Maps are a geographer's tool for representing the three-dimensional real world in two dimensions. While flat maps necessarily distort features like shape, size or direction to varying degrees, cartographers use map projections to minimize these distortions. Common projections include cylindrical, azimuthal, conic, and pseudocylindrical projections. Topographic maps produced by the USGS precisely represent land elevations and relief using contour lines. However, what and what is not represented on maps can reflect biases and influence how we perceive the world.