HORIZONTAL. The most common instruments of this type currently used are compass-clinometers from Suunto or Silva.Cross staff: The cross staff is used to set out the perpendicular directions for offsetsPrism square: An optical square is a hand instrument used by surveyor's to lay off RIGHT angles that are multiples of 90° or of 45°. They normally comprise of TWO optical GLASS penta PRISMS in a sturdy housing. Used for placing points on a line, offset measurements, setting our curves or determining horizontal plans.