Stadiametric rangefinding Stadiametric rangefinding , or the stadia method is a technique of measuring distances with a telescopic instrument . The term stadia comes from a Greek unit of length . Stadiametric rangefinding is used for surveying and in the telescopic sight s of firearm s , artillery piece s , or tank gun s , as well as some binoculars and other optics . It is still widely used in long - range military sniping , but in many professional applications it is being replaced with microwave , infrared , or laser rangefinding methods . Although much easier to use , electronic rangefinders can give away the shooter ' s position to a well - equipped adversary , and the need for accurate range estimation existed for much longer than electronic rangefinders small and rugged enough to be suitable for military use . Principle . The stadia method is based upon the principle that in similar triangles homologous sides are proportional . This means that , for a right triangle with a given angle , the ratio of adjacent side length to opposite side length ( see tangent ) is constant . By using a reticle with marks of a known angular spacing , the principle of similar triangles can be used to find either the distance to objects of known size or the size of objects at a known distance . In either case , the known parameter is used , in conjunction with the angular measurement , to derive the length of the other side . Since a radian is defined as the angle formed when the length of a circular arc equals the radius of the circle , a milliradian ( sometimes called a mil ) , is the angle formed when the length of a circular arc equals 1 / 1000 of the radius of the circle . An object 5 meters high , for example , will cover 1 mrad at 5000 meters , or 5 mrad at 1000 meters , or 25 mrad at 200 meters . Since the radian expresses a ratio , it is independent of the units used ; an object 6 feet high covering 1 mrad will be 6000 feet distant . Surveying . Stadia readings used in surveying can be taken with modern instruments such as transit s , theodolite s , plane - table alidade s and level s . When using the stadia measuring method , a " stadia rod " is held so that it appears between two " stadia mark s " visible on the instrument ' s reticle . The stadia rod has measurements written on it that can be read through the telescope of the instrument , providing a known length for the distance calculations . An instrument equipped for stadia work has two horizontal stadia marks spaced equidistant from the center crosshair of the reticle . The interval between stadia marks in most stadia instruments gives a " stadia interval factor " of 100 . The distance between the instrument and a stadia rod can be determined simply by multiplying the measurement between the stadia hairs ( known as the stadia interval ) by 100 . The instrument must be level for this method to work directly . If the instrument line of sight is inclined , the horizontal and vertical distance components must be determined . Some instruments have additional graduation s on a vertical circle to assist with these inclined measurements . These graduated circles , known as " stadia circles " , provide the value of the horizontal and the vertical measurements as a percentage of the direct stadia measurement . This system is sufficiently precise for locating topographic details such as rivers , bridges , buildings , and roads when an accuracy of 1 / 400 is acceptable . The stadia method of distance measurement is primarily historical for surveying purposes , as distance nowadays is invariably measured by electronic or taping methods . Total station instruments do not have stadia lines marked on the reticle . Traditional methods are still used in areas where modern instruments are not common or by practitioners whose primary task is not surveying with sophisticated instruments .