[OpenTRV-interest] Count crowds with Bluetooth Connectivity Kit

Damon Hart-Davis dhd at exnet.com
Wed Dec 30 09:25:23 GMT 2015

So, the issue remains what fraction of people have their portable BT device in some sort of non-radio-slient mode so that its presence can be passively detected.  I don’t want to track individuals, but it would be noce to look at device type info to exclude car-based and home-based devices that might be near a bus stop, or even particular devices that remained in range for many hours to be selectively ignored.

One reference suggested that up to ~30% of BT mobile devices are sensable this way; a BT engineer told me that that was very firmly nonsence, as mobile phones now clamp down on BT transmissions for energy saving and security reasons.

At > 10% of people in London waiting for buses carrying a passively-detectable BT device I think we can use it as one strong indicator of presence (alongside others).  Much less and I think it is unlikely to be valuable for our application.

Opinions on that number please?



> On 30 Dec 2015, at 09:13, Simon Hobson <linux at thehobsons.co.uk> 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.
> 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.
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