Create an alignment that follows the most logical path through a series of feature lines, COGO points, or AutoCAD lines, arcs, points, or blocks.
You can adjust the command to obtain the desired results based on the type and the accuracy of the source data.
For example, when you use COGO points as source data, AutoCAD Civil 3D determines the most logical sequence of the COGO points, and then reverses the order of any that do not correspond to the sequence of the other selected COGO points.
When two paths of roughly parallel COGO points are used, an AutoCAD spline (1, 2) is created through each path of points. The points from each spline are projected onto the other spline; points where the two projections meet are used to create the centerline (3).
Next, AutoCAD Civil 3D calculates curves (1) based on the sequence of the points and the specified Maximum Radius For Curve Detection setting. AutoCAD Civil 3D approximates the curvature by linking the sample points together with linear segments. To increase the accuracy when using either feature lines or AutoCAD entities, intermediate sample points may be either weeded from the original input data or created between each pair of sample points. When a sequence of points has a radius that is greater than this setting, the points will form a tangent (2). The apparent intersection of the tangents (3) are the points of intersection for the alignment tangents.
When the Create Spirals option is selected, AutoCAD Civil 3D creates a symmetrical pair of spirals (1) between each curve and its tangents. If spirals that meet the specified R/A value do not fit between the tangents (2) and the curve (3), then the curve radius is reduced (4) until a solution is found. When you specify this option without the Match Tangents option, the spirals will be symmetrical, but the solution may have greater error.
When the Match Tangents option is selected, AutoCAD Civil 3D creates an asymmetrical pair of spirals (1, 2) between each curve (3) and its tangents (4, 5). If a spiral that meets the specified minimum R/A ratio does not fit between a curve and a tangent, then the tangent (4) is moved toward the curve until a solution is found. The minimum R/A value enables you to restrict the spiral lengths; a higher value produces shorter spirals, while a lower value produces longer spirals.
To create an alignment by best fit
- Click Home tab
Create Design panelAlignment drop-down Create Best Fit Alignment 
. At the Command prompt, enter CreateBestFitAlignment. - In the Create Best Fit Alignment Dialog Box, Under Input Data, specify the type of data from which to create the best fit alignment.
- Next to Path 1, click
.
- In the drawing, select the objects from which you want to create the best fit alignment. Press Enter.
- If you wish to create the best fit alignment between two paths of objects, next to Path 2, click .
- In the drawing, select the objects from which you want to create the best fit alignment. Press Enter.
- If you chose either Feature Lines or AutoCAD Entities as the Input Data, click Weeding Options.
- In the Best Fit Alignment Weeding Options Dialog Box, specify whether to either weed extraneous vertices or insert intermediate vertices. Click OK.
- Under Best Fit Options, specify the following parameters:
- Maximum Radius For Curve Detection—Specifies the largest acceptable radius of curves that are created from the input data.
- Create Spirals—Specifies that spirals are created between the alignment tangents and curves. The spirals on either side of a curve are symmetric unless the match tangents option is selected.
- Desired R/A for Spirals—Specifies the target R/A ratio for spirals that are created.
- Create Spirals And Match Tangents Where Possible—Specifies that the solution will follow the tangents as much as possible. If no solution is available, then the tangents are moved toward the circles until a spiral solution is found between the tangent and the curve.
- Minimum R/A For Spirals—Specifies the minimum R/A ratio for spirals that are created. This value controls the length of the spirals:
- A higher value produces shorter spirals.
- A lower value produces longer spirals.
- Under Creation Settings, specify the following parameters:
- Alignment Name
- Site
- Alignment Style
- Alignment Label Set
- Layer
- Use the Show Report check box to specify whether to show a report of the best fit regression points.
- Click OK.