A variety of string and torpedo levels or outside fence and roofing applications

Torpedo levels are carpentry tools that are utilized to ensure that a surface or angle is level or plumbed properly. As one of the modern incarnations of the spirit or bubble level, the torpedo level comes in handy for everything from hanging a picture to checking the alignment of joints during the construction of a wall or floor. While traditional models utilize a glass vial or bubble to determine the evenness of the slope or angle.

A string line is one of the oldest and most basic of hand tools and is commonly used in masonry and carpentry to help the user create a straight line between two reference points. The shortest distance between two points is a straight line. A string line enables the user to mark the shortest distance, which helps to avoid sags or tilts in a structure. A tool that is hundreds of years old, a string line is typically either used in conjunction with a plumb bob, commonly referred to as a plumb line, or is a chalked string line.

String levels are fairly accurate for basic stuff like setting posts for a deck or leveling a driveway or something along those lines where minute accuracy is not required.  For a fifty foot line you would have to stretch the line pretty tight for it to be effective.  You need to have your line tight enough that when the level is hanging on it the string doesn't sag.  To use the level, just hang it on the string and when the bubble in the level is in the center, your line is level.  You can put the string level pretty much anywhere on the string but I recommend at least a third of the way between points.  If you are too close to an end the opposite end might be affected since the accuracy will be diminished on the longer string side.  Once you have established that your line is level you can slide the level down the string to check it in a couple of spots to make sure