With a lively mixture of scholarship, anecdote, and humour, this book tells the story of the shock troops of settlement - the surveyors of Ontario. It brings to light for the first time the crucial role played by surveyors in the history of the province. From the early cartographers and explorers such as Champlain and the Jesuits to today's specialists with their maps based on satellite measurements, the surveyor's role in the development of the province has been prodigious and largely unnoticed. Surveyor's set the stage on which we live out our lives. They laid out the townships, farms, cities and towns, roads, railroads, highways, and hydro lines. Early surveyors were men of enormous endurance. Robert Bell and his party faced starvation when they ran the first survey line south of what is now Algonquin Park in the winter of 1847-48. In 1872, Hugh Wilson walked 250 miles before he began his survey of one of the first townships south of Thunder Bay. Incorporated in 1892, the Association of Ontario Land Surveyors is one of the oldest professional bodies of its type. it is the self-regulating organization for professional land surveying in the province of Ontario. Its membership is comprised not only of the traditional cadastral, or legal boundary, surveyor, but also of hydrographers, photogrammetrists, geodesists. They Left Their Mark celebrates the centenary of the Association.