OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. ~ William & Flora Hewlett Foundation
For a more comprehensive overview on OER, see:
This is a sample of several OER Land Surveying & Geomatics textbooks and course materials available. Additional OER may be found by searching the resources listed on the ACC Libraries primary Open Educational Resources Guide.
Sample 2: Topographic Change (OER Commons)