We never had high accuracy :wink:
I started traversing timber sale units in the woods in 1988 during selective availability. Now, I use a survey grade 12 channel ProXR GPS Unit at work when I need to survey my road construction, landings and harvest units in the woods to make timber sale maps and compute acreage for logging appraisals and expand timber cruises from time to time, (good work if you can get it).
I adjust my Elevation Mask to 30 degrees, Position Dilution of Precision PDOP to 8.4 and Signal-to-Noise Ratio SNR to 4.0 and maneuver the antenna in between trees and branches to record at least 4 satellites per "hit" to obtain 3-D coverage from satellites which I can "hear clearly". Heck, I’m even wobbling trying to collect a hit on 50% slope with rain dripping down my collar, …was that a spider? I want to filter out bad data before I store it and move on. I’ve got to collect at least 30 3-D hits to make 1 raw position.
I download my roving raw data into a computer with some GPS software and differentially correct it against a GPS Base Station which collected GPS hits from the same satellites during the same time, but from a fixed location with a surveyed position, Portland State University maintains a GPS Base Station, and are online for this purpose.
Sometimes I look at the raw data of a fixed position, and yes, there is wobble, but I have a lot of trees, limbs and needles or leaves in the way, deflecting the signals. Anyway, so I compare my rover files to the Base Station, and that's what I call Differential Correction. Kind of like a science experiment between a variable and a control. The Base Station will also indicate a wobble, but with a clear view of the constellation of satellites. The Differential Correction process computes out the differences between the two wobbles to identify correct data.
From the book, several factors affect accuracy even with Selective Availability turned off. Briefly, occupation time at a point, number of Satellites visible, satellite elevations, Signal-to-Noise Ratio, PDOP, Multipath or signals bouncing off of stuff, and the distance between base station and the rover receiver.
My corrected data for my last survey work in timber last March had errors. A brass cap section corner way upslope of the North Fork Reservoir with 1/2 view of blue sky with about 50 hits from at least 4 satellites had a standard deviation of 13 feet, 95% Precision Horizontal 3 feet and vertical 6 feet. The next day, mapping a spot on the Hillockburn Road in wet timber with sheets of hail with about 60 hits corrected to a standard deviation of 7 feet, 95% Precision horizontal 6 feet and vertical under 13 feet. This is using very expensive equipment, and logging about 30 to 60 hits to establish 1 raw point, downloading the raw data, downloading the base station data, and subtracting out the differences between wobbles with a high end computer, to establish just one point on a map with some reasonable confidence.
I’ve compared my GPS survey work to information collected by “Recreation Grade GPS Units” in the woods without Differentially Correction capability, and the differences will be at best, 200 horizontal feet apart 95% of the time. I don’t worry about vertical GPS differences on a 2 dimensional map in the woods, its all relative. No doubt accuracy is best out in the clear to pick-up the most number of satellites, like out on the water.
By the way, my first personal “Recreational Grade” handheld GPS unit should arrive in the mailbox this weekend :dance:
[ 06-13-2003, 12:12 AM: Message edited by: Jet Drifter ]