I'm not too sure what you mean, but hopefully this will help;
Select your object and highlight the square where you want it to be over (or under). Hold down the shift key when you are placing an object (scenary, start of a ride, path, etc...) and then when you move your mouse your object will go onto a vertical axis. When you think your object is at the right height click the mouse button and it should be placed in mid-air (rotate the camera to check this).
If you want to stack walls on top of each other (to get a vertically straight wall) select your scenary object and highlight the square you want to be over and hold shift and place the first piece on the ground (or in mid-air if desired) and continue to hold the shift key, move your mouse up a bit and another white hologram of your piece should appear.
If your still having problems you could try raising the ground level placing your object, lower ground level and continue.
The first method is far better though. Hope I helped