Besides Dead batteries, and mechanical failures,
This thread pertains to accuracy.
Kids move your base nail a few feet. OR somebody else set a BASE nail on the SAME hill, and you are not finding YOUR nail. This is for multiple days, on the same base site, maybe even several months, or years apart. (check to something)
You are staking a corner. Mild woods. You get within 10 ft of the corner, with a float, and set it up, and wait for a fix. It says xxx brg and dist. You use your compass, and box tape to move the 6 feet to the corner, and then move your gps system to the corner, and it says FIXED, move 0.10' and you set your corner, and you ASSUME you are good. BUT you did not loose lock between the 2 observations. They were BOTH wrong by the same amount, and the same direction. So, you carried your bad initialization between the shots, and set your corner wrong by the amount of error, at the 1st fixed observation.
You set your base station up near a metal roof barn. Multi path is coming from that roof. Your corrections are gonna bounce around all day. Search carefully so that you do NOT set your base in a multipath environment.
If you need to observe with your rover, in a bad environment, for multipath, then walk AWAY from the bad environment, to CLEAR sky, re-initilize, and walk back to your needed point, in the bad environment, CARRYING your fix. This usually gets it right, but the LOCAL accuracy is poor. That is, it is not wrong by 6', but only wrong by 1/2 a foot. I am thinking about roof corners here.
When it goes FIXED, usually your 1st fix is bad, because it has not had time to update the computer. Wait for a later fix. IF it goes float, the whole shootin match is subject to question.
The DOD Sats that we as surveyors use, orbit the earth about 2x a day. So, there are many combinations possible. If you observe once, and wait 6 hrs, then you are observing with new sats. By doing separate observations, 4-6 hrs apart, then you can develop quasi "Closed Loops" for your observations. This would be good, for extremely important shots. Even waiting 1 hr between shots, develops a good check, but 4-6 hrs is better.
OK, tell me yours.
Nate
Once saw a shot on a control point in a plant that was off 0.5' in the vertical because someone else in the plant had a voice radio that was overriding the RTK radio - but not for long enough for the data collector to notify the user, just long enough to have latency in the corrections. No telling how many other shots were taken under similar circumstances and with similar results.
GPS is [sarcasm]neeeeeevvvvvvverr wrong[/sarcasm].
That being said, we are on a network now, so the base corrections come via cell phone. I was told that if a weather front is moving thru it can play games with the atmosphere so for critical shots, it may be best to wait.
Joe - Here in Louisiana it often happens that the weather is drastically different at your location than it is at the nearest network station(s), and that can definitely cause a tropospheric bias. As you said, best to wait if you can.
GPS is one of the coolest tools I've seen come along in my career. It is also the most abused. As we continue to demand things the equipment cannot do, manufacturers are selling us versions that appear to work magic. The plain and simple truth is getting lock more often likely means an exponential increase in bad inits. The surest way to avoid this is to read a book written by someone OTHER THAN THE SALES TEAM. My 02 Tom...
Localizations can really bite. I avoid them like the plague. But sometimes a necessary evil. Set up base with autonomous derived coordinates, localize to known point(s) with rover. Store base point after localization (now related to known coordinates). Come back tomorrow, with localization still applied, set base on same point, now using position based on known coordinates. Everything is off by the localization. It didn't bite me (I didn't mess up any work), but it sure took an afternoon of head scratching to figure out why I was off a few feet with a fixed solution. The localization was correcting something that had already been corrected. I just needed to turn the localization off. Check method - if I don't have a check point nearby, I stake out the base station point, get within three or four feet to the base station point and use the tape to see if the stake out distance is proper. I also evaluate the cut/fill, does it look right? Not a precise check obviously, but definitely a good smell test.
I have never had it happen but I often wonder about if you accidently connected to someone elses's base and didn't realize it. Obviously you would have to be on same frequency and had same base code or be set to any so it is not likely but is possible.
I was working in an area a few weeks ago and saw 4 bases's set up within radio range of where I was working.
