We were shooting grade lines on a property, and picked up the dwelling roughly using a stadia rod. When we plotted it, it was clearly out of square and the boss went nuts. Sent us back to re-shoot the house. Same results - the 25x50 house was about a full foot out of square.
I use my main halyard to get the mast square in the boat - measuring the distance to the toe rail at the chain plates. Since the halyard is a bit stretchy, I add a fish scale to the rig and actually measure the load on the scale to make the hook touch the rail. Get the same load on each side and things are good.