Understanding Survey Measurement Terms

Have you ever tried to read an old survey or map without success? Terms like ‘rod’ or ‘township’ were once commonplace, and indeed are still in use in specific applications, but most people have no idea what they mean. How can you visualize a property without having a basic understanding of these distances and measurements?


Today’s surveyors continue to use measurement systems that were developed by long-ago surveyors. A rod (sometimes known as a pole) is a measurement of 16.5 feet. Each rod is comprised of 25 links. A two-pole chain is twice the length of a rod, or 16.5 feet. Likewise, a four-pole chain, which is also known as a surveyor’s chain, measures 66 feet (or 100 links). Originally, these measurements were based on the equipment used by surveyors. The chain consisted of 100 links, each measuring 0.66 feet, for the total length of 66 feet. If a distance on a plat map is represented in chains (usually abbreviated Ch), multiply by 66 to determine the number of feet.


The surveyor’s chain was was devised in the seventeenth century by an English astronomer so that ten square chains are equivalent to one acre. The pole, or rod, was a wooden pole used for survey measurements. Abbreviated P, it was also known as a ‘perch.’ The more modern term for this piece of equipment and its associated measurement is a ‘rod,’ although the actual wooden rod has not been used by surveyors for over 200 years. Still, the particular length of the rod, or 16.5 feet, had an enormous influence on surveying and land development. According to folklore, the distance of the rod was established in the sixteenth century as the combined length of the left feet of the first 16 men out of church one Sunday. And yet, references to a pole or perch measuring 16.5 feet can be found as early as the thirteenth century.


Survey measurements are also commonly stated in miles or feet. This measurement system is more easily understood by people today, but even this system can be converted into chains and rods. For example, a quarter mile measures 20 chains or 80 rods. One mile, or 5280 feet, can also be stated as 80 chains or 320 rods.


Especially when reading plat maps, you may run into the section and township system of measurement, which is related to the property’s measurement in acres. In this system, one township contains 36 sections. Its total area is 36 square miles. Each section within the township is one square mile, or 640 acres. The sections can be further divided into 1/2 section or 1/4 section.


Today, land measurements are generally in acres. Each acre is equal to 43,560 square feet, or 10 square chains, or 160 square rods. Still, many people cannot visualize the size of an acre. A parcel of land exactly one acre in size and perfectly square measures 208.71 feet along each side. A rectangular acre with a width of 100 feet would measure 435.60 feet long. Each square mile has exactly 640 acres, which is also a section under the township system. Although older surveying measurement systems use seemingly odd distances, most can be easily converted into acres.


Still, there are many variations that may show up on surveys or maps, especially those from centuries past. Even in the relatively standardized acre, there are still variations, including the Scottish acre and the Irish acre, which measure 1.27 and 1.6 English acres, respectively. Regional terms abound, from the Morgen (used in Germany to represent the amount of land that could be plowed in one morning) to the Colpa (an Irish term representing the amount of land required to support a horse or cow for a year).