How to disable Garmins auto chopping (distance and moving time) "feature"?

Mostly Yes; the Fit file has a distance stream that is like you described, but it also has available the GPS points for making a map, or for re-calc of distance based only the GPS points such as Stravas "recalculate distance" function. But it's been hard for me to prove that the watch is using the accelerometer this way, and that's why I cast it as an "opinion". Note that watches that didn't have accelerometer (such as 201, 305, 310xt) also had a distance stream that differed from the GPS point total, so originally my theory was it was based on some filtering they might be doing for distance calculation. Another reason I think they use accelerometer is that in a forest trip where you don't have gps lock, the distance will still be counted, it will be sort of close - even without a single GPS point. 

carleson said:

But it's been hard for me to prove that the watch is using the accelerometer this way

Right, it does not, since it cannot. An accelerometer, as the name suggests, measures the acceleration, not the velocity. The acceleration could be theoretically used to roughly estimate the speed, but since the watch being on the wrist is permanently in motion in all axes (regardess of the direction of the body velocity), and the direction of the motion cannot be detected without having also a 3D gyroscope, so that you could create the velocity model properly positioned in the space. Instinct does not have a gyro.

Instinct (and other Garmin watches) simply reset the distance when the velocity as measured by GPS (distance between two GPS keypoints devided the time difference) is below certain threshold. Additionally, it also sets the keypoint distance to zero anytime the aquring of GPS signal fails, even if it could easily interpolate the missing point from the surrounding keypoints.

Thanks for the information. 

Counting my steps before and after a GPS recorded activity, and over known distances, I calculated that I have an average stride length of 0.79.

The watch does not seem to use my custom stride length to Measure distance when doing non GPS activities though. It only seems to use that figure to give a distance in the steps widget on the watch and on connect. 

If it used my custom stride length, I would probably not use GPS for most walks, as counting steps before and after then multiplying by 0.79 gives me an accurate distance often the same as the GPS track. 

However, if I do a walk and turn the GPS off beforehand, it doesn't use my custom stride length at all and is over 20% out. 

I've tested it over known Distances on many ocassions. 

Tom said:

If it used my custom stride length

Instinct does not support Custom Stride Length. Devices compatible with this feature are listed in the document What is the Stride Length Feature Found on Some Garmin Fitness Devices?, and Instinct is not among them.

I did a test the other day and tried to walk as slow as I could (it is harder then you'd imagine :D) and the distance was spot on on the usual running track. So looks like the logic is not built on pace alone, probably has something else to do with the GPS point patterns.

The first screenshot below is from my superslow but straight walk while the second is from hiking (pace against distance (minutes/km)), you can see that pace hits rock bottom when hiking (this repeats constantly - see 3rd image).

That's the incorrect link.  The Garmin Instinct does support 'Custom Stride Length'.

https://support.garmin.com/en-GB/?faq=RAsAiDCmNM2mxSkOcPQCQ8

Tom said:

That's the incorrect link.

OK, thanks for the link. Did not know there are contradicting versions of the document on Garmin's website. Now we can at least chose which one of them we want to trust  However, as you have shown on your data, it looks more like it does not work.

The very same document (the same ID, different locals), shows no Instinct either:

https://support.garmin.com/fr-FR/?faq=RAsAiDCmNM2mxSkOcPQCQ8

Yes, all the custom stride length seems to do is provide a distance reference for the step widget on the watch and in connect.

Out of curiosity I did a GPS walk with the dog today.

The walk recorded by GPS was 2.94 miles (4731.47 metres). 

The watch recorded 5,895 steps, starting on 266 and ending on 6161 steps.

4731.47 metres ÷ 5,895 steps = 0.80 metres average stride length. This nearly matches my custom stride length I have set at 0.79 metres. 

If my Instinct incorporated the average stride length multiplied by steps taken, to calculate distance during a non GPS walking activity, I probably wouldn't need to use GPS at all for walks. It would be an ideal option for walking holidays, long day hikes, saving battery whilst still keeping track of distance, being able to record Ascent, Descent, altitude and everything else you can do in a normal walking activity profile.

I would make the argument that it would be an even better option than GPS if under heavy tree cover or other difficult GPS conditions make up a significant part of the route.

As it is, none GPS walk activities seem to massively under record distance and ignore the custom stride length completely. 

Yes it can measure the distance from the accelerometer only, besides the anecdote I cite above, this article Garmin indicates the accelerometer is used to measure distance on treadmill. https://support.garmin.com/en-US/?faq=zk9G4O5DkS4znMm9KSV2VA. You can go outside and run in treadmill mode and get a distance with no GPS. You can run in tunnel with no GPS available and get a distance in the tunnel. Also, in the so-called UltraTrac mode, they documented in the Forerunner 920xt manual that the accelerometer is used for speed and distance when the GPS is off.

     They could be calculating the distance with it just from cadence and then an internal calibration from when you have GPS on it calculates a stride length. It's maddening to not know what's going on in there.