There are two types of accuracy that we generally talk about when describing the accuracy of GPS; Relative Accuracy and Absolute Accuracy.
When measuring the area of paddocks/fields, the relative accuracy has the greatest effect on your results. Think of relative accuracy as the difference between two points you’ve recorded when all other variables are held constant. For example. If you record one point, then you walk 10m north and record another point. In this time, the GPS will likely be using the same satellites to calculate position, and the atmospheric conditions have not changed much. Therefore, all the factors affecting the radio signals from the satellites to your GPS unit remain relatively constant, so the measured distance is quite accurate.
In trials we have done, we've found that if your main reason for wanting to use a GPS is for general farm mapping, the relative accuracy is well adequate to calculate areas accurate enough for seed, fertilizer and feed budgeting.
The image to the right (click it to zoom in) shows the difference in area reported by a hand held Garmin GPS60 (measured several times), the area calculated from an ortho-corrected photograph, and a 10cm accuracy professional "RTK" GPS (actual area). You really must deicde whether the extra accuracy of a professional GPS map is worth the added expense over a DIY GPS map or a map created from an ortho-corrected aerial photograph.
Absolute Accuracy is the accuracy that is reported on the screen of your GPS. Think of Absolute Accuracy as the ability to go back and find a marked waypoint sometime in the future.
If you bury your treasure and mark the position of your treasure with your GPS that has an (absolute) accuracy of +/- 5m, you can presume that the point you just marked is accurate to somewhere within about 5m of the true position about 95% of the time. Therefore, if you were to come back with even the most accurate GPS available, the data you are using to find your treasure is only as good as what the GPS that recorded it was. For example, in a poor case scenario, you could mark the point with +/- 5m accuracy and it records a point 5m from the true position, then when you go back to find it and it is also accurate +/- 5m, it is possible that your GPS could guide you to 10m away from the true position.
It is common for handheld GPS units in the Southern Hemisphere to display an accuracy of +/- 5-15m 95% of the time. In the northern Hemisphere where WAAS (USA), EGNOS (Europe) or NSAS (Japan) are available, the accuracy can be expected to be +/-1-3m 95% of the time.
It is the absolute accuracy that you need to consider if you are going to be using your handheld GPS for jobs like marking where a buried water or drain-line is, then expect to find it again. Although marking with a handheld is better than nothing, you may still need to do a bit of digging to find what you buried.
In order to combat poor absolute accuracy for jobs that require precision (eg, marking out new fences or centre-pivot irrigators), we use RTK (Real Time Kinematic) GPS which has an accuracy of 2cm.
What are you using the paddock area information for? If you are going to be cropping, will you get right up to the fence line, or will you be half a metre out?
If you are using a plate meter and the size of the paddock to calculate feed in a paddock, how well do your plate meter results represent the entire paddock?
Is the paddock completely flat? Trials in gentle rolling hill country show that the hills in a paddock can lead to a greater increase between 2D planar area and 3D surface than the error introduced by your mapping technique.
With this in mind, you must decide whether you want to pay the premium for a professional GPS farm map, or for the sake of plus or minus a few centimetres around each fence line, should you go for a less expensive option like the DIY GPS farm mapping kit (where you get to keep the GPS for work and play after you have mapped your farm) or a digitised map from ortho-corrected photography with the end product being a colour photograph (showing paddock areas) that you can hang on your wall. Note that wall maps, map pads and map whiteboards can all be made from a DIY mapping kit file.