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

Mike Stirling mail at mikestirling.co.uk
Wed Dec 30 09:32:02 GMT 2015

On 30/12/15 09:13, Simon Hobson wrote:
> 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.

But how many people would have BT turned on _and_ advertising all the 
time (else it's as good as turned off from a detection PoV)? These days 
I'd say that number is close to 0 - certainly Android only advertises 
for a limited period of time (older versions) or while the BT settings 
menu is open (later versions).  This is as much a battery saving 
exercise as it is privacy protection.
> 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.
For Bluetooth Smart (BLE) it is more likely you'd be able to detect 
devices as they tend to be advertising most of the time; they can do so 
with a far lower power budget than BT Classic.  For example, a year or 
so ago when I started doing some BLE work in the office there were no 
devices other than my own, but now I see Fitbits, Apple watches, etc. 
presumably from people in the offices downstairs.  The problem here 
though in relation to MAC addresses is that for BLE they are often 
temporary (random):



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