Case 1:
I know a surveyor, who does NOT set anything below his base. No Nail.
He does set his base with autonomous coords, Runs Local coords. Runs on "Geodetic at the base".
What he does, is set his base up, and localizes each time out. It is a ONE POINT LOCALIZE.
Now, I can actually see this working.... IF his base is close to the same location each time.
And, it probably works ok, if his base is radially north, or south of each setup. Or, is approximately in the same location.
But, I can see this going to pot, if his new base station is much east or west of last time. Convergence around here is around 55" per mile, EW.
(This scene has so much wrong with it, that it is not worth explaining. If you don't understand what is wrong with this... well nevermind!)
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Case 2:
Not carefully checking to see if there is another base nail, in the vicinity of your base. RTK'ers think alike, ie bald hilltops, good sky view, and good place for radio.
You go and work, come back 2 yrs later, and THINK that's your nail, and it's not. Yours is 6' west of it.
The cure for this is to search carefully, and then to take a check shot, on some previous point.
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Case 3:
Surveyor sets up his nice new 30-50k of gear up, and starts working. Base is set on autonomous point.
Publishes plat.
Plat says Basis of Bearing, True north, via GPS.
Problem is, convergency.
Goes down the road a mile, and does another survey, and does not understand why they are not on the same bearings.
( I see quite a bit of this)
I have seen all three of these scenes.
Nate
Oh the Horrors!!!
Not sure I have all that much trouble with Case 1, as long as he makes the appropriate adjustments. It does seem like losing out on the opportunity to build a long term database of recoveries and ties - the sort of thing that might actually have some value at retirement.
For case 2 - I'd sure like to use something more substantial than a nail. To me, a nail is something I expect to be good on the day I set it, not years later. Not even tomorrow. Set an iron rod with a cap with your name on it. And then check that.
Case 3 is just incompetence.
Case 4 is setting up the base autonomous and not bothering to localize at all. Then setting up the next day, carefully plumbed over the same mark, and running autonomous again. Saw a lot of that in OK.
I do Case #1 all the time, but I also set up each time very close to the first base point, or north/south of it. Typically I can see the scuff marks in the dirt, or the indentations in the pavement.
I have a situation, where I'm pretty sure Case 1 happened. He went, set his base in the near same location. Did a one point localize. While on Geodetic bearings, ground. But, something went wrong. Survey number 2, by same surveyor is some 5 to 5.5' different than survey number 1. Wrong initialize?
This is at present likely. I have more field work to do. And, I may never know for sure. Unless he shared his raw data file with me, which is not too likely. His ability to survey is greatly diminished at present.
But, if you actually understand what is happening, you are not going to get into trouble. Trouble strikes, with a bad init. And, not enough double checks. And, trouble strikes, when the base is moved EW, a large amount, and now convergency gets you onto a different bearing system.
N
Nate The Surveyor, post: 387821, member: 291 wrote: ...5 .5' different than survey number 1. Wrong initialize?
5 feet could simply be the difference between 2 autonomous solutions. Case 4.
Ya know, it is possible.
The purpose of this thread is to educate. Not throw rocks.
Bruce Small, post: 387820, member: 1201 wrote: I do Case #1 all the time, but I also set up each time very close to the first base point, or north/south of it. Typically I can see the scuff marks in the dirt, or the indentations in the pavement.
But why not just set a nail ? It only takes 30 seconds, reduces the chance of errors and provides for repeatability.
Those are ways of looking for trouble. Almost as if they want something to go bad
I can see skipping the base nail, IF you have a javad, and you "reverse shift" it in. Or, m local. And, you leave the base mounted to your truck... And you know what you are doing... Or, if you know what you are doing with any brand... But no 2x checks... Spooky. Ctrl points do get moved... I was a kid once...
See Law of the Seven P's
Proper Prior Planning Prevents Piss Poor Projects.
If one's RTK stereo player is one cycle off (double differences RTK model), one's coordinates shown to him will be perfectly normally distributed, one's, "fixed" vector components will show perfect normal distribution of their deviations from epoch to epoch...
.. but if ambiguity is resolved wrong even by one cycle - it results in decs to meters wrong perfectly normally distributed coordinates.
(do not use RTK in bush)
Nate The Surveyor, post: 387887, member: 291 wrote: I can see skipping the base nail, IF you have a javad, and you "reverse shift" it in. Or, m local. And, you leave the base mounted to your truck... And you know what you are doing... Or, if you know what you are doing with any brand... But no 2x checks... Spooky. Ctrl points do get moved... I was a kid once...
Even sitting on the truck you still need a physical base monument. Actually you need at least two, it's no big deal to do it, so why not?
Biggest mistake with RTK I see around is with more recent converts that are accustomed to doing everything on local plane projections and then pushing those projections out as far as their radio will reach. May work fine for a local site but get out too far and things get warped pretty quick.
It has always been my practice to check into a couple of the previous day's points. Using GPS works the same as any instrument. Good practice creates good works. The error should have been caught in the office at least and corrected by another trip to the field if necessary.
It's nice to have an RTK CORS network without worrying about these base/rover issues, but I worry about continental drift. I assume I will someday I'll have to apply a vector shift? Am I the only one that worries about this?
Mark O, post: 387993, member: 11591 wrote: It's nice to have an RTK CORS network without worrying about these base/rover issues, but I worry about continental drift. I assume I will someday I'll have to apply a vector shift? Am I the only one that worries about this?
I set Bar & Cap Control Stations (RTK Base), or use USGS/NGS/PLSS, whatever Monuments, so I don't have to worry about such things.
Loyal
Mark O, post: 387993, member: 11591 wrote: I assume I will someday I'll have to apply a vector shift? Am I the only one that worries about this?
You might look into attending one of the NGS programs presented around the country...
[MEDIA=youtube]oPGsyEJmkv8[/MEDIA]
Got me today. Staking a section line easement, running rover in the open and pulling our stakes into heavy dense brush, very difficult to look back on previous stakes to make sure things are lining up. Bad initialization I guess, 3 in a row out by 4-6'. DC says fix is good. It lied. Checked in to control before and after and hit spot on. Check and then check again. Murphy and the ghost in the machine have my number.
My experience with RTK, (non javad) was that there were 2 times in the day that it would lie. As it transition from a POOR DOP, to a GOOD time, and when transitioning from GOOD to POOR. During these transition times, it would consistently lie.
N
