[OpenTRV-interest] Fwd: Count crowds with Bluetooth Connectivity Kit

[Simon Hobson]

Nigel <nigel at discreetsecuritysolutions.com> wrote:

> Counting people though? Bad idea - without any other way to check it, it's going to basically be a random number.
...
> So no: you can't get even a rough guess on crowd numbers with Bluetooth without loads of extra data. 

I think your conclusion comes from incorrect assumptions.

Yes, for small numbers you are correct. In (say) a group of 10 people, some will not have taken their phone with them, some will have BT turned off, and you really wouldn't know.

But for a large crowd, and bear in mind that they are talking about 10s of thousands here, it becomes statistically valid to say that "on average X% of the population carry a phone with BT turned on, we've counted Y BT devices, so there are very roughly about Y/X people here". The key thing is knowing X - I don't know it, but I bet there are experts in the field of counting people who do have a good idea. People have been counting people for longer than I've been on this earth - it wouldn't be hard to run old and new alongside each other to empirically determine a value for X under different circumstances.

So if you are in the business of counting people, I don't think X is hard to determine (and it'll vary by situation and demographic). Once you know X, you just knock up a device to count Y and you've got a pretty good idea of total numbers. Not "there were 54,157" people there" accuracy, but at least "there are 50-55k people there" levels of accuracy which is probably more than accurate enough for this sort of situation.

BTW - for counting devices you don't look at the device name, you'd look at the MAC address which, if the manufacturer isn't a complete idiot, is globally unique.