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Oct 04 2011

Hydro Survey: GPS? Who Needs That?

Guest Post by Joe Robinson, Hydro Survey Crew Chief

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If you have spent some time in the field of modern day surveying, you probably know one of the key limitations with using GPS – Obstructions overhead that might considerably reduce otherwise available satellite reception. GPS currently works by means of line of sight connections between your field location and the many satellites that are out there in orbit. If you lose that line of sight, you’re simply out of luck. The technology is certainly progressing, but in today’s world of GPS surveying, line of site and a full sky of satellites is definitely the ideal blend for maximum accuracy and efficiency in the field. So what do you do if you have a situation where your job site is located near a dam, or tight canyon, or even under a bridge? For a traditional ground survey, it’s pretty simple – you can use readily available alternative methods, such as a standard total station, or even advanced methods such as a 3D Laser Scanner. But what do you do when it’s a hydrographic survey? One method that you might think about trying is the use of a Robotic Total Station (TPS) in conjunction with hydrographic data collection software to tackle these hard to reach places. This little modern day gizmo definitely has some amazing field capabilities, and it is possible to incorporate the use of TPS into a more challenging hydrographic survey environment.

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Some time back, we ran into a situation where this combination of tools proved to be very effective. The project that involved obtaining bathymetry on the Sacramento River at a design sight located directly adjacent to an overhead interstate crossing. To correctly model the effect of our designed bank protection at the site, we needed the underwater data under the bridge. We had already dealt with issues involving consequential gaps in collected survey data at another site location further downstream, and knew that we simply could not get around it here. A portion of the site extended directly under the interstate crossing, which greatly reduced satellite reception. We needed a more complete and current bathymetric dataset, and were able to accomplish this task by the use of the Robotic TPS.

Once we discovered how to effectively use the robotic for hydrographic surveys in obstructed areas, the applications grew from there: at the base of a sizeable dam, other bridge sites, including a bridge within a tight mountain canyon, and even sites where we were able to fill in areas that fell under tree canopy where, before, GPS methods would simply cut us off from acquiring valuable data.

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Using the robotic total station can sometimes take a bit more patience in the field due to its primary limitation: the likelihood of multiple setups to obtain a complete dataset. You really have to get out there and look at your site carefully, plan out survey control locations to try and maximize efficiency, and limit the amount of setups needed. Areas where we’ve completed hydrographic surveys using this method are generally places where wading can be impossible or extremely dangerous, and using a more conventional total station would take significantly longer due to the lack of automation. The robotic total station has become an indispensable tool and an excellent alternative means for conducting hydrographic surveys where thinking outside the box becomes a must.

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