This study investigates the strategies and assumptions that college students entering an introductory physical geology laboratory use to interpret topographic maps, and follows the progress of the students during the laboratory to analyze changes in those strategies and assumptions. To elicit students' strategies and assumptions, we created and refined a topographic visualization test that was administered before and after instruction to 26 students during the first semester of the study and to 92 students during the second semester. To more deeply understand how students think about and conceptualize topographic maps, we focused on eight individual students who were interviewed about their pretest and posttest answers as well as videotaped during three laboratory sessions. We found that even students who claim never to have worked with topographic maps often perform impressively on their pretests by making useful assumptions about symbolic topographic information. Some students, however, begin with less productive assumptions that may be unfamiliar to some instructors (e.g., thinking that the spacing of contour lines indicates elevation instead of slope). Initial success should not be misinterpreted, however, as an integrated understanding of topographic maps. Only in posttest interviews do most students express explanations integrating multiple normative assumptions. In addition to highlighting the strategies and assumptions that college students use to interpret topographic maps, we outline the implications of these findings for the design of learning objectives, curricular activities, and assessments for topographic lessons in introductory college geology courses and the training of future geoscientists. (Contains 3 tables and 6 figures.)