[datatable-help] Select all districts within a 100 km radius of a district

[repidemiologist]

Dear R Users! I need to limit my dataset to all districts within a 100 km
radius of a selected district (geographical area of interest).

I have many variables in the dataset and among them geographical coordinates
of centroids, e.g.:
Longitude Latitude
1  -61.68667 17.02444
2  -61.88722 17.10527
3  -61.79445 17.16333
4  -61.68667 17.02444
5  -61.72917 17.60861
...


Now, I need in the dataset only these observations (districts) which are in
the radius of 100 km around the first one (-61.68667 17.02444). How to do
this? Can you help me?

Many thanks... 



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