I have a robot and a 4'+ pole. I HAD to get a shot on an iron rod under a tree limb of a triple something or other that is about 3.5' off the ground. What did I do? I pulled out my trusty plumb bob and mini prism and got that shot under that limb. As I walked away, I actually thought to myself what many surveyors would do in that same situation... especially solo.
Carl
I send my instrument man out with specific instructions on where to set the base and what to tie. 'Make sure you check in to something before you get going', I tell him. We have tons of control in the area so this shouldn't be a big deal. I get a call late in the afternoon after he's taken some 200 shots. 'Something's wrong, I just checked in and I'm missing by 13'!!! What do I do?' (The sound of panic in his voice). Long pause. 'You didn't check in before getting started like I asked you to, did you?', I asked. Nooo. I told him to go back to the base and measure from the point he was set up on the bearing and distance he was missing the control by. Five minutes later I get a call back,' He'd set up on the wrong mag nail. BTW, he is no longer with me and is someone else's problem now.
My horror story didn't cost me any money, but the client time. Inclined plane.
I had 5 bench marks on a straight road about 400 ft from my site, the site was another 400 ft wide. I did calibrate horizontally to the found corners and all was good. Like a dummy I calibrated vertically to those 5 BM's.
When the out of town surveyor goes to stake it using his total station, he notes a vertical bust ranging from 2 ft on the road side to 4 ft across the site in plane like manner, perpendicular to the row of BM's. Thus the term "inclined plane", as the world tips.
Basically, they had to re-design all the grading. It was about a 5 acre site, so that went quick. I just went "whew"
I was kind of new in the RTK world then and learned quickly not to do that again. If calibrating use a one point calibration and just check the others, and then run a box around my site with my robot for construction control.
Inclined Plane. Um hm. That could be a big one!
N
I was fortunate enough to watch someone else make that mistake. It taught me to read the entire localization report and learn what each item meant. I will never forget 'slope north 0.04'per mile slope east 240' per mile'...
We had an incident just the other day where a control point set by gps was off by about 40mm horizontally and 110mm vertical.
Yet the quality of position statistics suggested an expected error of 7mm and 13mm respectively.
The site, a small earthquake rebuild, was tight, the point was being set so we could use a total station for the survey. But the control point had good sky view, 8 sats were observed, a three minute occupation time was used.
We caught the error because we ALWAYS double check our control points - sometimes by a re-occupation or in this case by the total station backsight shot showing up the height difference. The other backsight control point was fine
It occured on the second day they had issues with the Glonass satelite ephemerides. We think we must have been tracking just the russians at the time.
I generally like and trust our equipment, but is easy to get complacent. Even the very best seems to throw up a wrong number every now and then.
> I have never had it happen but I often wonder about if you accidently connected to someone elses's base and didn't realize it.
I have had that happen. Both were based on State Plane, and pretty close coordinate wise, not enough to be "bad" but just different enough to have us wondering if something was up.
I passed another surveyors GPS last wk, on the side of the hwy. Near a store. GPS was in front of a volunteer fire dpt.
When I passed it again later, there was an 18 wheeler, parked right beside the RTK base. I am sure that those came, and went several times that day. And, that multipath from the big trucks dithered his RTK corrections.
IF we are going to use, and trust RTK gps, we should KNOW all we can about what can and does go wrong.
Nate
Nate ...you're wrong.
Those ob-stacles simply funneled more signal to the base. Therefore, the base was more accurate in its location.
😉 😉
Them **** Pterodactyls, getcha every time!!
N
Nate ...you're wrong.
Yeah, and so now I put sheet metal up around mine!!
🙂
> I passed another surveyors GPS last wk, ... there was an 18 wheeler, parked right beside the RTK base. .. that multipath from the big trucks dithered his RTK corrections.
It doesn't have to be parked. If you set up your base right beside the road you get that multipath effect from every truck that passes by, and one may be passing by at the moment you hit "record". I am working hard to convince our field crews to set the base well away from the road.
> > I am working hard to convince our field crews to set the base well away from the road.
[sarcasm]And WALK??[/sarcasm]
DDSM:-)
