This data set includes the median distance between the borders of the origin and destination country of migrants. It is calculated by first taking the minimum distance between the borders of origin and destination countries for each migrant, and then calculating the median of these minimum distances.
The number of international migrants and their origin and destination countries are based on the UN DESA International Migrant Stock data. The geospatial data for the borders of countries is retrieved from the Natural Earth project (https://www.naturalearthdata.com/downloads/110m-cultural-vectors/), specifically the admin 0 - countries and tiny country points datasets.