Your landing zone is determined by purchased land. It's centered left and right, and up and down (no matter how you look at the NSEW debate :)) on the outermost piece of land you bought. So, if you buy another space moving out east, then your landing zone will move slightly to center your LZ in the middle.
This is why you see a lot of people buy one run of land off to an edge and do nothing with it until they start to fill out the rest. This is so they can have their LZ centered right off the bat, and design around it.
Hope that helps. :)