Accuracy guide
How accurate are online map measurements?
It's a fair question to ask before you trust a number off a screen. The honest answer is that online map measurements are more accurate than most people expect — and there are a handful of specific reasons they're never perfect. This guide explains exactly where the figure is solid, where it can drift, and where you should put the tool down and call a professional.
The short answer
The calculation behind a map distance is not the weak link. When you click two points, the tool works out the great-circle distance between them — the shortest path across the curved surface of the Earth — using a formula (the haversine) that is accurate to the metre. Give it two exact coordinates and it returns an essentially exact distance.
So almost every bit of error you'll ever see comes from somewhere else: from your inputs and from the map, not from the arithmetic. In practice, with careful point placement, a straight-line distance is typically within a fraction of a percent of the truth — easily good enough for a run, a garden bed, a fence line, a rough catchment, or sanity-checking a plan. Area measurements are a little more sensitive, for one specific reason we'll get to. Below, each error source, plainly.
What can move the number
Point-placement precision
This is the big one, and it's entirely in your hands. The tool measures between the exact pixels you click, so if your click lands a few pixels off the spot you meant, that offset becomes real metres on the ground. Zoomed right out, a single pixel can be worth tens of metres, so a sloppy click can shift a result by tens of metres on its own. Zoomed in, the same pixel might be worth a metre or less. The fix is simple: zoom in before you click, place each point on the precise feature you mean — a doorway, a corner, a trailhead — and your accuracy improves immediately.
Map projection
Web maps, including this one, are drawn using the Web Mercator projection. To flatten a round planet onto a rectangular screen, Mercator stretches the map more and more the further you get from the equator. For a short, straight-line distance this barely matters — the tool measures real ground distance, not screen distance. But it matters a lot more for area: because Mercator inflates the apparent size of land at high latitudes, an area figure measured far from the equator can be noticeably larger than the true ground area. If you're measuring a polygon up near the poles, treat the area as an estimate, not a survey figure.
Sphere versus ellipsoid
Great-circle maths treats the Earth as a perfect sphere. The real Earth is an ellipsoid — very slightly flattened at the poles and bulged at the equator. Modelling it as a sphere introduces a difference of up to roughly 0.3% on long lines, and far less on short ones. For a 5 km route that's a handful of metres; for a transcontinental line it's more. Some tools correct for this with ellipsoidal formulas, which is why two tools can disagree by a whisker on a long distance without either being wrong.
Terrain and slope
A map distance is a plan distance — the horizontal, bird's-eye distance, as if the ground were flat. The real, over-the-ground distance up and over a steep hill is slightly longer, because the path climbs and descends rather than cutting straight through. On gentle ground the difference is tiny and you can ignore it. On steep terrain it grows, so a hilly trail walks a little longer than the flat map distance suggests. If climb matters to you, measure it directly — see our guide on elevation gain for a route.
Base-map and imagery alignment
Satellite and aerial imagery is stitched and georeferenced, and it can sit slightly off from the true ground position — sometimes by a few metres. If you trace a feature by eye from imagery, you inherit that offset. It's usually small, but it's another reason a careful trace isn't the same as a survey.
Accuracy tip: zoom in until the feature you're measuring fills a good part of the screen before you place any point. Most of the avoidable error in a map measurement is placement error, and zooming in is the single most effective thing you can do to shrink it.
How to get the most accurate result
None of the above means online measurements are unreliable — it means a little technique goes a long way. To get the tightest figure the tool can give you:
- Zoom in before every click. Place each point on the exact spot you mean, not roughly near it.
- Use more points on a curve. For a road, river or trail, click frequently along the bends so the measured path hugs the real shape instead of cutting corners.
- Switch to satellite view when you're tracing a physical feature, so you can line up on the real edge rather than a generalised map line.
- Re-place a point if it looks off. Use Undo to remove the last point rather than living with a click you're not happy with.
- Sanity-check the total. If a number feels wrong, it usually is — a stray click far off the map can quietly add a big segment.
Do this and a distance result will normally land within a fraction of a percent of the true ground distance — close enough that the rounding in the readout is doing more than any real error.
A note on elevation accuracy
Elevation figures deserve their own caveat. The height at any point comes from a digital terrain model (DEM) — a grid of ground heights — not from a survey of that exact spot. Because the model has a fixed resolution and is smoothed, small bumps and dips get averaged out. That means a total climb figure can differ from what a GPS watch records on the same route, and neither is the absolute truth: the watch catches noise the model misses, and the model irons over detail the watch picks up. Treat elevation totals as a good estimate, not a measured certainty.
When you should NOT rely on it
Honesty matters more than the sales pitch here, so let's be clear about the limits. An online map measuring tool is built for planning and everyday questions. It is not a substitute for professional measurement, and you should not rely on it for:
- Legal property boundaries. A traced line on imagery is not a title plan. For anything that affects ownership, fencing disputes or land registration, use an official boundary source or a licensed surveyor.
- Construction and engineering. Building work needs survey-grade measurements with known tolerances, not pixels off a web map.
- Surveying. If the figure needs to stand up to scrutiny or sign-off, it needs a survey, full stop.
- Navigation and aviation. Lives and legal requirements depend on certified data; an online tool is not a certified navigation source.
For all of those, go to a professional survey or an official source. For runs, walks, gardens, rough sizing and planning, the tool is exactly right — just place your points with care.
FAQ
How accurate is an online map distance measurement?
With careful point placement, a straight-line distance is usually accurate to within a fraction of a percent. The underlying maths is exact to the metre; the small errors come from how precisely you click and from the map itself, not the calculation.
Why is my measured distance slightly different from another tool?
Different tools can place points a few pixels apart, treat the Earth as a sphere or an ellipsoid, and use slightly different base maps. Those small differences add up to tiny gaps between tools. None of them is wrong; they're just rounding the same real-world distance in slightly different ways.
Are area measurements as accurate as distance?
Area is more sensitive than distance. Web maps use the Web Mercator projection, which stretches areas the further you are from the equator, so an area figure can be noticeably inflated at high latitudes. A short straight-line distance is barely affected by the same projection.
Can I use an online map measurement for a property boundary or building work?
No. Online tools are great for planning, runs, gardens and rough sizing, but they're not a survey. For legal property boundaries, construction, surveying, navigation or aviation, use a professional survey or an official source.