A knotted cord is an ancient device once used for measurement in surveying, before it was replaced by more accurate measuring tools. Evidence of such devices can be seen in records from several ancient societies, and the basic premise behind the knotted cord is still usable today. Some people use a version in quick and dirty measurements, or in educational units designed to inform students about surveying and the history of distance measurements.
The knotted cord consisted of a length of sturdy cord with knots at set intervals. Each knot marked out a unit of distance, such as a cubit in Egypt. Surveyors could use the cord in observations to take distance measurements by stretching it between points of interest. They also used a device known as a surveyor's cross to orient themselves and make sure measurements were taken appropriately. Surveyors needed to develop complex math formulas for some of their activities, like measuring distances that could not be covered on foot.
Egyptian tomb paintings show surveyors in action with the knotted cord and cross, providing information about how this tool was used in the ancient world. The Greeks also wrote about uses of the device. This evidence allows historians and researchers to learn more about ancient systems of measurement and surveying practices. This can be useful for activities like locating ancient sites or understanding the mathematics used in architecture and major construction projects.
This measuring device had a significant drawback; rope is flexible, and stretches while in use in addition to stretching out with time. As a result, measurements could be off considerably, depending on who performed a survey and the condition of the rope used. This was not satisfactory for measurements requiring a high degree of precision, and the knotted cord began to be replaced by the chain. Chains, while not ideal for measuring distance, do not give when pulled, and thus allowed ancient surveyors to take more precise and accurate measurements with fixed distances.
Knotted cords can be useful for quick frames of references on some projects. They are also useful in education, where hands-on activities with knotted cords and other ancient surveying devices can allow students to learn more about ancient cultures. Students may identify some of the flaws with the technology and can develop alternate proposals. This may help them understand the development of surveying science through to the modern day and the use of equipment like lasers for measuring with a very high degree of accuracy and speed.