How accurate is a golf simulator? What is measured and what is computed

Behind the question about accuracy there is usually another one: can I trust the numbers? The answer has two halves. What a device measures, it measures more precisely than you swing. What it computes is a model, and models have assumptions. Here is which number falls into which category, why two venues show different values, and how to check in ten minutes whether a bay measures cleanly.

Measured or computed

Number	Camera system	Radar indoors
Ball speed	measured	measured
Launch angle and direction	measured	measured
Spin and spin axis	measured from the dimple pattern	derived from club and impact
Carry	computed from the launch values	computed from the launch values
Total distance	computed, with an assumed ground	computed, with an assumed ground
Impact location on the face	measured on some devices	only with an added camera

The third row matters most. A camera photographs the dimple pattern several times and reads the rotation directly; Foresight calls the method spherical correlation (foresightsports.eu). A radar measures spin from the flight, and indoors four metres are not enough for that, so it derives it. Both produce usable numbers, but only one of them is a measurement.

Normalisation: the number that never happened

Every carry on the screen is a calculation, and that calculation has settings. TrackMan calls the feature normalisation: it gives the value for calm conditions at the altitude and temperature the user enters, and it assumes a premium ball. The defaults are sea level and 77 degrees Fahrenheit, about 25 degrees Celsius (TrackMan).

That is not a flaw but the purpose: a normalised number is comparable because it takes the weather out. It is simply not the number your ball would have produced outside. Play at Sion, 500 metres above sea level, while the software sits on its sea-level default, and the figure differs from what the course later shows. TrackMan also puts numbers on the wind effect itself: with a 6-iron a headwind hurts almost twice as much as a tailwind helps.

Why two venues show different numbers

• The ball. A premium ball spins differently from a hard practice ball. If you want to compare, bring the same one both times.
• The settings. Altitude, temperature and the assumed firmness of the ground sit in the software and are rarely identical.
• The mat. The club slides across it instead of cutting into the ground; attack angle comes out flatter than on grass.
• The setup. A unit that is not squarely aligned to the target line shows a direction that does not exist. Which is why good venues mark the ball position.
• The strike. Two millimetres off centre cost ball speed and tilt the spin axis. That is not the device being imprecise, it is you.

The least stable number

Spin. It varies more from shot to shot than any other value, and it is at the same time the number that decides most about ball flight. On a radar indoors it is derived; on a camera it depends on clean images of the ball.

In practice: take spin as a tendency across ten shots, not as the value of a single one. If the driver averages 3,500 revolutions and you are looking for 2,500, that is a statement. If one ball shows 4,100, it is usually the strike and not the device.

What the software makes of it

Between the measuring device and the screen sits a flight model, and that belongs to the software. The same launch values produce slightly different carries in two programs, because each computes differently and assumes different things about ball and air.

Then there are the settings of the course you happen to be playing: ground firmness, green speed, wind. They do not change your measured values, but they change everything the ball does on the screen afterwards. Which is why the practice view with bare numbers is more honest than a round when measuring is the point. Which programs exist is in Playing real courses in a simulator.

When the numbers do not match the feel

It happens, and this is the order in which you find the cause. First the ball, because a hard practice ball flies shorter and spins less. Second the settings, meaning altitude and temperature in the software. Third the strike, because two millimetres off centre cost ball speed without the swing feeling any different.

If a difference remains after those three, it is rarely the device. Usually it is memory: on a course you count the good shots and forget the average ones, and the screen counts all of them.

What the makers publish, and what they do not

What is interesting is what the technical sheets leave out. For the TrackMan 4 and the iO, TrackMan describes the technology, dual radar and optically enhanced radar tracking, and lists the parameters captured, but names no tolerances in yards or per cent (TrackMan, tech specs).

That does not mean the devices are inaccurate. It means any claim of the form “accurate to two yards” that you read somewhere does not come from the maker. Anyone who wants to know how well a bay measures therefore checks it themselves, and that is quicker than any research.

The ten-minute test for a bay

1. Ten balls with the 7-iron, hit calmly. Watch ball speed: it should vary by a few miles an hour, not by ten. Big jumps at the same feel point to a device not capturing individual shots cleanly.
2. A club you know. Take the club whose distance you know outdoors. If the carry is systematically ten metres out, look at the settings.
3. Look at the settings. Altitude and temperature are in the software. Ask the venue what they are set to.
4. Check the alignment. Lay a club on the mat parallel to the target line. If the screen shows a deviation on a straight shot, the alignment is off.
5. Ask about the ball. Are you playing your own or the venue's? For a distance table it should be your own.

What a good venue does about it

Three things you notice on the way in. The ball position is marked, so the device sees the ball where it expects it. The mat lies parallel to the screen, not at an angle. And somebody can tell you what altitude and temperature the software is set to.

Where that is true, the numbers usually are too. Where the mat wanders and nobody knows the settings, the bay is good for playing and not for measuring.

What you can believe

Everything that is a comparison inside the same hour. Whether your second block of ten flies further than the first, whether the spread narrows, whether the smash factor rises: the same device measures that under the same conditions, and that is what simulators are built for.

Equally solid is the direction over weeks. Hit ten balls every Wednesday in the same bay with the same ball and write down the median, and you have a curve that means something. What the individual values mean is in Understanding launch-monitor numbers.

What you should not read into them

Absolute distances for a course, without thinking about the conditions. Total distance, because it assumes a ground nobody has seen. And the comparison with your friend's numbers from another venue: different ball, different settings, different device.

The most useful attitude is therefore an unexcited one: a simulator is a precise instrument for the moment of impact and a model for everything after it. How the two designs work is in Radar or camera, and which venue runs which system is on the map.

One last point that often gets forgotten: a simulator measures what you do, not what you intended. Ten balls therefore always show more spread than the evening feels like, and that is not the device being imprecise, it is the first honest feedback many players ever get.

Frequently asked questions

How accurate is a golf simulator?

For what it measures, more accurate than your swing: ball speed, launch angle and direction. Spin depends on the design, because a camera reads the rotation directly while a radar derives it indoors. Carry and total distance are always computed, with assumptions about weather and ground.

What does normalisation mean on a launch monitor?

The value shown applies to calm conditions at the altitude and temperature that are set, and assumes a premium ball. TrackMan defaults to sea level and 77 degrees Fahrenheit. That makes the number comparable, but it is not what your ball would have done outside.

Do the makers publish tolerances?

The technical data for the TrackMan 4 and the TrackMan iO list the technology and the parameters captured, but no tolerances in yards or per cent. Claims such as “accurate to two yards” therefore do not come from the maker.

Why does every venue show different distances?

Because of five things: the ball, the altitude and temperature settings, the mat, which flattens attack angle, the alignment of the device, and your strike. So only values from the same bay with the same ball are comparable.

How do I check whether a bay measures cleanly?

Ten calm balls with the 7-iron: ball speed should vary by a few miles an hour, not by ten. Then take a club whose distance you know, ask what altitude and temperature are set, and check the alignment with a club laid on the mat.

Sources

• TrackMan: Normalization erklärt (Windstille, Höhe und Temperatur als Eingaben, Premium-Ball unterstellt)
• TrackMan: Tech Specs, TrackMan 4 und iO (Technik und Parameter, keine Toleranzangaben)
• Foresight Sports Europe: Different types of launch monitor (Spin über sphärische Korrelation)