Measuring Distance Traveled in a Dive

profunditas said:

Really? How do inertial guidance systems work then?

Basically the intertial guidance systems estimate the velocity and direction from the changes detected by the accelerometer and gyroscope, which works relatively fine on large scale, but the accuracy is very limited anyway - in hundreds of meters. See for example the graph here showing the accuracy of an navigation inertial system after one hour to be 650m. It may be fine for the estimates of a motion of a large objects like a ship, plane, submarine or even a vehicle, but at a diver there are many additional factors reducing the accuracy - especially the sensitivity of the miniaturized sensors, the way the device is worn (on the arm permanently moving and twisting in many direction), and the external factors like currents - most of the influences can be detected (to certain extent), but each of them makes the estimates less and less accurate.

So you have entry point, exit point, 9 degrees of freedom, extremely accurate pressure reference system and enough compute for any non-linear filter. I really don't really see a feasibility problem. Might look like a rocket science, because it is.

Either that, or you simply generate a random coefficient for multiplying the entry-exit distance and call it "travelled distance", and it will have about the same effect - some people will be happy to have more numbers to look at, others will be happy they can complain about even more nonsense in the stats, and Garmin will save on development costs, and on processing power.

The one I am thinking of mostly is the Ratio Computer. I have heard the Poseidon M28 does this as well from a user, but have not confirmed it.  

Mark_H20 said:

The one I am thinking of mostly is the Ratio Computer.

Well, I checked the documentation of Ratio Computers, and it indeed claims it brings you back there where you started your dive, but it is a GPS function helping you for example to find the boat, once you are on the surface. The manual clearly states that it does not work underwater. So that's something MK1 can do without problems too.

Umm NO, the Mk1 never made that claim and as it took Garmin 18 months to get the compass to point in the right direction I am not sure I would trust my life to it. But hey, go for it. 

Mark_H20 said:

Umm NO, the Mk1 never made that claim and as it took Garmin 18 months to get the compass to point in the right direction I am not sure I would trust my life to it. But hey, go for it. 

GPS navigation on surface works just fine and reliably on any Garmin GPS enabled device, so why shouldn't it work on MK1? Ratio Computers do not offer anything else than that - GPS navigation on surface.

First off I have not used a Ratio computer, I was told it will navigate you back to your starting point underwater by the dive shop. No idea how well this works, or even if it works, but Garmin does not support this feature. Garmin only has surface navigation.