Posted by Ice on Dec 2, 2005

Still don;t know what thet 5 cm. accuracy thing is.

Here is my e-Mail to my son with a copy of my post on GPS system accuracy I had posted elsewhere.


November 23, 2005

Here is a copy of a post I put on a Kayak Forum a couple weeks ago. They were argueing about the accuracy of the GPS System in general along with which cheap handheld units were the most accurate. This is what we were talking about today. Didn't realize you were using a mobile base and rovers tied to a sounder to check on the dredge work. Got a few disbelivers with the hundredths of a foot accuracy, so I sent them to some websites. At $60K I would hope you had that type accuracy. Also, how many people have used a $20K graph readout depth sounder? Dad

--- Forum Post ---

GPS units vary all over the lot as far as precise accuracy. Cheap consumer units vary with type & speed of the processors and number of satellites received, the manufacturer, etc. etc. etc. As price goes up (to many thousand dollars - like $60K-$75K), generally so does the accuracy. The price of cheap consumer units is generally tied to more bells and whistles though. I'm not talking about kayak applications here but just about the accuracy of the system.

Hand-held GPS units for mapping ('drawing maps') not only receive the GPS signal, but also receive a correction signal via radio from a Corp of Engineer base station This allows for accuracies of three feet and under, and is used for mapping trails, wetlands, well locations, or any other application where limited accuracy is needed. The information is collected in a data logger which is then downloaded to a GPS software program. The data can then be exported into a GIS usable format for map drawing. This is somewhat similar to what DGPS units do, but a lot more accurate.

Survey GPS units are designed for acquiring very accurate coordinates and measurement. Accuracy of hundredths of a foot can be obtained in just a few seconds of observation. This is achieved by setting a mobile base unit on a control monument and measuring various points with rover GPS units. By using RTK (real time kinematics) method you get instantaneous coordinates on measurements. This is accomplished by having both the base receiver and the rover receiver collecting GPS signals from the same satellites at the same time and then the base station broadcasting a correction signal to the rovers via radio signal.

In Response to: Re: GPS by Ice on Dec 1, 2005



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