31. The contour line image would need to become semi-transparant so that the map image shows through. 32. Elevations followed by a plus sign ( + ) were interpolated using topographic map contour lines . 33. It has been the primary method to plot contour lines on topographic maps since the 1930s. 34. The contour lines played a linking role. 35. But the region had remained unproductive before the innovation of building a segi along a contour line . 36. A level set is a generalization of a contour line for functions of any number of variables. 37. Part of the track follows the 750m contour line and there are rest benches along the way. 38. Because gas floats above oil, the thin red contour line marks the gas / oil contact zone 39. Because gas floats above oil, the thin red contour line marks the gas / oil contact zone. 40. Then I go in with a finer brush or pen to add details, texture or contour lines ."
