The data file that you upload in Gabmap does not necessarily have to contain information on some specified linguistic variables. You can choose to upload difference data instead, that is, linguistic distances between sites. When you upload difference data in Gabmap, the linguistic distances can be measured in any way that you like to use. In this case, your data table contains the pair-wise linguistic distances between all pairs of sites.
The table is symmetrical along the diagonal, with zeros on the diagonal. No negative values are allowed and there can be only one single value in each cell. Empty cells are not allowed.
Example of difference data:
| Amsterdam | Groningen | Utrecht | |
| Amsterdam | 0 | 1.42 | 0.63 |
| Groningen | 1.42 | 0 | 1.38 |
| Utrecht | 0.63 | 1.38 | 0 |
You can upload a difference file in RuG/L04 format, too.