It seems a universal interest of go players to want to know how their ranks compare between online servers and face to face.

The linked post (Chinese) from the Baidu weiqi BBS has an interesting analysis comparing Tygem ranks with Chinese adults ranks.

Tygem 1 dan = Chinese 1 dan (a weak rank, a catch all really)
Tygem 2 dan = weak Chinese 2 dan
Tygem 3 dan = weak Chinese 3 dan
Tygem 4 dan = Chinese 3 dan
Tygem 5 dan = Chinese strong 3 dan
Tygem 6 dan = Chinese strong 4 dan
Tygem 7 dan = Chinese 5 dan
Tygem 8 dan = Chinese strong 5 dan
Tygem 9 dan = pro or 6 dan

A few of the interesting comments. The OP felt that below Tygem 5 dan, the difference between online ranks was about a half dan. He commented that the difference between Tygem 4-5 dan was a relatively small one, while the difference between 5-6 dan was significant. The gap between 8-9 dan is the largest of all.

I am only 4 dan on Tygem, but in my experience this post is quite accurate.

What is the correlation between tygem and wbaduk?
I am kgs 2-3d and weak-average 6d on wbaduk

I can only say anecdotally. At my level, I am 4D on both servers, but I feel wbaduk is softer.

I can't seem to access the linked post, but that seems a bit overoptimistic of offline ranks. I think it is sort of an ideal mapping, but in reality offline ranks can be much softer.