I have not come across this practice before but it seems to be popular at my place of work.
at start of survey take a shot on benchmark.
do survey
takes shot on bench mark at end of survey.
adjust all survey Z values so that the benchmark shots come in closer to the benchmark Z value.
What are your thoughts?
To be clear - all of the benchmarks are hard levelled (optically). We are using a network RTK with fluctuation in Z about plus/minus 15mm.
The practice is an attempt to reduce the effect of this fluctuation.
How fast does it fluctuate? If it were a smooth change over the duration of the job and the adjustment was made according to the time the shot was taken, this would be a great technique. If it typically wanders back and forth several times during the job, then you have improved only by averaging two bench mark shots instead of one, and most of the other shots still have the fluctuation.
We set the base onsite when needing decent elevations for tight verticals. The base sits on a benchmark, I don't imagine I would use a network rover for that kind of work. And when things need to be really tight like curb and gutter, blue tops, sewer MH's ect., then it's the instrument and or level.
If the offset is the same direction, it MIGHT be O.K. to do that. Meaning if you checked into that benchmark with your network RTK and it was always a 0.10' low. As a rule, it sounds like a good way to make mistakes. Suppose you got out to do some staking later?
A soap-box of mine - unless you are working in Earth-Centered Earth-Fixed coordinates a bench mark will not have a "Z" value. Orthometric heights are referenced as H. There is no room for colloquialisms in precision surveying. -- Reference "Use H not Z" from Professional Surveyor magazine
base9geodesy, post: 425769, member: 7189 wrote: A soap-box of mine - unless you are working in Earth-Centered Earth-Fixed coordinates a bench mark will not have a "Z" value. Orthometric heights are referenced as H. There is no room for colloquialisms in precision surveying. -- Reference "Use H not Z" from Professional Surveyor magazine
Noted!
MightyMoe, post: 425663, member: 700 wrote: We set the base onsite when needing decent elevations for tight verticals. The base sits on a benchmark, I don't imagine I would use a network rover for that kind of work. And when things need to be really tight like curb and gutter, blue tops, sewer MH's ect., then it's the instrument and or level.
This is my feeling. If more accuracy is needed use a short baseline base or optical survey. We are told to work with what we've got and the above method passes for a pro tip apparently.
