Establishing horizontal and vertical control

big-al · 2012-02-13T16:28:16Z

In the absence of an explicit standard for a job that requires obtaining horizontal and/or vertical control from known benchmarks in the vicinity, and where there is freedom on the part of the surveyor to choose how he establishes control on the project, what is the best/most expedient way to establish horizontal and vertical control? Any good how to manuals on the subject?In my area, CORS stations are typically at 25 miles or more from a project. A single dual frequency GPS receiver set up for 2 to 4 hours, in this area, with a good view of satellite vehicles, will, by my experience, generate (by OPUS or other post processing software) solutions, constrained by CORS stations, in the vicinity of 2-3 cm horizontal and 6-10 cm vertical.I'm wondering how that could be improved upon. I am aware that longer occupation times will theoretically improve the results, so that is one option to consider. However, will using additional dual frequency receivers in the vicinity of the project theoretically improve upon those results?I understand that there is a limitation on the quality of the vertical solution based on the current geoid model in use which is still to be improved upon based on gravimetric modeling currently underway, so the vertical solution (i.e. orthometric height) will have its limitations. That is, no amount of additional observation time will eliminate the present errors associated with the geoid model.What about the use of passive benchmarks that lie closer to the project than the CORS stations? In my area, there are passive control marks, typically 3rd order benchmarks (both horizontal NAD27 and vertical NGVD29) that were set in the middle part of last century, which have published coordinates to the hundredth of a foot, and which are often located within a few miles of a project. Some of these have since been destroyed, but some remain. These marks are often far enough away from the project that locating them directly by traditional leveling and/or traversing methods would prove time consuming and expensive, but it could theoretically be done. These passive benchmarks were typically set during a time period when GPS did not exist, and so often are not ideal locations for the setup of a GPS receiver. And, there remains the fact that the published coordinates for these marks also contain errors that I suspect will exceed the errors obtainable by GPS methods. Nonetheless, one method I can envision is to recover one or two of these benchmarks in the vicinity of the project, and to occupy with dual frequency receivers a couple of nearby points (in the vicinity of each benchmark) where there is a good view of the sky, and to use these GPS observations to constrain the project control by the passive benchmarks (local level and/or traverse loops would need to be run in order to tie the GPS observation points to the benchmarks). Does this have merit? What if there happens to be 2nd or even 1st order passive benchmarks closer to the project? For example, there is a 1st order vertical benchmark at a distance of about 10 miles from my typical project. It's on the wing wall of a bridge, and so won't be able to occupy directly with GPS receiver. Could I use the published elevation of this point to improve upon the solution at my project?I know this is a rather rambling post, but I would appreciate any thoughts or comments. I am still learning about GPS, so please take it easy on me.

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Norman_Oklahoma

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Along with Bill Hennings webinars you should check out Bill Hennings very informative booklet on establishing control with single base RTK:
Single Base RTK

As far as tying to the NSRS vertically, I'm usually just as confident in the average of 2 or 3 long OPUS sessions as I am in levelling some miles from a piece of brass that hasn't been retied to the NSRS in my lifetime.

Posted : February 13, 2012 10:16 pm

big-al

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Yes, those guidelines were referenced in the webinar. Another excellent resource.

Posted : February 13, 2012 10:34 pm

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Establishing horizontal and vertical control

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