How do I unify lighting across meshes stitched together?

I am creating a terrain my stitching meshes together, but am unable to get the lighting to blend seamless across the meshes (see image).

Any idea why?

Make sure your vertex normals on the stitched vertices are the same for both meshes. You could simply combine them by adding the two normals together and renormalize the result. This would be used for both vertices. Of course at a corner where 4 meshes meet you have to combine the 4 vertices that meet at that corner.

Doing that will make the surface continuously.

In addition you might want to apply some kind of smoothing around the stitched edges before you unify the normals. That way the the sharp differences will disappear.

For this you would simply take the difference in height between the two vertices and join them at half the distance. This height reduction should be propergated with a smoothing factor along the few next vertices. This will raise / lower the entire area around the edge to get rid of the seams.