Textures on joined objects

Hi All,

I’m trying to create a 3d map using DEMs and map an image on top of it.

The DEM came is several tiles which I converted into .obj using AccuTrans, I then imported them into blender, layered them out correctly. Then joined into a single object.

I removed all the materials, and created a new one with an image for a texture.
Image Mapping Extension = Clip
Mapping Coordinates = Generated
Projection = Flat

As you can see in this sample render, towards the bottom of the large island there is a sharp line where two of the original tiles met. Why are these two tiles that are now joined treat the texture differently?

Secondly on the image map that I am using I have draw several coloured line to help with orientation, for some reason the horizontal lines dont appear on the render.

Any ideas as to what is happening here?

Blender 2.69
MacOS 10.6.8


Render Sample

Image Map