Chaining, Precision and Boundary Analysis

Chaining, Precision and Boundary Analysis

Probably some of the young surveyors are thinking what is this guy talking about! How are these three words related! I am in the last or close to the last generation of surveyors that spent any significant amount of time using a surveyor’s chain (No, not the Gunter’s chain – that was before my time). The early distance meters became affordable and in use in the late 70’s early 80’s. Up to that time, our traverses were chained with usually a 200 foot steel tape and angles were wrapped four times with a 20 second transit. With average ground conditions, no temperature corrections, and no tension correction, we were successful in obtaining a precision of 1:3,000 to 1:5,000. This means that on a mile long line we might be 1.3 to 1.7 feet different from what distance you would measure today with modern equipment. So when you run across these differences don’t malign us old surveyors, we were doing good work with the tools available!

With recognition of the precision of our work and the resulting accuracy, it is important to consider the time that original deeds were surveyed and the impact of the tools used at that time to measure and/or monument tracts. With these variances in measuring accuracy, you should recognize the importance of monumentation and why monuments rate higher over the called distance in the “Dignity of Calls”.

Now, when you are comparing your current electronically shot data to a deed from let’s say in the 60’s, and you find a difference for 1.5 feet on a 5,000 foot leg, would you expect it to be longer or shorter? Well, read your deed! If the distance was between two found iron rods, you would expect the more accurately shot distance to be shorter because the 1960 surveyor’s chain was usually shorten by sagging and slight bows in alignment. This shortened chain resulted in a longer distance measured between to fixed monuments. If the distance in the deed was to a set iron rod, you would expect the more accurately shot distance to also be shorter for the same reason. The 1960’s surveyor’s chain was shorten by sagging and slight bows in alignment so when he set the iron rod at 5,000 feet it was actually set short.

Now comes the decision time in your analysis. Of course if the called for irons are found and identified you would hold the monuments and show your measured distance being shorter than the called distance in the deed. Sorry if this screws up your best fit analysis (a later story) but it gives you a comfortable feeling as to why the distance is shorter! What happens if one of the monuments is missing and you have to reset it. Would you set it at the called for 5,000 feet or would you adjust it to the precision of the tools used to set it originally which would mean you would set it shorter than the called for 5,000 feet. Interesting question, huh? Of course other data might help you make that decision such as a closing angle, or a well monumented line that the end point should fall in. If not you might be able to determine the average precision from other measured lines in the survey and use that ratio to shorten the called for 5,000 feet and set the corner where the original surveyor set it.

The answers are not always easy or clear in boundary analysis! Just don’t lose site that your primary duty is to retrace the footsteps of the original surveyor! Yes, usually the original surveyor was out there dragging a steel chain and not standing behind a total station or sitting at a desk rotating data points in a computer!