Basically, distances derived from unprojected coordinate systems (i.e., WGS84, epsg: 4326, lat/lon, etc.) can't be easily converted because the length of a degree of longitude varies with latitude. This will be more apparent the farther you are from the equator. If we were masochists, we could try to check inputs for projection and reproject, but that would be error prone and take us forever.
So, instead, we'll just have to be clear in the documentation that the inputs must be projected with proper distance, rather than angular, units. If we're feeling generous, maybe we can try to do a bit of introspection and throw some exceptions if people try to load data with angular units.
Basically, distances derived from unprojected coordinate systems (i.e., WGS84, epsg: 4326, lat/lon, etc.) can't be easily converted because the length of a degree of longitude varies with latitude. This will be more apparent the farther you are from the equator. If we were masochists, we could try to check inputs for projection and reproject, but that would be error prone and take us forever.
So, instead, we'll just have to be clear in the documentation that the inputs must be projected with proper distance, rather than angular, units. If we're feeling generous, maybe we can try to do a bit of introspection and throw some exceptions if people try to load data with angular units.