Establishing farmland geometric boundaries is a critical component of any assistive technology, designed towards the automation of mechanized farming systems. Observing farmland boundaries enables farmers and farm machinery contractors to determine; seed purchase orders, fertiliser application rate, and crop yields. Farmers must supply acreage measurements to regulatory bodies, who will use the geometric data to develop environmental policies and allocate farm subsidies appropriately. Agricultural parcel delineation is the process by which land parcels are geofenced into their individual field geometric boundaries. The problem scenario addressed in this paper is to automatically discern when a tractor towing a) a silage baler b) a mower implement, moves between adjacent fields while the towed implement remains active. We present a novel method for automated agricultural field segmentation. When towed implements are considered, their power drive (usually from a Power-Take-Off shaft) can often remain active when the machine operator traverses between multiple sites that are in proximity. The solution herein can automatically discern between adjacent sites, solves for field geometric boundaries and applies a job context to machinery operational data on a field-by-field basis. The method uses historical coordinate data, coupled with a PTO activation signal, recorded on agricultural harvesting implements over several months. The algorithm shows how adjacent fields in a land parcel, which cannot be distinguished using machinery sensors, can be spatially segmented with an accuracy of 92.6% on a baler dataset and 80.2% on a mower dataset. Data was aggregated from a full season of silage baling in East Galway, Ireland. The method is shown to outperform the state of the art within the literature at a fraction of the cost, and the technique can be applied to several agricultural implements or self-propelled machines. No aerial surveillance or undesirable land traversal is required to produce the results shown, offering an efficient, environmental and cost-effective geofencing method to agricultural stakeholders around the globe.