Overview 

The Mark Centerline tool generates a precise centerline between two selected polylines. These polylines typically represent the left and right edges of linear features such as roads, rivers, canals, or corridors. The centerline serves as a backbone alignment, crucial for downstream processes like profiling, cross-section generation, corridor modeling, or GIS data analysis. 

Manually creating such a centerline is error-prone, especially when the edge curves differ or have varying segment lengths. This tool automates the process using geometric interpolation. 

Key Use Cases 

• Road and Highway Design: Define horizontal alignment from road shoulders. 
• Hydraulic Modeling: Extract thalweg (deepest path) from riverbanks. 
• Corridor/Civil 3D: Use centerline as control baseline for assemblies. 
• Mapping/GIS: Convert area features (e.g., riverbanks) into linear features. 

Workflow 

1. Launch the Command by typing command  GCL_SP or from the SPCAD ribbon  

2. Select First Polyline 
   Click the first polyline (e.g., left riverbank or road edge). 

3. Select Second Polyline 
   Click the second polyline (e.g., right riverbank or opposite edge). 

4. Result 
   The tool interpolates between the two selected polylines and creates a centerline. The new polyline is placed in the drawing as a smooth path equidistant or averaged from both edges. 

Technical Details 

• Supports 2D polylines only. 
• The algorithm samples vertices or segments from both polylines and computes midpoints or average normals between corresponding sections. 
• The centerline maintains overall curvature fidelity to both inputs. 
• Output is a lightweight polyline, ideal for downstream use as an alignment or path. 

Tips 

• Ensure both polylines are oriented consistently (start to end) for best results. 
• Works well even if polylines are slightly uneven in segment counts. 
• Use JOIN or PEDIT to unify edges before centerlining if they consist of multiple segments.