GNSS in Canopy

shawn-billings · 2017-02-13T21:59:00Z

A few years ago, I was afraid that some of the "art" of surveying would be lost to new technical developments. I was concerned that there wouldn't be much required skill in operating a GPS receiver. It's pretty binary - fix or float, and there doesn't seem to be much a user can do to affect that. It's not like operating an optical instrument which seems to have a more synergistic exchange between machine and man. With optical equipment, you absolutely cannot ignore the significance of the quality of the instrument, but you also cannot discount the importance of a skilled operator behind it. Which is more important? Debatable.But with GNSS, and RTK in particular, many have come to understand that the magic black box provides a position. Most of the time, it's a good position, sometimes it is not, but because it's magic in a black box, the user is separated from the result.In some places this new era is acceptable. In open environments, procedures don't matter very much. Observation duration and age of the fix can be ignored and the magic black box can provide reliable positions.Once the user presses the GNSS receivers into more hostile environments - under trees, near buildings, etc., good results are not a product of magic but of synergy. Is a good receiver more important than a knowledgeable operator or vice versa? Debatable. What could be said though is that the results will only be as good as the weakest link, whether that weak link is the receiver or the operator.I've been using RTK under some incredibly dense canopy.[USER=640]@John Hamilton[/USER] mentions in another thread that the fix (or convergence, I suppose, for an R-10) for a point under canopy dissipated after less than a minute. This is a good sign that the fix is bad. As the satellites move in their orbits and the multipath changes, it becomes less possible for a receiver to maintain a bad fix as the distance-distance intersections no longer intersect where they once did. This actually works to the operator's benefit as a long lasting fix can be evidence of a likely good fix. Even better is repeating the fix after a few minutes. I cannot speak to all receivers, but my own experience with my own receivers suggests that a bad fix cannot be repeated after 180 seconds. This means that if I have two fixes separated in time by 180 seconds that the fix is likely good. You mileage may (will) vary depending on the receiver being used.What doesn't work is a 10 second observation from a single fix. In the open, such procedures may produce acceptable results. In the woods, a different approach must be taken. Post-processing base-rover vectors and comparing the results to real-time results is also very useful. Often the RTK processors will evaluate data differently from the engine in the post-processing software. When the two agree, the likelihood that the results are good increases dramatically.If I were to sum it up, I'd say that there is as much need for the "art" of fieldwork when working with RTK as there ever was with an optical instrument, even if it is in an entirely different way. The profession will increasingly require professionals that understand how to deploy RTK in places previously thought impossible. Demands on quality will require the results to be good while market competition will require that surveys are completed more efficiently. Professionals that can deliver both quality and speed will, in my opinion, be in the best position for the future.

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GNSS & Geodesy

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nate-the-surveyor

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Its not just taking GPS shots in those photos. It is ALSO quantifying them. ie, a pretty accurate error ellipse to boot. Stuff surveyors love.

Posted : February 18, 2017 8:52 am

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GNSS in Canopy

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