When you create a model from contours and then generate contours from this model, in some cases, AutoCAD Civil 3D may not generate the same set of contours.
In the most common situation, surface data is missing in either a low or high area, such as near the top of a hill, at the bottom of a valley, or on the edge of the site.
To correct this problem, place spot elevations or another contour line near the place where the surface data is missing. For example, if a contour line is missing along the top of a hill, place a new contour, breakline, or spot elevation at the estimated maximum elevation of the hill. The recommended practice is to place any interpolated data, points, or contours on a separate layer.
If you create a surface using only contour information, you may need to place breaklines or spot elevations in areas where the contour line changes direction drastically. Areas where this is likely to happen include: a crowned roadway, a swale, or a ridge. In these places, contours tend to triangulate onto themselves. In most instances, the Minimize Flat Areas operation resolves this problem. For more information, see To Minimize Flat Areas in a Surface. In severe situations, you may need to add additional point or breakline data.
For best results, always follow the suggested order of operations for adding contour data to a surface. See About Contours.