I thought I would post over here after reading the Radial GPS thread in the main forum particularly because it appears the Mike Margolis with Ashtech did not think there was sufficient accuracy, or confidence in your results to do a radial RTK boundary without multiple shots. We do it all the time and now that I see people stating we should not I am wondering what kind of errors are you talking. I am going to say that 80% of the shots we check are easily within .07'. We get the occassional 0.1' and I have seen on occassion up to 0.2'. On a boundary survey of any significant size even the 0.2' pretty insignificant and in a several mile conventional survey you are looking at that type of error unless your closures are in the 1:50,000 range so I would consider that acceptable in most cases especially since it seems to be an aberration when it happens. I have seen significant errors when we have tried to force readings around trees, so we generally do not try if the unit does not want to keep a fix. I work in West Texas so that is generally not a concern. I am especially asking the Ashtech guys. All my experience shows me the accuracy is there. Is it not? I am asking your your opinion about RTK equipment in general, not Ashtech. Now I have seen errors brought about by Localizing (Using TDS) on bad control, especially vertical But that aside I have not seen any what I would consider critical horizontal errors when we were not doing something wrong. I know you get better results static, but I think you can get the equivalent of 1:50,000 or better rather easily. Believe it or not Texas only requires 1:5000 for rural (if they haven't changed it) I am always tickled pink with 1:20,000 when doing conventional surveys and think RTK gives me that and usually much better. Let me put it this way. If all my points are with 0.1' relative to each other except 1 out of 100 being off 0.2'. I don't think I can do that conventional except on very small surveys. Thanks for your opinions.
Modified By Jon Cieszinski on 2/28/2001 at 9:04 PM

I am not sure I did misunderstand you Mike. I guess My question is when are checks overkill.(I realize you can never make too many checks, but) You say we are 99.9% confident that the answers are accurate. Well how confident are we at the 99.999% level. We quite often do RTK boundary surveys where we don't make redundant measurements to most points. I may dump the rover and check an occassional point again. The fact is we do not find significant errors. We do a lot of subdivision stakeout which we always check the dimensions between points and usually everything is a tenth or less but a tenth is common. But why do we need redundant checks if everything checks. Everybody seems to think we need the checks. In some ways I see less need for them in RTK than in conventional. I have seen a lot of errors in RTK and most of them have been caused by people doing something wrong. In fact if we are having problems almost inevitably we shot a point wrong, entered a number wrong or have a bad cable or something like that. A chainman can drop a foot that never gets found. I am not finding any errors that tell me I need to make redundant checks in my RTK data. Aren't there redundant checks already made when you have more than 5 sattelites?? What kind of errors am I looking at if I don't make redundant checks. If the error is insignificant or is small enough that it is does not affect the overall job, why do I need to make redundant checks? What magnitude of errors may I be missing if I don't make reduntant checks. If the magnitude of the possible error is less than the requirements of the project then why do they need to be made? I am not finding problems. Why do we need to create problems where they do not exist. I worked with people years ago that did not trust EDMs. Does that mean you need to tape every measurement you make with a EDM to be confident with it?

Jon, what I believe the concerns are have to do more with a major glitch than with the precision or accidental error. RTK has always been subject to a bad initialization, which means it got the integer count wrong. That usually results in a misposition of a couple of feet. It doesn't happen often, and the smarter the manufacturers get the more they can ensure against the bad init, but in the unlikely event you do have one...a single shot will not reveal it. Maybe I'm just too old fashioned, or maybe plain old old, but let me suggest this: ask your supplier or other question-answering guy if it's possible to have a bad initialization with your setup. If he says yes, and if you keep doing singles, you're a braver man than I am, Gunga Din. BTW, how long does it take to dump the antenna and shoot it again? Just wondering.

I have never timed it, and it varies but I would guess when things are going really well and you have 7 or more sattelites, a couple of minutes, when they are not going so well and you only have 5 or 6 satellites, it can take a lot longer. When they are not going well is probably I guess when you should really be getting those reduntant shots because that is when you may feel a little less confident with your data. But I guess your point is is it worth it to dump the rover and get another reading just for peace of mind. I guess the better solution if all points were easy to get to is shoot all the points , dump the rover then shoot them all again, but you know if you have been doing this very long, it seems you are always running out of time or daylight when you are doing boundary surveys, just seems to go with the territory.

We recently had our first extended session with the new ZXtremes, and compared with the previous model (Z-Surveyor) init and reinit times are cut by factors of 10...By the time I set up the base, hook up the Ranger and load a job the danged thing is at a fixed solution. If I walk into the trees and lose to a float, walking out of the canopy problem and check my watch, I look up and it's fixed already before I can tell the time.....Caveats: 6 sats or more, pdops of 4 and short baselines (wihin reach of my 2 watt radios). I have not tried extended length baselines yet and cannot tell you the reinit times yet on those...bob in maine