How did mapping and measurement act as technologies of improvement? By the early seventeenth century, a professional class of surveyors had emerged in England, promoting concepts of geometric justice in print. They also integrated their services into crown estate management, promising to make forest and fen commons profitable. Much has been written about the spread of cartographic literacy among early modern elites, but relatively little is known about how local communities interacted with maps, surveys, and their makers. Fen projects brought the geometric techniques of improvement into contact with local customary knowledge. Examining maps and surveys of the northern fens across three centuries, this chapter traces how they were produced; how they re-organised social environments; and how fen communities negotiated these processes. It situates surveying as one epistemological tool within disputes over the redrawing of land and water in Hatfield Level, which involved legal officials, written documents, crowds, experiential knowledge, and oral testimony. Intended to author and authorise improvement, the boundaries that maps and surveys demarcated did not prove stable.