GE topography accuracy & alteration of contours

[pgarchnz]

I am wanting to model a large area of land about 10km square. I presume the only way to do this is to still stitch smaller grabbed pieces together?

Does anyone know how accurate the topography of GE is?

Is there a way to alter the topography in the GE view if an excavation is a part of the project?

Thanks in advance.