# Using Alpha Shapes in Surfer

Surfer has a new object type: Alpha shape. This new feature creates a polygon boundary around a set of selected data points. You can control the tightness of the polygon around the points by entering an *Alpha value*. The larger the *Alpha value *is, the more convex the polygon around the points will be. The smaller it is, the more concave the polygon around the points will be. The numerical value of the *Alpha value* will depend on the data.

To create an *Alpha **Shape* see the steps below:

- Select 3 or more points in a
*Base(vector)*layer, or points free floating in the Plot document. - Click
**Features | New Features | Alpha Shape.** - In the
**Alpha Shape**dialog, enter an*Alpha value.*Remember, the larger the value the more convex the polygon will be, and the smaller the number the move concave the polygon will be. - If you'd like to see what the resulting polygon will look like, click
*Preview.* - Click
*OK.*

**Additional information and tips:**

*Alpha Shapes*can only be created from*Point*objects in a*Base(vector)*layer or free floating*Point*objects in the Plot document. Three or more points must be selected.- Note that
*Alpha Shapes*can create holes in the middle of data if there is no data there. - The
*Alpha value*is based on layer units (not map units nor page units) - Once the
*Alpha value*is large enough to create the convex hull, setting an*Alpha value*larger than that will not inflate the convex hull. From that*Alpha value*setting to infinity, all you get is the convex hull. No matter how large the*Alpha value*, the*Alpha Shape*won't extend past the points (past the convex hull). - If the default value isn't working for a particular dataset, a good starting point is an alpha value of 1/3 the shortest dimension extents (either X or Y), in layer units. For example, if the extents are 9 units in X and 15 units in Y, then a good starting alpha value might be 3 (1/3 of 9). From there, the experiment changing the
*Alpha value*to higher and lower values to get the desired shape.

See **Assign NoData outside of Alpha Shape** for instructions on how to Assign NoData using an alpha shape!

*Assign NoData with an existing alpha shape or directly in the Grid Data or Grid from Contours dialog! *

## Comments

0 comments

Please sign in to leave a comment.