Accuracy of mapping, whether it be professional GPS mapping, 'Do It Yourself' GPS mapping, or mapping from aerial photography causes much confusion among farmers. Wheresmycows.com Farm Mapping conducted trial to see what sort of accuracy can be expected from various mapping techniques. This trail showed that in our opinion farmers are often paying more for a smaller increase in accuracy than they are lead to believe they are getting. "You really have to weigh up the zero-point-zero-something of a hectare difference in accuracy between a professionally GPS mapped paddock and the paddock area you could calculate yourself with a DIY GPS mapping kit or ortho-corrected aerial photograph mapping kit".

The image above shows several paddocks that were measured using a high accuracy RTK GPS unit (red lines and text, accurate to 2cm) at the same time as recording points with a Garmin GPS60 from our DIY GPS farm mapping kit (black lines and text). These points were recorded on a summer day in New Zealand with fine weather and constant atmospheric conditions (Ideal GPS conditions). As you can see in the image, the average paddock size error is approximately 0.01ha, and total area difference is 0.04ha.

The above figure 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.

Unless your professional GPS mapping company is using sub-10cm accuracy GPS equipment (not many do for general GPS farm mapping), error could still be up to 1% of the total paddock area in an average paddock. For example; a 2ha paddock with 1% error could be mapped as 1.98ha or 2.02ha. A square 2ha paddock has 565.69m of fence surrounding it, whereas a 2.02ha paddock would have 568.51m of fence surrounding it. This difference is the equivalent of moving all the fences surrounding that paddock outwards by 35cm (just over 1 foot). Of course, as you measure larger areas, the relative accuracy is going to improve. You will find that over your whole farm you may have some paddocks reading slightly more than their true area, and some paddocks slightly less than their true area. The errors do NOT add up to an increasingly inaccurate map of your whole farm, but in fact tend to even out.

Some things to consider when selecting a mapping method:
• 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 inside the fence anyway?
• If you are using the area to calculate the amount of feed in a paddock/field, the error in your final calculation introduced by your pasture measuring technique is likely to be greater than that introduced by your mapping technique.
• Is the area completely flat? Trials in gentle rolling hill country show that the hills in a field 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.

Please fill in the form below if you have any questions, or you would like any further information about GPS accuracy.

Garmin GPS60 handheld GPS
Available in our DIY GPS Farm mapping kit