Garmin has some distinct products. Their sport, fitness and aviator wearables are one of them. Many of their wearables have an altimeter, barometer and compass – and are commonly known as an ABC watch. I can’t resist the opportunity to try, test and use instruments and gauges. I went and out and got the D2 Bravo – an aviators watch. The D2 Bravo was in essence a Fenix3 modified with a new software platform and functionality meant specifically for aviation. An example of an aviation feature was the inclusion of an airport database. Another was the configuration of two right-side buttons to be the ‘Direct To’  and ‘Nearest’ buttons. The D2 Bravo didn’t stay with me for very long. Suffice it to say that I had to replace the D2 Bravo four times for various reasons which are beyond the scope of this post

After waiting a few days, I got the Fenix3 hoping that without the added aviation features, the Fenix3 by itself would work just fine for sport and fitness use and then the Fenix too had an altimeter, barometer and a compass built-in – the 3 things I was looking for. It also had built-in GPS.

I have always been an avid student of the variability of atmospheric pressure with altitude (as also temperature with altitude). Of course, it is imperative that aviators at any/all levels understand such variability really well. Not doing so can lead to serious trouble. The above relationships are simple and complicated at the same time.

The Fenix3 brought with it a period of use and study of the watch and the curiosity to figure out how it works or why it behaves a certain way. In particular it was the altimeter and barometer that were built into it. Part of it was the continuation of my interest in altimetry. For the sake of providing some context, here is what was happening. The altimeter would drift over time even if the watch and person wearing it would be in one place. As a means of calibration, the watch allows for calibrating altitude by inputting known elevation. but it does not allow us to use sea-level adjusted pressure as a means of calibration. This means that in order to use the watch for measuring elevation (also known as true altitude), the only way to do so is to know the elevation of a location and use that number to calibrate the Fenix3. If you observe closely, once you enter the elevation, the indicated sea-level adjusted pressure number in the pressure widget will change (expected, of course).  As to why the Fenix3 was not designed to accept a sea-level corrected pressure number for calibration as the other option (say as published by the weather website or the local airport reading) baffles me. I have tried reading up but i have not found an answer.

In simple terms, there are 3 variables in this equation – Ambient Pressure, Elevation and Sea Level Adjusted Pressure.

Ambient Pressure +(-) Adjustment for Elevation = Sea Level Adjusted Pressure.

As can be seen above, one impacts the other two. Knowing two, we can find the third variable. Hence I cant figure out why Garmin decided to leave out the ability to set sea-level adjusted pressure as a calibration option.

There is a ton of material written in various forums varying from questions, answers, opinions, demands and rants around this topic – and there maybe no need for one more addition from me! I couldn’t resist writing up my view on this.

Lets for the time-being ignore that the watch (or the individual) has a GPS or any other device to measure altitude. The only device we have is this watch and the altimeter on it.

The first thing to note is that an altimeter will indicate some altitude at all times – the correctness of that indication is something we will come to further down in this post. Stated simply, the commonly found altimeter is actually a barometer that senses pressure. It’s just that the dial on an altimeter is calibrated to indicate altitude. The barometer senses ambient pressure (local pressure) at that location.

How does it then know what the altitude is?  Well, we know that pressure is 29.92 inHg at sea level. We also know that pressure decreases with increasing elevation – approximately 1 inHg per 1000 ft of elevation. Then it becomes easy to infer that if the ambient pressure at my location read 28.92 inHg, I must be at a 1000 ft of elevation above sea level. Easy?

That MAY BE correct. That is NOT ALWAYS correct. Why?  Because while on a standard day the above rule of pressure reduction may hold, there is hardly a standard day and even if there was, they are far and few. For one reason or the other things go non-standard very quickly. Imagine a storm moving through the area. What would happen to pressure in that situation? The pressure would begin drop. If you want to verify this, check out the surface weather charts for the US and compare it with the Doppler radar images (the ones that show green patches for rain, blue for ice etc). You will notice that the green patches will match up with the areas that have low pressure ‘L’ indicated on the surface charts. 

Lets go back to our example. We are at a location and pressure indicates 28.92. Ideally this should be a 1000 ft elevation. Will it always be so – NO. That’s the first thing to note and understand.

Then what is the actual elevation – also referred to as “true altitude” of that location? We will not know unless we can calibrate the altimeter in one of  two ways – a) local pressure adjusted to sea level OR (b) definite elevation of that location (as measured or surveyed). There is a 3rd option – GPS – but we already said earlier in this post that we are leaving that one out

Lets discuss the options above.

(a) – Local Pressure Adjusted to Sea Level

This number essentially is the pressure the location would be at if located at sea-level. This method of normalizing pressure at any geographic location (regardless of altitude) serves as a means of using the number for comparative purposes. To make this point clearer, imagine a location at sea level. Say the pressure at this point is standard – i.e. 29.92. Think of boarding an elevator at that exact location and going up a 1000 ft. On a standard day, the pressure at 1000 ft should be 28.92 (remember the thumb rule provided above – 1 inch drop per 1000 ft). While the ambient pressure at 1000 ft is 28.92, the sea level corrected pressure at that location will still be referred to as 29.92. This is very important to understand. Even if you went up another 1000 ft and the ambient pressure measured 27.82 (another inch less for the 1000 ft), the sea level corrected pressure at that point will still be 29.92. This is the reason that aircrafts arriving into an airport terminal area are provided the sea level corrected pressure (even if they are flying at that time at 10,000 ft). Pilots use that number to calibrate their altimeter (which is in fact a barometer) and thereafter they can rely on the indicated readings from that altimeter. Similarly, we could this option to calibrate any altimeter.

What happens if we are unable to know sea level corrected pressure for a certain area? We could use the other option.

(b) – Use known elevation of that location to calibrate the altimeter.

What is known elevation of a location? – Land surveys, topographic maps provide surveyed altitude of various locations. This is even more prevalent in areas where terrain is uneven. For example, while climbing up alpine mountains like Pilatus or Jungfrau, you will notice elevation marked at regular intervals. We could use these numbers to calibrate an altimeter.

If we don’t have either (a) or (b), then we indeed cannot calibrate our altimeter. This does not mean that we can’t use the altimeter. All we have lost at this point is the ability to determine our true altitude (elevation) from sea level. The altimeter is still useful in that it can be used to measure ascent or descent – in other words, to determine how many feet we have climbed or descended. How does this work? Remember an altimeter is a barometer that senses pressure. Regardless of whether we have been able to calibrate the altimeter to reflect true elevation, it is still sensing ambient pressure. If we climb from one point to another, it will detect a pressure change (a drop in this case) and by virtue of that drop, it will reflect a change in altitude (an increase in this case). Likewise, if we descended, the opposite cases would apply.

What does this mean? It means that if we are unable to calibrate our altimeter, we have lost the ability to measure our elevation (true altitude or height above sea level). However we are still able to measure altitude changes.

Given all of the above, the Fenix3 altimeter works exactly as expected.  There are several individuals with this question and this post is to provide my understanding on how the Fenix3 altimeter works. In summary, Fenix3 faithfully records altitude changes caused by climbing or descending. If calibrated manually (or via GPS), it indicates those changes in terms of elevation. It sets sea-level pressure correctly when provided a known elevation.

Note that I am not providing any view on the accuracy of the Fenix altimeter over varying usage. I have not been able to get to testing that accuracy.

Note also that in the above discussion, I have stayed away from the topic of how the watch behaves when ambient pressure is varying simultaneously with change in altitude. That’s for a different post.

CP Jois