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!