6 engines fixed probably the whole time. RMS stats were good, 0.03' or less; Confidence 3, Consistency 44.95. When I compared the RTK result with my adjusted terrestrial network position (and after accounting for the small datum shifts between the project coordinate system and the RTN coordinate system), the differences were 0.00N, 0.01E and 0.08 elevation. Pretty darn close enough for me.
The next point, 60, is about 8' feet east an 8' chain link fence and a few feet clear of a leafless tree to the northeast. I recorded 64 epochs in 66 seconds, with similar RMS values as at Point 59. Confidence 2, Consistency 14.25. However, the differences with my total station position are 6.36N, 5.69E and 5.32 elevation. Ouch.
The third point, 56, is under some overhanging evergreen limbs (pine and live oak), though it has decent sky view to the south. I recall having trouble getting fixed here, but the stats say that I recorded 155 epochs in 170 seconds, so I guess it wasn't that bad. RMS values were bigger, around 0.1'. Confidence 8, Consistency 118.5. But the differences were 2.58N, 1.91E and 11.23 elevation. Yikes!
So, what should I do differently in order to produce more reliable results, or at least positively identify unreliable results?
Thanks!
I recently hit a number of control points in a project with RTK using single-base RTN. The RTN base station is close by, less than 4,000 feet from the site. I use the Stop button rather than timed shots, with a minimum of 2 fixed engines. I've been playing around with observation times, and haven't really settled on a scheme. I started off trying for 180 epochs for control points like this, but lately I've reduced that to 60 when I see that the spreads are small. However, when conditions are dicey I often lose patience before I get to 60, especially when I know I'll have other means of verifying the position.
After the RTK observations I went through the points with a total station, using my normal procedures (D&R, careful setups and a pole-mounted miniprism forsight). I was quite surprised by what I found in a couple of instances. I don't like those kinds of surprises, so I'm hoping someone will be able to let me know if I'm dealing with anything unusual, or if this is just the way things are with RTK.
I'll use 3 of the points as examples. All dimensions are in feet.
The first point, 59, is pretty much wide open, and is representative of what I find with clear sites and good SV coverage. I clicked off 60 epochs in 65 seconds, with 5 or 6 engines fixed probably the whole time. RMS stats were good, 0.03' or less; Confidence 3, Consistency 44.95. When I compared the RTK result with my adjusted terrestrial network position (and after accounting for the small datum shifts between the project coordinate system and the RTN coordinate system), the differences were 0.00N, 0.01E and 0.08 elevation. Pretty darn close enough for me.
The next point, 60, is about 8' feet east an 8' chain link fence and a few feet clear of a leafless tree to the northeast. I recorded 64 epochs in 66 seconds, with similar RMS values as at Point 59. Confidence 2, Consistency 14.25. However, the differences with my total station position are 6.36N, 5.69E and 5.32 elevation. Ouch.
The third point, 56, is under some overhanging evergreen limbs (pine and live oak), though it has decent sky view to the south. I recall having trouble getting fixed here, but the stats say that I recorded 155 epochs in 170 seconds, so I guess it wasn't that bad. RMS values were bigger, around 0.1'. Confidence 8, Consistency 118.5. But the differences were 2.58N, 1.91E and 11.23 elevation. Yikes!
So, what should I do differently in order to produce more reliable results, or at least positively identify unreliable results?
Thanks!