Check elevations before merging !
One caveat to beware of...Check the height of each map before merging. Most are built at "0" elevation but, many are at "real" elevation which could be hundreds of feet. Obviously, you would have trouble merging if the elevations were that much different.
Check elevation by opening the top tab in surveyor and clicking the "get height" button. It will read out the height at the location of the surveyor compass.