Hacker News Reader

> 1. How do you stop someone from finding the actual property location by querying that map a bunch of times and averaging the positions returned?

Back when YikYak was a thing, I was scraping the data for university campus through their API and found that their location data was randomized upon access. This meant that popular posts (they survived for longer) would end up producing pretty accurate location data.

In one case, I confirmed this by walking into the computer lab in the building where the Geography courses were located and asked "who was asking for lab help on YikYak?" and one student sheepishly raised his hand. I helped him and then also gave him a lesson on real world applications of the central limit theorem.

> 2. Are there any negative consequences for the people whose property a displaced marker ends up on?

That's definitely plausible: https://splinternews.com/how-an-internet-mapping-glitch-turn...

To answer (1), if you seed the randomiser with a known static value—e.g. the co-ordinates, or a database record ID—then repeat queries will return the same mangled result.

Or an even simpler, more trivial solution would be to store mangled values next to the true values. Then all of your APIs and services can pick the appropriate value for the circumstance.

To make it more obvious that the data is imprecise, I would also ensure that the mangled geo co-ordinates returned with a minimum number of decimal places. Three decimal places gives you a precision of 50–100 meters, depending on latitude. Four gives you 5-10 meters, which is the most you'd ever need for an approximate house location.

The concern that comes to my mind is the size of the randomisation and whether they factor in the size of surrounding properties. The level of imprecision required in a dense city will have little to no effect on farmland properties. I would dynamically adjust the random amplitude so that the circle of confusion covered a minimum number of properties. At minimum you'd find a database that listed median property sizes for a given postal code/zip code.

We used a slightly modified version of the code in the article. Instead of using random numbers we used pseudo-random numbers so the positions didn't change across requests.

Is the pseudo-random number generator cryptographically secure?

Every set of coordinates uses a different seed, so I would say yes.

Not if there is more entropy in the coordinates than the seed...

On 2. the consequences can be real, as in the article linked below where people keep accusing a couple of stealing their phones. I believe in that case their house was located at the centroid of a zipcode and whatever tool was used to map it simply put a pin at the centroid. This suggests that just lowering accuracy is not sufficient to protect innocent parties if that lowered accuracy isn't reflected in the visualization, and an approach like highlighting a region is probably a better way to go than giving a precise but inaccurate location.

https://www.washingtonpost.com/news/the-switch/wp/2016/01/26...

https://gizmodo.com/how-cartographers-for-the-u-s-military-i...

https://splinternews.com/how-an-internet-mapping-glitch-turn...

In those cases there were negative consequences because low-precision markers were not displaced.

Amusingly, this is the first time that I've seen the Law of Large Numbers being treated as a negative thing.