HDOP measures how favourably satellites are arranged in the sky, acting as a multiplier on positioning error — the same measurements give a far worse fix when satellites are clustered.
HDOP stands for Horizontal Dilution of Precision. It describes how well the visible satellites are spread across the sky, and it works as a multiplier on error rather than as an error figure itself.
The insight it captures: the number of satellites is less important than where they are. Eight satellites clustered in one corner of the sky produce a worse fix than four spread evenly around the horizon.
A receiver calculates position by intersecting distance measurements from several satellites. Each measurement has some uncertainty, so each defines a shell rather than a sphere, and the position is wherever those shells overlap.
When satellites are spread widely, the shells cross at sharp angles and the overlap region is small and well-defined. When they are clustered together, the shells cross at shallow angles and the overlap is a long, smeared region. Same measurement quality, far worse position.
HDOP quantifies that smearing. Actual error is roughly the raw measurement error multiplied by HDOP.
| HDOP | Rating | What it means |
|---|---|---|
| < 1 | Ideal | Rarely seen outside open sky with a good receiver |
| 1–2 | Excellent | Positions suitable for anything |
| 2–5 | Good | Normal outdoor conditions |
| 5–10 | Moderate | Usable, but expect noticeable error |
| 10–20 | Poor | Positions may be tens of metres out |
| > 20 | Unusable | Discard |
A useful worked example: a receiver with 3 metres of measurement error and an HDOP of 1.5 gives roughly 4.5 metres of position error. The same receiver with an HDOP of 12 gives around 36 metres — from identical hardware, in identical conditions, purely because of where the satellites happen to be.
HDOP is one of a family. VDOP covers vertical accuracy, which is always worse than horizontal because satellites are only ever above you, never below. PDOP combines both into a single 3D figure.
Horizontal is what matters for geofencing, since zones are drawn on a flat map and altitude is irrelevant to whether a device is inside one.
Satellites move. The constellation overhead at 9am is different from the one at noon, so HDOP at a fixed location varies through the day on a repeating cycle of roughly 24 hours.
This explains a genuinely confusing observation: a tracker that reports accurately most of the time but produces a bad position at the same time every day. Nothing is broken — the satellite geometry from that spot is briefly poor.
Buildings make it worse by blocking part of the sky, which is why urban HDOP is routinely 3–5 times higher than open-sky HDOP. Using several constellations at once is the main defence, because more satellites overall means more chance of a good spread — see GNSS.
If your tracker reports HDOP — many do, in their status output — it is the most useful quality signal available for deciding whether to trust a position.
A reasonable rule: treat positions with HDOP above 5 as approximate, and discard anything above 10 rather than testing it against a geofence boundary. A position with HDOP of 15 crossing your zone edge is far more likely to be geometry than movement, and acting on it produces exactly the false alerts that alert debouncing exists to suppress.
Where a device reports a simple accuracy radius instead of HDOP, that figure already has the geometry factored in — see GPS accuracy.
Last reviewed 7 August 2026.
GPS accuracy is how close a reported position is to the true location, usually expressed as a radius within which the real position probably falls.
GNSS is the umbrella term for all satellite navigation systems — GPS, Galileo, GLONASS, and BeiDou — of which GPS is only the American one.
GPS drift is the apparent movement of a stationary device caused by errors in its reported position rather than by any real change in location.
A cold start is a GPS receiver powering on with no usable satellite data, forcing it to download orbit information slowly from the satellites themselves.
Draw a zone, assign a phone or GPS tracker, and get a blip the moment it crosses the line. Free for 7 days.