How does a speedometer measure distances

What actually is a total station?

«What can it do and how do you read it? »

A total station is one of the most common additional functions of wristwatches. It is especially popular with sporty chronographs. We have summarized how it works and how to use it correctly for our readers in this article.

Total station or speedometer?

The term total station originated from a combination of the two ancient Greek words tachýs and métron, which mean fast or scale or measure. In practice, it means two fundamentally different things. In geodesy, a total station is understood to be an optical or electronic device with which horizontal directions and vertical angles can be determined. In contrast to theodolites, surveyors with total stations can also determine inclined distances, that is, distances measured obliquely to certain target points.

This device owes its name to the fact that the measurements in question can be carried out relatively quickly and easily with it. In contrast to this, a tachymeter on a watch is a function for measuring speeds, which, more correctly, should actually be called a tachometer. The fact that the same term is not used for this in practice as for the speed measuring device of a vehicle, for example, certainly has traditional reasons.

On the other hand, it also indicates that there is a considerable difference between the two. Because the "speedometer" in a vehicle continuously measures the speed driven based on the distance traveled and directly displays the value determined. The tachymeter scale on a watch, on the other hand, works a little differently.

More estimation than measurement - this is how it works in practice

First of all, it should be noted that the tachymeter ring of a watch is more of an aid to estimating speeds than an exact measuring device. In addition, users must be able to recognize or read off fixed, known route lengths if they want to determine their approximate speed.

In practice, milestones or kilometer signs are mainly used for this on the edge of roads, railroad tracks or racetracks. In practice, the measurement is carried out in such a way that the stop-second hand is activated at the twelve o'clock position when passing a kilometer stone and stopped again after one kilometer. The speed can then be read from the point on the tachymeter scale where the pointer has stopped.

It requires a certain concentration to press the button exactly when passing the milestone, so that the driver should delegate this task to his passenger or another person on board for safety reasons and not take it on himself.

It is certainly understandable that fans of retro-design watches with a passion for motorsport can get excited about this type of speed measurement. But even in a modern vehicle with a speedometer, situations can arise in which this method is used. The reason for this can be explained quickly: The speedometer built into the dashboard as standard shows the current speed, but this often varies greatly during the journey, for example in city traffic or on winding roads when you have to brake frequently and accelerate again.

The overall resulting average speed can differ considerably from this, but is very important information for the crew of a rally car or for participants in longer car races, for example, and is of much greater importance for the final outcome of the race than the speed driven at a certain point in time.

What watch lovers should know about the accuracy of total station measurements

If you want to use your chronograph with tachymeter scale to determine speeds, you should keep an eye on the achievable accuracy and the reading logic on which the scale is based. What is read on the tachymeter ring is actually the reciprocal of that part of an hour that has passed since the second hand left the twelve o'clock position. The 120 at six o'clock means that exactly 1/120 of an hour has passed when the second hand arrives there. The unit read on the tachymeter ring is therefore "per hour" and it can be multiplied by any length or unit and then reflects the respective change in the relevant parameter per unit of time.

In addition to the speeds specified in kilometers per hour, it can also be used, for example, to measure flow rates in liters per hour. The user of the total station saves having to divide his measured variable by the measured time specified in seconds and then multiply by 3,600 seconds per hour, which ultimately results in the measured units per hour. The fact that the tachymeter ring usually does not start at the top in the middle at the twelve, but only at a certain distance to the right of it, is related to the fact that it is assumed that the measurement to be carried out takes at least 15 seconds because shorter measurements are due to the human response time would be too imprecise anyway and would be subject to significant measurement errors. This position of the scale may seem confusing at first, but those who use it frequently will usually get used to it quickly.

If the measurement error is one second, this would mean an inaccuracy of almost seven percent for a measurement lasting 15 seconds. The procedure is therefore not suitable for exact laboratory measurements, but in everyday life it delivers quite useful results. The tachymeter scale is usually positioned on the edge of the dial or on the bezel. Models with the ring can be found at all common brands, such as Tag Heuer, Rolex, Omega, Breitling or Hublot and many others.

BreitlingHanhartOrisTachometerTachymeterTachyTeleTag Heuer2019-05-22