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tricks for tighter control?
i am still very green, 2 years surveying. i really like boundary and topo surveys, but lately ive been doing more construction. most of the time i get search points from the office of previous surveyed control points, go to the job, try to find a couple based on desc, scaling…then occupy one and back sight the other, try to angle check another. if it under a tenth, i roll.
however, i am partial to setting up jobs using VRS GNSS. i have GPS+GLONASS subscription so i get a lot of satellites. and when you can set up and calibrate a job without traversing all day its nice. BUT….everytime i do this, the next time im out on the job to stake something, and i set up the total station, my setup deltas are sometime more than a tenth off which is no good for staking gravity flo pipe, curb, grids….or at the very least it bugs me. i can get under a hun with a gun setup, but GPS setup jobs are way less accurate. i typically occupy the point for a full 3 min obs. lately i will run back through the points and re observe them and avg the obs together, which seems to help.
any tricks or comments about setting up a job with GPS???
thanks
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6 Replies
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Have you tried raising your Elevation Mask to reduce the satellites at the edge of the horizon? I usually set mine to 15-20, it gets rid of the satellites that are “noise” that affect your residuals. Also, of course, check your rods/tribrachs for plumb and prism constants.
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Another consideration is time spread between the shots you average. An hour between shots should give enough time for the satellite constellation to sufficiently change. Otherwise youƒ??re just averaging shots off the same setup, basically.
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There are no tricks to getting tighter control, you do it right and do not expect anything else to help you skate by.
Paul in PA
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Gps is a statistical measuring tool. Measuring FROM moving objects in space, (satellites) to spots on an earth, that is wobbling, and spinning. The measurement signals, are passing through a liquid/gaseous substance, called earth’s atmosphere. Which is not the same, at all locations.
For this discussion, we’re going to leave out bad inits. (These are blown gps shots, that are wrong, by 0.5′ to 15′, typically)
If you take a 3 minute observation, it’s giving you “the truth, as best it can, within that pile of data”.
If you come back, and re-observe, it will give you the best it can, for that data set. And so forth. You can average several observations. This compiles data, from differing times.
Assuming there is nothing interfering, eash additional observation, statisticaly improves the data. It can really yield tight data, over long distances.
TBC…. To be continued…
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First, let me say that much better results that a tenth can be achieved. Errors of maybe half or a third of that are very achievable without heroic procedures.
It could be a great many things. For a good background please download, read and understand the following NGS document on using RTK – User Guidlines For Single Base Real Time GNSS Positioning. The guidelines apply to RTN work as well.
Some other likely culprits, in something of an order of likelihood, to look into:
- Adjustment of your rods and tribrachs for plumb & centering, already suggested by others.
- Prism constant misapplied. Many prisms have a constant of 30mm, which is very nearly 0.10′.
- Mixup of International foot and US foot settings in you dc. Errors from this source are often on the 1-2 foot magnitude, but it depends on the magnitude of the coordinates.
- Issues with State Plane grid setting vs. local coordinate systems. If you are doing your GPS with the settings on grid, and your total station is using those coordinates as local, the magnitude of the errors easily be in the area of 0.1′. The converse is also possible. If the GPS derived grid coordinates have been scaled to ground, and you are using them as grid coordinates, the errors might be in the 0.1′ area.
- When collecting RTK points – using GNSS – your PDOP should be under 2. If not you can occupy to until the cows come home and you won’t get optimum results. If you were using GPS exclusively (ie/no GLONASS) then a PDOP of under 2 would be a rare thing. It is possible in a GNSS receiver to turn off the collection of GLONASS , GALILEO, etc. constellations. If so, your PDOP would likely be above 3, and the precision would be significantly degraded.
There are, of course, dozens of other possibilities. Best of luck. Be sure to let us know if you find anything.
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As Nate alluded, there are multiple sources of error. Here’s my intuitive version of what’s going on.
The best relative position vector between nearby points is obtained with receivers simultaneously working on two (or more) of your project points and receiving the same satellites, so that many of the errors are nearly the same for those points and the relative position vector between them subtracts out most of those errors. This particularly applies to iono/tropo propagation errors and satellite clock error.
If you use one receiver and a network, the solution is based on the difference between your receiver’s data and simultaneous data from one or more stations that are probably a lot further away than the size of your project. That distance means the errors are less alike and subtracting doesn’t get as good a result between project points.
If you then move to a different project point with your one receiver, the errors will probably have changed in the elapsed time, so that vector will have different errors than the first one, and the vector between your two points has more error than either of the vectors from the network stations.
If you are limited to one receiver, then the best you can do is to do multiple sessions on each point, and longer sessions, with enough time between to get independent errors so that averaging brings each one of them closer to the truth.With too little time, errors due to multipath and some other sources will still have some resemblance between sessions, but that won’t be the same for different project points.
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