allow hiding portions of gpx data before upload #2

Strava has the pretty simplistic method of masking the first and last 200 m of an activity

https://support.strava.com/hc/en-us/articles/115000173384-Edit-Map-Visibility

I feel like there should be a better way! But it will have to balance partial anonymization with loss of part of the exercise data. With enough tracks from a user I suspect that any anonymization effect will be severely reduced. Gotta aim for better rather than perfect here.

Some steps I would take, in roughly ascending order of privacy gain + utility loss:

1. User applicable setting of "cut the last X m of my activity".
2. User applicable setting of "add a randomised perturbation to the start time of my activity of X-Y hours" (if you run the same route, timing can be de-anonymising!)
3. "Randomly shift my whole activity by X-Y km" (fun!) maybe needs some pseudorandom shift of the individual points too to stop road shape being de-anonymising?

When doing a random cut of the first/last X m, I propose dividing the area up into boxes of X km and cutting all activity within the box that the activity started in. This avoids the radius problem. If (like strava) you just cut the first and last 200 m, with enough activities you get a nice bullseye with the start point (your house) in the middle of it

At some point the efforts to anonymize GPS data butt up against the reality that there is no way to anonymize a non-trivial amount of location and time data without removing all the value from said data of greater granularity than average speed 🤷