Question Terrain offset when using heightmap

Hi,

I'm new to the unity and just trying to use heightmap to create a terrain as a background of my scene. My heightmap is a 500x500 raw file and when I create the terrain, I find there is a offset in my terrain.

I have some objects in the scene but they always hit my terrain at first, then I realize the offset when adjusting my camera position. Right now, I have to set the camera at (0,0,-10) to get match the origin of my terrain.

I guess there is something related to the heightmap resolution as I find the only option might be 513x513 pixels for my case but is anyone know about that?

Thanks,
Weiping