Conversion of miles to latitude and longitude degrees using geopy

Question

Background

I want to add a model manager function that filters a queryset based on the proximity to coordinates. I found this blog posting with code that is doing precisely what I want.

Code

The snippet below seems to make use of geopy functions that have since been removed. It coarsely narrows down the queryset by limiting the range of latitude and longitude.

    # Prune down the set of all locations to something we can quickly check precisely
    rough_distance = geopy.distance.arc_degrees(arcminutes=geopy.distance.nm(miles=distance)) * 2
    queryset = queryset.filter(
        latitude__range=(latitude - rough_distance, latitude + rough_distance), 
        longitude__range=(longitude - rough_distance, longitude + rough_distance)
    )

Problem

Since some of the used geopy functions have been removed/moved, I'm trying to rewrite this stanza. However, I do not understand the calculations---barely passed geometry and my research has confused me more than actually helped me.

Can anyone help? I would greatly appreciate it.

Answer 1

It looks like distance in miles is being converted to nautical miles, which are each equal to a minute of arc, which are 1/60th of an arc degree each. That value is then doubled, and then added and subtracted from a given latitude and longitude. These four values can be used to form a bounding box around the coordinates.

You can lookup any needed conversion factors on Wikipedia. There's also a relevant article there titled Horizontal position representation which discusses pros and cons of alternatives to longitude and latitude positioning which avoid some of their complexities. In other words, about the considerations involved with replacing latitude and longitude with another horizontal position representation in calculations.