This is indeed very similar to how initial sorting of the database is done. To improve accuracy (over performance) further sorting is done. Like mentioned the

This is indeed very similar to how initial sorting of the database is done. To improve accuracy (over performance) further sorting is done. Like mentioned the non-linearity with regard to latitude makes standard trigonometry useless. In my case I opted for the haversine formula (https://en.wikipedia.org/wiki/Haversine_formula https://en.wikipedia.org/wiki/Haversine_formula) this is pretty accurate on the small range of 1km