Does anybody have advise to a way to weed through inconsistent shots? Our field crew goes out on Monday, shoots elevations on control stations set 10 years ago, (that have been checked-out to have good NAVD88 elevations. They take 50 shots on each station using RTK, come in with the info and one might be within a few hundredths, the other may be 0.10' high, the last one might be 0.26' high. A week later, go back to the same control stations, again, take 50 shots on each one and be a tenth on the first, 0.08' on the second, 0.28' on the last. Are we doomed to these errors or are we heading about it wrong? Any suggestion will be welcome!
This Content Originally Published by a land surveyor to Land Surveyors United Network
With the results your stating, I am going to assume you are using a VRS RTK setup? If so, that's the results I would expect. If that is the case, one of the things we will have our guys/gals do is to shoot them in when set and again later in the day. It will show you the problem points and give you corrections you can try to average out. However, if you are getting 0.24' between the 2 readings, you are hoping that the average is closer to true than one of the readings.
Another thing with VRS is weather. Your reference station could be 15 miles away. If it, or you, are in weather, you should not proceed with control.
I have instructed our crews, if relying on multiple point VRS origination, they had better run some levels through or Rapid Static at each. I have instructed most crews, when originatng off VRS, do a single Base pt. Then go to local RTK to set the pair point or continue setting perimeter control. I handle equipment for about 50 crews in 30+ offices and this is something I have really had to push. VRS will never be as accurate as Local RTK...the vectors can be huge, the signal path can be riddled with environmentals, multipath is increased, etc. The big complaint with local base is having to setup a base and break it down. I can setup my base, level the fixed legs, and start it in under 5 minutes. I can break it down and put away in about 2. Let's call it 10 minutes total with drive time. Now, how much time has been spent worrying about vertical, questioning work, etc. Not saying this to you bc I don't know your situation but just want to put it out there. All it takes is that one mistake and all that "saved" time and money is gone!
If this is happening with Local RTK, base and rover, it could very well be environmental or the receivers could be out of phase center. I wouldn't be shocked to see 0.10' with local RTK but if you are getting 3", it is either procedure, environmental, or an equipment issue. You have to ask yourself some questions about the environment the recievers are in...near treelines, tall buildings, large bodies of water? Additionally, are you getting ground or grid elevations and are you sure you are sure?!?! Are you working off a localized plane or geoid?!?! These are all things to look into. If I get some more info about your particular situation, I may be able to help more!
Oooohh Myyyy - the young ones today are sooo spoiled!!! Your comment "The big complaint with local base is having to setup a base and break it down." is hilarious!! Can you imagine these same crews had to carry a 20 pound gun and 8 pound battery for a couple miles and doing a setup every 2000 feet back in the day?
I carried a Geodimeter in the case on my back, a five pound battery, my wooden tripod, CPSs, a 4# sledge, flagging, markers and my radio...more times than not in the woods, rolling terrain. Carried lunch and a canteen for water, too. Set ups were approximately 700-800' apart....all day long.
I hear you Kevin Allen!!
One quick response, Gary....then I'm off to go to sleep. Several years ago, I performed a retracement survey by myself...rugged and timbered, along township and range lines, sections 35 and 36. I used GPS one morning and had just set up the base station when a four wheeler, hauling a gentleman with two little girls (daddy and daughters) rolled up and stared at the yellow legs, plane and antennae. First time for them to see such a contraption. The father asked what I was doing and I replied, "Contacting the Mother Ship." They left in a hurry, never to be seen again.
Regarding RTK elevation data collection. How many satellites are you using? Are you shooting between say 10 a.m. and 3 p.m.? Are you using a bipod/tripod. Have your tried a static shot? Are trees to the south? Is your mast set correctly? Is your measure up correct? These are a few questions I'd ask before Monday morning's work. .... and of course, the higher the shot count per station, the more error you're throwing in.
AHHH Daniel.... You and so many others ask the same thing from time to time. There are several components to your answer starting with the fact that GPS in inherently less accurate vertically than horizontally. "But why??" is the question that comes back to me. First, look at the GPS constellation, 100% of your "Base points" ie satellites are above you or all on one side of your observations. If you were to conduct a ground traverse and 100% of you observations were always to your left, then you would have a much less accurate closure. Same with GPS as there are no satellites "below" you to balance the ones "above". Old school strength of figure here. Second, for all practical purposes, every GPS equipment manufacturers specs state the the vertical accuracy is twice that of the horizontal. Take the classic Trimble R8 10mm H. vs 20 mm V. is as good as you can get with that receiver. Third, the variations in the orbits from day to day will add to that if you do not "check-in" to a valid point, one that you will hold over all others. When doing this type of RTK work you always need to calibrate to a known point(s) and adjust to them. This will go a long way to reducing the day to day variations you are describing.
As far as weeding thru your shots goes, try setting your data collector to a precision condition instead of recording a stream of points. I set this function and let to point take as much time as it needs ( within reason), this way you are "weeding" your points as you collect them.
Hope this helps
Those differences in elevations (0.10' +/-) are consistent with RTK. If you want consistence, set a pair of points with the RTK, then observe all the critical points with a Robot, or conventional instrument. These results will be within 0.01 to 0.02', or should be.
Various DOT's (Dept. of Trans.) refuse to use RTK processes for this very reason. Lack of consistency.