Why is my “80 Acres” really only 79 acres?

When the original government surveyors began the task of laying out (surveying) the public lands of Oklahoma, one of their instructions was to make each section as close to 640 acres as possible.

excerpt from the BLM manual of Survey Instructions

Chapter 1-21. The basic provisions require that the public lands "shall be divided by north and south lines run according to the true meridian, and by others crossing them at right angles, so as to form townships six miles square;" that "the townships shall be subdivided into sections, containing as nearly as may be, six hundred and forty acres each;" and that "the excess or deficiency shall be specially noted, and added to or deducted from the western and northern ranges of sections or half-sections in such townships, according as the error may be in running the lines from east to west, or from south to north." The system of rectangular surveys fits the basic requirements to the curved surface of the globe.

To obtain 640 acres, they would attempt to make the sections 80 chains in length on each side. One (1) chain equals 66 feet so 80 chains equals 5280 feet. A section that is exactly 5280 feet square would contain 640 acres.

(5280’x 5280’=27,878,400 square feet/43,560=640 acres) <------ MATH

The curvature of the earth did not allow for the north side of the section to be the same width as the southside of the section, typically resulting in a distance of less than 80 chains.(Click here to read more about Oklahoma Land Descriptions)

Surveying in the late 1800’s in Oklahoma was also not a walk in the park. Distractions such as hostile native Americans, rough terrain, inclimate weather and distance measuring equipment that was somewhat lacking, All led to less than perfect results across various parts of Oklahoma.

The compasses used by early surveyors were fairly accurate devices and produced some results that would rival modern survey methods. The Gunter chain used to measure distances, was not as precise, especially across rough terrain with much elevation differences. Don’t get me wrong, they did a great job with what they had to work with.

Curvature of the earth, less than perfect measurements in the original surveys, better measurement techniques used today and corner monuments that have since been replaced, account for many sections that do not (likely never did) contain 640 acres EXACTLY.

Subsequently, the original surveyors divided these sections into portions which contained an aliquot part of the original acreage, whatever that might have been.

For example a Quarter Section is commonly believed to contain 160 acres. Half of that is sometimes referred to in the vernacular as “an 80”. If the original section was, in fact 5280 feet square the subsequent “Quarter” section would have been 160 acres but what if it only really contained 635 acres? The quarter section would then only contain 158.75 acres and the resulting “80” would really only contain 79.38 acres. Make sense?

A lot of deeds have been written through the years with the acreage being reported on hypothetical measurements and not actual measurements. So if you have a deed that states “…contains 80 acres more or less”, you should not rely on that acreage unless it has been surveyed and confirmed by a Professional Land Surveyor