GPS is a great technology, but the bottom line is, if you are looking for the best level of accuracy (cm), then you cannot operate in trees (period).  There is no GPS equipment or technology that will allow you to do that.  In an urban canyon, you may be able to operate more efficiently if including GLONASS (provides additional satellites, but multipath (signal reflection off buildings), can and will cause problems that far exceed the cm level.

Also, the only way to know what level of accuracy or precision you are getting on any position, is to have an independent occupation to check/average.  The reobservation needs to be at least two hours later (but not the same time the next day).  You could make the case that shots taken in the open are good (maybe true), but any shot taken in a challenging environment definitely needs a check.

One alternative to trying to take the shot under the tree, is to use one of the number of offset strategies to create a point that exist within the data collection software.  One that I like quite a bit is the "extend a line" method.  Picture this as a scenario:

You want to locate a water shutoff that is under a tree.  You know that the  GPS will be crap, so you take out your 100' or 200' cloth tape.  Spike one end at the water shutoff, then extend the tape out into the open away from the tree and other obstructions.

Take a shot anywhere on the tape, walk toward the water shutoff and take another shot at some even foot interval.  Then tell the software to create a point by extending the line that even number of feet.

There are a number of different offset methods.

My other strategy is  to set control points and use a total station.

Hope this helps.

Dan

On Tue, Feb 23, 2016 at 12:30 PM, Andy Smith-Petersen <[log in to unmask]> wrote:

Friends,

 

We have a goal of shooting 50,000+ services and countless other bits of infrastructure with cm-level accuracy. Our Trimble R8 units normally work well for this task if satellite visibility is good. But in tree cover and urban canyons, or on difficult-to-occupy targets, fixes are difficult and even the estimates of error can be off.

 

What GPS technology or strategy/ies should we be looking at as part of our work process to achieve this goal? What’s worked for you (or tried and failed)?

 

Especially interested in hearing from other utilities on this topic.

 

Thanks,

Andy

Andy Smith-Petersen Business System Analyst - GIS Portland Water District Phone: 207-523-5417 E-mail: [log in to unmask] http://www.pwd.org Portland Water District NOTICE & DISCLAIMER Confidentiality Notice: THIS MESSAGE IS INTENDED ONLY FOR THE USE OF THE INDIVIDUAL OR ENTITY TO WHICH IT IS ADDRESSED AND MAY CONTAIN INFORMATION THAT IS PRIVILEGED AND CONFIDENTIAL. Please notify the sender if you have received this message in error. Recipients should be aware that replies to this message may not be considered confidential under the Maine Freedom of Access law, 1 M.R.S.A. §402, and may therefore be subject to public disclosure upon request.

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