Model falls through heightmap terrain

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Hi,

I'm using gazebo 1.5 on gentoo linux. I installed it from dev branch about a month ago.

When I create a terrain using a heightmap and try to insert a model onto it, the model falls through the terrain and out of sight. I've tried it with the pioneer robots and with various other models - same result. However, with the Brickbox, Bookshelf, and Cabinet, they stay where I put them. I don't know how to solve this problem.