Our recreational GPS's aren't as accurate as TV will have you believe. To many things that interfere with that weak signal coming back from space. 33 feet is as accurate as you should hope for any one point. Multiply that by all the other points on your ride and that can add up. Your gps's have a read out that supposedly tells you what it's accuracy is from moment to moment, however the makers don't really tell us what this really means. For Garmin's it's been suggested that perhaps it means 50 percent of the results for that position came back within the radius reported for accuracy. However you are moving most of the time, so it's not getting many plots of that same spot to compare against.
Survey grade GPS's use a supplemental transmitter/s... I believe, that is/are located at a known position on the ground to help it more accurately digest the satellite information. What agricultural GPS's do I have no idea, but certainly it must be more along the lines of survey grade instruments.
So how accurate is your need? If you are doing studies that need survey quality, then you need to fork out the big bucks for survey type equipment. $10,000 might get you started.
As for the wheel sizing you did..... is there a wheel sensor on your bike that connects to both GPS's? If not, then wheel travel is considered more accurate. But only if calibrated correctly. I will either put a drop of white out or paint on my wheel and just ride it for two full turns or so, then measure the distance between first and last spot, divide by appropriate number and then convert to mm as my tape measure is not metric. Most devices I've seen want that number in mm of wheel circumference.
Don't know what you were getting at with the 200 foot measured course. Who measured it? Was it straight? If not, staying to the inside of turns might throw some inaccuracies into the mix if the measure was down the middle of turns, and enough turns are involved.
And then there is also where did you look at the data? On the device themselves or from data uploaded to website or software? Some websites and software will derive their own figures for distance from the log file data. Log data on most devices is at best 1 second apart. If your route involves sharp turns or doubling back, then corners may be missed or part of where you doubled back. Remember at just 13 mph you are going 20 feet per second.
For recreational stuff, your figures are good enough. Don't dwell on it too much.
Last edited by Iride01; 04-13-20 at 01:01 PM.