Objectives Obtaining valley (ridge) lines from contour data has important practical significance and high production value for terrain analysis, slope aspect calculation and reservoir area designing. In view of the fact that current contour-oriented geographic lines extraction mostly adopts feature point direct-connecting method, and the extracted geographic lines are prone to spatial logical conflicts with contour lines. Meantime, in areas where the contour lines are more curved and more complex, the extracted valley (ridge) line does not conform to the bending characteristics of contour lines, and even deviates dramatically in certain local areas.

Methods Therefore, after overall shape and local shape characteristics of the contour line are analyzed sufficiently based on Tobler's first law, a novel methodology of extracting geographic lines from contour data based on Morphing transformation techniques is proposed, which takes the contour line on both sides of a valley (ridge) into consideration, as well as constraint of feature points’ location. First, a contour tree is constructed based on the elevation and hierarchical relationship of contours, and the contour curve segmentation is carried out according to the concave and convex attributes of nodes on the contour line. Then, the terrain feature points are extracted efficiently and accurately. Second, each contour line is split into two parts with the terrain feature points, whereafter, an optimal correspondence of the nodes on both parts of a contour line are computed. Third, with respect to the constraints of terrain feature points, the geographic lines are extracted by performing Morphing transformation with a gradient shifting distance value.

Results The experimental results show that the methodology extracts complete geographic lines accurately, which are consistent with the actual terrain and take into account the overall shape and trend of contour lines.

Conclusions The geographic lines extraction based on Morphing transformation techniques method can take into account the overall shape and trend of contour lines and unique shape at feature points to extract complete geographic lines consistent with the actual terrain, and it is robust. When the local terrain changes are complex, the contour shape is also complex. At this time, the allocation of Morphing transformation value for parameter $t$ considering more factors will be involved, which is also the content to be further studied in the follow-up work.