So the other day I was in Brooklyn, NYC on a construction site doing some topo for an environmental remediation/marine construction firm...loose stuff just for general grades on gravel in the snow, etc...they were just looking for a quick two foot contour map to see where they stood with flow. I had clear skies 2/3 of the way around centered mainly on the southeast. The northwest was a 30ish foot conc. wall.
So I whip out the ol' Leica GS15 with my CS15 controller and go to work. I'm on the NYSNET CORS network maintained by NYS DOT...there are 5 or 6 coors stations within 10k of me. I was around 1k from the nearest. In general my DC is reading 0.03' to 0.05' in vertical accuracy. More than good enough. And before anyone chimes in about not trusting the DC, we've compared those readings to total station readings so many times over the past three years. We have rarely found more than 0.02ish' diff in horizontal and 0.04ish' diff in vertical to what the GPS reads in real time. Sometimes a bit more but rarely.
So I finish everything except along the wall and I decide to give it a shot before I pull the total station out. Right up against the wall I was getting vertical accuracy readings of 0.10 to 0.15. This was good enough for the task at hand but I was worried about multipath and the accuracy of these readings so I decide to pull out the total station anyways. Guess what...pretty much dead on. Results where +/- 0.1 with the GPS readings.
I fully understand that this could have been a fluke and I try to avoid multipath situations but I'm just sayin' 😛
Tom
> So the other day I was ... doing some topo ... for a quick two foot contour map to see where they stood with flow. ..... getting vertical accuracy readings of 0.10 to 0.15.
For that kind of precision, and if you keep a really sharp eye on things, yes, it will work. And work pretty well. But the minute you let your guard down its going to bite you on the a$$.
I am the licensed person who's seal is on the map, so I try to keep a pretty good eye on that stuff!
It's all good, until you get burned.
Been working on a job all week that should have gone a lot smoother. Reusing another guys work done in '02 to get going and save some time. Just my luck the one monument I found and set my base on to get going was out by 8.75' because of a bad RTK shot. Fortunately it was just an old DOT control point and didn't really control any boundaries, but sure had me scratching my head. So much for saving time.
Again, it all comes down to knowing when to and when not to use the tools in our box based on the task at hand. I knew what they where using it for and what my acceptable tolerance was. I would not set control from something like that but the data met the requirements of the task. I checked it then and I will check it again in the future but it's good to know the capabilities and limitations of your equipment.
It is good that you are checking your work under different constellation. If you don't, one day they will show up and haul you away.
I have an antique clock on the wall in my office that does not run, but it is dead accurate twice a day, and even though it hasn't ticked in 50 years or more, it provides the time within 3 hours 50% of the time, and adjusts to daylight savings time automatically.
I like that and it's always good to remember. I put a lot of faith in technology, probably more than some, maybe it has to do with age, but I have had a computer since before I can remember and own half a dozen right now. My grandfather has been a computer scientist/engineer since the early 50's and has been a part of teams that advanced computing quite a bit. It's kind of ingrained in me. That said, I also have my feet firmly on the ground. I like to test equipment (and software, and employees) to learn limits. Once you know the limits you can make educated decisions on what/when to accept liability and what/when not to. I'm not afraid to check something a different way if I'm not comfortable with a result.
Good post!
fielsdwork, it's all about checking and knowing the limits of your tools.
:good:
Christof.
+1
Good on you for checking.
With my equipment I watch those RMS values creep up and I know my results are less trustworthy. I feel confident that when I'm seeing .01-.02 horizontal and .02-.04 vertical, I'm very likely doing fine. If I start seeing 0.10 horizontal and 0.2-.03 vertical, I know it's probably not working, may have a bad fix, need to reinitialize, or return under better constellation or get the gun out. Like Christ said, it's about knowing your tools. If you can repeatedly see that your RMS (or equivalent statistical error estimate) generally matches the actual differences you see, then that's great. For me(with my equipment and own experience) it's more likely to simply tell me something is amiss.
> Once you know the limits you can make educated decisions on what/when to accept liability and what/when not to. .
Unfortunately we are liable ALL the time, and on everything.
Exactly. Thats why you check with other methods until you are confident in the tool.
