Geofence radius and reporting-interval planner

What does the geofence planner tell you?

A tracker does not watch a vehicle all the time. It sends a position every few seconds, and the platform usually decides from those positions whether the vehicle entered or left a geofence. If the vehicle crosses a small fence between two positions, no position lands inside and the alert never comes.

This planner turns that into numbers. The radius tab gives the smallest radius that catches every pass and shows one pass drawn to scale. The reporting rule tab shows what your triggers mean in records, SIM data and the gap between records. The point test tab says whether a coordinate is inside, outside or too close to call.

Why geofences get missed

• Speed times interval is a distance. At 90 km/h a report every 30 seconds leaves 750 meters between two positions. A fence narrower than that can be crossed with no position inside.
• GNSS error eats the fence. A position is only surely inside when the true position is at least the error away from the edge. The usable radius is smaller than the drawn one.
• A road that skirts the fence is shorter inside it. The further the road is from the center, the shorter the path inside, and the bigger the radius you need.
• Reporting every second is not free. It multiplies records and SIM data. The reporting rule tab shows the cost before you change the setting.

Inputs you need

The tool opens with typical values for the GNSS error, the network delay, the extra records and the bytes per record. They are not Tracom device specifications. Replace them.

How it is calculated

Radius

The worst case for catching a position is when the two positions that straddle the fence fall one on each side of the center. The tool draws that case. A real pass can be luckier, but a fence sized for the worst case never misses.

Reporting rule and bends

Here a is the road length between two records and R is the bend radius. Summing straight lines between records is short on a bend. The tool uses the exact circle geometry, not the small-angle shortcut, which fails on tight bends.

Point test

Distances between coordinates use the haversine formula on a sphere. A point is inside a polygon when a ray from it crosses the edges an odd number of times. When the point is closer to the edge than your GNSS error, the tool does not call it: a real device could report either side.

Worked example: 90 km/h, a report every 30 seconds

• Gap: 90 km/h is 25 m/s, so 30 s leaves 750 m between two positions.
• Smallest radius: with no GNSS error the road through the center needs 375 m. With a 10 m error it needs 385 m.
• A 300 m fence: after the 10 m error the path inside is 2 x 290 = 580 m. That is less than 750 m, so about 77% of passes get a position. To catch all of them at this radius, report every 23 s or faster.
• Delay: 30 s plus 10 s of network and platform time is 40 s in the worst case, inside a 60 s limit.
• Reporting rule: a record every 30 s or 300 m, 40% urban at 30 km/h, 40% highway at 90 km/h, 10 ignition hours and 20 extra records per driving hour give 2,080 records a day. At 140 bytes that is about 8.7 MB a month. Every second would be 36,000 records and about 151 MB.
• Gap at highway speed: the distance trigger fires first, every 12 s, so two records are 300 m apart and that rule needs a radius of 160 m.
• A bend: two positions 100 m apart in a straight line on a 100 m bend have 104.7 m of road between them, so straight lines are short by 4.7%. At 300 m on a 200 m bend the shortfall is 13.1%.
• Point test: a 300 m circle centered on 24.7136, 46.6753. The point 46.67777 is 249.5 m from the center and 50.5 m inside the edge: inside. The point 46.67832 is 305.1 m out and 5.1 m past the edge, less than the 10 m error: too close to call. The point 46.67886 is 359.6 m out: outside.

The planner opens with these values, so the numbers on screen match this example.

Reading the results

• Radius shows a lamp for your planned radius: ready when every pass gets a position, check when half or more do, stop below that. The table lists the smallest radius for common speeds and intervals.
• Reporting rule shows records and SIM data, the gap between records at each speed, and the radius that gap needs. If your fence is smaller, the tool says so.
• Point test gives inside, outside or too close to call, with the distance to the edge. A polygon of the right size for a platform that takes only polygons comes with the circle.

Common mistakes

• Drawing the fence around the building. Size it for the road and the speed, not for the yard.
• Using the speed limit instead of the real speed. Read the top speeds from trip history.
• Entering the GNSS error of an open field. Between buildings and under trees it is larger. Measure it where the fence is.
• Counting on a lucky phase. Positions do not line up with the fence. Plan for the worst case.
• Cutting the interval to 1 second to fix a small fence. A bigger radius costs nothing in data. Check the cost on the reporting rule tab.
• Trusting a point near the edge. Inside the GNSS error, the answer can flip from one report to the next.

Assumptions and limits

Results are estimates. They assume a straight road, a steady speed, one report per interval and the GNSS error you enter. Some trackers also report on turns or on a distance rule, and platforms differ in how they decide entry and exit. The default values are typical, not Tracom device specifications, and the planner does not say what a particular tracker will do: read that from your device settings and your own trip data.

Further reading (links and content checked on 3 November 2026):

• GPS.gov: GPS accuracy, and factors that degrade it such as sky blockage
• Published geofencing guidelines: radius and missed events
• Haversine formula
• Circular segment: chord, arc and sagitta

Verify any rule of your platform against its current documentation.