Last updated: February 16, 2026
Why Flat-Map Math Fails on Real Land
You trace a parcel in Google Earth, export the coordinates, and run them through a simple shoelace formula. The result says 24.8 hectares. A licensed surveyor later measures 25.6 hectares—a 3 % gap that, on farmland worth $12,000/ha, quietly adds $9,600 to the real price. The difference comes from treating a curved planet as a flat sheet. A GPS coordinate area calculator built on the WGS84 ellipsoid removes that distortion. Paste or draw your lat/lon polygon, and the tool returns geodesic area in hectares, acres, sq ft, and more—plus perimeter and centroid—all computed in your browser with no data leaving your machine.
The output is a planning estimate, not a deed. It tells you whether a parcel is roughly the size a seller claims, how much fencing the perimeter needs, and whether two sites in different countries are actually comparable. For anything that ends up in a contract or a permit file, you still need a licensed surveyor to stake the corners and certify the boundary.
Geodesic vs. Planar: What Changes
| Method | How it works | When it breaks |
|---|---|---|
| Planar (shoelace) | Treats lat/lon as flat X-Y grid | Above ~1 km² or past 30° latitude — error grows with distance and latitude |
| Spherical | Models Earth as a perfect ball (R ≈ 6,371 km) | Poles and equator differ by ~21 km in radius; adds up on large parcels |
| Geodesic (WGS84) | Uses the same ellipsoid GPS satellites reference | Accurate to < 0.1 % for areas under 10,000 km² |
At 45°N a 100-hectare field measured with planar math can read 2–3 hectares high. Geodesic math eliminates that drift because it follows the actual curved path between each pair of coordinates instead of pretending the surface is flat.
Run Through It Once
Scenario: A landowner near Ames, Iowa (roughly 42°N) wants to check a listing that says “62 acres.” She has four corner coordinates from the county GIS portal.
Coordinates (decimal degrees):
- 42.0340, −93.6200
- 42.0340, −93.6140
- 42.0300, −93.6140
- 42.0300, −93.6200
She pastes them into the calculator and hits Calculate.
- Geodesic area: 248,400 m² ≈ 24.84 ha ≈ 61.4 acres
- Perimeter: 1,994 m ≈ 6,542 ft
- Centroid: 42.0320°N, −93.6170°W
The listing says 62 acres; the geodesic result says 61.4. That’s within normal rounding for a county parcel map, so the claim checks out. If the tool had returned 55 acres instead, she’d know something was off—maybe a misdrawn boundary or a road easement subtracted from the total—and could ask the seller before spending on a formal survey.
Running the same four points through a flat shoelace formula gives roughly 63.2 acres—about 3 % over the geodesic answer. At $12,000/acre that 1.8-acre phantom overshoot represents $21,600 of land that doesn’t exist.
Four Ways GPS Area Estimates Go Sideways
- Swapped lat and lon. Paste coordinates as (longitude, latitude) instead of (latitude, longitude) and your polygon lands in the ocean or on the wrong continent. Always glance at the map preview before reading the number. If the shape is nowhere near your site, the columns are probably flipped.
- Self-intersecting polygon. Click vertices out of order and the boundary crosses itself, creating a figure-eight. Most geodesic algorithms still return a number, but it’s meaningless. Trace the perimeter in one continuous direction—clockwise or counter-clockwise—and check the preview for any crossing lines before you calculate.
- Too few decimal places. A coordinate rounded to two decimals (42.03, −93.62) pins you to a ~1.1 km grid. That’s fine for a 1,000-hectare ranch but useless for a half-acre house lot. Use at least four decimals for parcels under 50 acres and six decimals when sub-meter precision matters.
- Sparse points on a curved boundary. Three or four points work for a rectangular field. But if the boundary follows a river or a winding road, straight-line segments between sparse points cut corners and undercount the area. Add a vertex every 20–50 m along curves to keep the polygon faithful to the actual shape.
Inputs, Outputs, and Limits
What coordinates does the tool accept? Decimal-degree lat/lon pairs (e.g., 42.0340, −93.6200). If your GPS device exports in degrees-minutes-seconds, convert first: 42°02′02.4″ = 42 + 2/60 + 2.4/3600 = 42.0340. Most modern devices and Google Earth already default to decimal degrees.
How many points do I need? Three minimum (a triangle). For a clean rectangle, four is enough. For irregular shapes or natural boundaries, use as many as it takes to follow the outline without cutting corners—20 to 50 is typical for a field with one curved edge.
Does point order matter? Yes. List them in sequence around the boundary. The tool auto-closes the polygon by connecting the last point back to the first, but if the order zigzags, the shape self-intersects and the area is wrong.
Is WGS84 the right datum for my region? WGS84 matches consumer GPS receivers worldwide. If you’re working with survey data tied to a local datum (NAD83, GDA2020, SIRGAS), the difference is usually sub-meter—negligible for planning but worth noting if you later compare against a cadastral survey.
Can I use this for legal paperwork? No. The result is a planning-grade estimate. Property transactions, permit applications, and boundary disputes require a licensed surveyor with calibrated equipment and an official datum.
Precision Notes
Geodesic area on the WGS84 ellipsoid is accurate to better than 0.1 % for polygons under 10,000 km². The main source of real-world error is coordinate precision, not the algorithm. Consumer-grade GPS is typically accurate to 3–5 m; for boundary-level work, use survey-grade GNSS (centimeter accuracy) and get a licensed surveyor to certify the result.
Got the area but need it in regional units? Convert to kanal, marla, bigha, and more or estimate fencing with the fence length planner.