Hi Yoz
But i don’t know the next steps, what are the best settings for unwrap tool ? I have made a first try, with setting fixed detail (1cm), then generated textures and exported OBJ file (1Go).
There are no best settings 
Simplification set to 15 million triangles is OK.
As for the settings you have used (fixed - 1 cm), it is mostly OK for GIS. But depending on the model size, the texture can be still pretty big. And that is then the problem regarding your GPU RAM.
Best if you set 1 cm, and set a 4-8k texture size, because 4-8k textures are easier to manage by your GPU and can work easier with it in an external application. This can be a problem as many external applications are not prepared to handle too big textures (8k+).
Set GUTTER to 1 (2 pixels among UV islands, more effective texture space use).
Try these settings and report back if it has helped to solve it…