How to optimize old meshes? I am working with a very old mesh; actually it is a terrain. I selected a part of it, where the surface of water is. You may think it does not look too good and you may be right as well.



The surface is like two surfaces on the same plain, so when it comes to render, it looks like this (I highlighted the area of overlap):



I used disolve faces on this, where this texture was used, but as you may see, it is still two surfaces:



Sure I could remove faces easier now, but I thought there may be a better way for this. I want to:


merge the surface into one, preserving the UVs OR 
delete one surface (problem here: bigger parts, what I would love to keep are
sometimes hiding the scattered surface, and I can not select that, to
delete) OR
optimize this magically, to "just make it work"
You can use the Decimate modifier to optimize the mesh and it should preserve UVs.

Select your object in Object mode

Add Modifier Decimate

In the modifier settings, select Planar and set a low angle limit (1 degree is enough if your mesh is truly flat).


Apply the modifier when you're done. Check the face count to see how many polygons you have now.


Note: The Decimate modifier affects the whole object