It is possible that some classified-ads-second-hand commerce apps were conceived as a location-first search engine. That means that for each search we are having, one critical factor for retrieval and/or ranking is how far the result is located from your current location. Think of it this way: If you are living in Helsinki, and you need to commute to purchase an item, would you purchase the first perfect result on your search if it's located in Rovaniemi?
For this search problem, distance modelling becomes important to rank the results properly, from nearest to further. For this example, Solr has been suporting for a while distance-based searches with some methods that help sorting their results by the distance from each other.
For distance-based searches in its most simple form, Solr uses the [Haversine](https://en.wikipedia.org/wiki/Haversine_formula) algorithmWIP