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Can someone tell me how can I refine the mesh in Matisse, I made a model for a river so I need to make the mesh 15 m near the banks and 25 m in the inner part.

Hi
Depending how you created your mesh...
The regular way is to select the nodes in the inner part of your river and define a criterion of 15m (default value = 25) and compute a mesh with this criterion...
You could also try to do the inverse solution.
Select the banks and define a criterion of 25m with a default value of 15m

Hope this helps