A few days ago I came across this link
I think that corresponds to my research but I stumble on bubble logic
Here is the excerpt that interests me
If the location of the things is stable you could store the distances in the database.
"
How to do it:
Create a new datatype “Relation” that will include reference to two things and a distance field.
Create a backend workflow that is triggered when a new thing is added. The workflow should iterate all things and create “Relations”.
then you can use “Relation” for sorting.
If it is not your use case and you need to calculate the distance every time within a certain radius then a different a bit more complex approach could be used. It requires more explanation. Let me know you need it. "
So I created a new type called relationship with this inside.
Does my distanceresult field have to be a number
And now I don’t see how I can fill this in my workflow
help please 
thanks