Measure a Field From GPS Points and Check It Against the Deed

Landowners, farmers and buyers often want to know how big a piece of land really is: to order seed or fertiliser, to price fencing or to compare with the figure in a deed or listing. Walking the boundary with a phone and computing the area is quick and usually good enough for planning. This guide measures a five-sided pasture, works out its area and perimeter, and then asks the question that matters: is the difference from the deed real or just GPS error?

Collecting the corners

Walk to each corner of the field, wait for the position to settle for 20 to 30 seconds and record it. For this pasture the corners, in order clockwise, are 42.10215, −72.58840; 42.10398, −72.58702; 42.10322, −72.58431; 42.10140, −72.58467; and 42.10102, −72.58690. Five decimal places of a degree is about 1.1 m, finer than the phone can actually measure, so there is no point recording more. Order matters: the points must follow the boundary around the field, clockwise or anticlockwise. Writing them in the order you happened to visit them can create a bow-tie shape and a meaningless area.

Step 1: area and perimeter

The polygon area calculator gives 74,108 m², which is 7.41 hectares or 18.3 acres, with a perimeter of 1,042 m. The five sides measure 233, 239, 205, 189 and 176 m. The calculation treats each edge as a straight line on the globe and uses a sphere with the same surface area as the WGS84 ellipsoid, so the formula itself contributes far less error than the positions you entered.

Step 2: how good are the corners?

A phone in an open field is typically accurate to about 3 to 5 m; under trees or near buildings it can be 10 m or worse. A useful worst-case estimate of the area error is the perimeter multiplied by the position error, because a boundary shifted outward by 4 m on every side adds a strip 4 m wide all the way round. Here that is 1,042 m × 4 m = 4,168 m², about 0.42 hectares or 1.0 acre, roughly 5.6% of the area. The real error is usually smaller, because errors at different corners partly cancel, but the estimate shows why phone measurements of small fields cannot settle disputes about a few per cent.

Step 3: compare with the deed

Suppose the deed describes the pasture as 18.0 acres. The measurement is 18.3 acres, 0.3 acres or 1.7% more. That is well inside the 1.0 acre uncertainty, so the measurement is consistent with the deed and there is no evidence of a discrepancy. If the deed said 16 acres, the 2.3 acre difference would be more than twice the uncertainty, and it would be worth checking whether you walked the right boundary, whether a corner sits on a road or ditch the deed excludes, or whether the deed is out of date after a boundary change. Deeds and surveys measure horizontal area; on sloping land, the walked surface is slightly larger than the deed area, about 1.5% more on a 10° slope.

Step 4: using the figure

For planning, 7.41 hectares is plenty precise. Seed sown at 25 kg per hectare needs about 185 kg; fencing the 1,042 m perimeter with a 3% allowance for corners and gates needs about 1,075 m of wire per strand. Mowing time scales with area and, for a given mower width, with the total length of passes, which is roughly area divided by the cutting width. Keep the corner list with the farm records so you can repeat the measurement after a boundary change.

Common mistakes

• Points out of order. A crossing boundary gives a wrong area with no warning sign.
• Too few points on curved edges. A stream boundary needs a point at every bend, or the straight edges cut corners.
• Repeating the first point. The shape closes automatically; a repeated point adds a zero-length edge and does no harm, but listing a point twice in the middle does.
• Recording on the move. Positions taken while walking lag behind; stop at each corner.
• Treating GPS area as legal area. Legal boundaries come from the deed and a licensed survey.

Field measurement checklist

1. Walk the boundary and record each corner after the position settles.
2. Enter the corners in order around the boundary.
3. Read the area in hectares or acres and the perimeter.
4. Estimate the uncertainty as perimeter times position error.
5. Compare with the deed and investigate only differences larger than the uncertainty.
6. Keep the corner list for future measurements.