There are two ways to do this. One way is using the Snap feature. This is the fastest way when your using the same or similar in size objects (fences, walls, sand bags...) to make a long string of them.
Snap It! Snap it good!
Check the Snap box on the main screen.
Set your Grid to a usable size (8m or smaller depending on what item your using)
Insert your Item. (If your zoomed in close enough then you should see your object align on a grid intersection, usually at the center of the object.)
If needed, adjust the attributes (pitch, heading, z) of that item and "right click/copy" it.
Insert/paste next item.
If you inserted a new item, you'll probably have to Move it.
If you pasted it, you should see the item aligning with the grids as you move your cursor.
Butt the items together and set into place.
Repeat 5-6 as needed.
To speed up the process even more, after placing three or four items, highlight all the pieces and copy them. Repeat 5-6 as needed.
Pixel Perfect
This method is a lot more time consuming for inserting numerous objects of varying sizes but it will guarantee seam less transitions.
Place two items as close together as you can by any means you can.
Center your view on the seam by holding down the ctrl button and right clicking.
High-light one of the objects.
Zoom in
really
REALLY
CLOSE
Use the arrow keys to move your object 1/256 of a meter or 0.391cm at a time. Holding down the CTRL key and pressing the arrow moves the object 1m.
Butt the items together and set in place.
Repeat 1 - 6 as needed.
Walls are easy though. When it become a problem is when your trying to align complex items. Then you have to find the index marks. NL very nicely and probably for some sanity sake put them in all the complex items. See examples below.
If the mouse over doesn't work for you
(I'm sorry because it's pretty cool, and I'm still trying to figure out
why it doesn't work for me in Netscape) then you can click HERE to see what your missing.