Hawth's Analysis Tools for ArcGIS

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 Summary

CREATE MINIMUM CONVEX POLYGONS TOOL
Input: point layer (typically animal locations)
Output: polygon shapefile
Features:

  • use only selected input features option
  • check for coordinates of 0 (x or y) and ignore those records (common in GPS collar data)
  • create multiple MCP's based on the unique values in a field in the attribute table, and assigns that value to the resultant polygon
  • any of the above three features can be combined simultaneously
  • an Area field is automatically added to the MCP layer
  • the MCP layer is automatically added to the ArcMap Table of Contents

Limitations:

  • no polygon can be created with less than three input points (this scenario sometimes arises when using the unique values of an attribute)


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 Example

Figures A & B. In the most simple example, a shapefile containing a set of point locations (A) is used to create one MCP polygon (B).

Figures C & D. In this case, the input data is coded with an attribute that splits the data into three categories (in this case season) as shown in C. An MCP polygon is created for each of these three categories (D) using the option "Create a different MCP for each unique value in this field".

Figures E & F. Finally, in the most complex example, all three options are used: only the selected input points are used (cyan in E), the program checks for coordinates with an x or y of 0 and omits them from the MCP generation, and the season field attribute splits the points into three categories. What results are two polygons (F). Note that the third (green) category is not represented in F because there are less than three of these points selected in E (a minimum of three points is needed to create a MCP).

 Help

Getting started. A point layer must be loaded into ArcMap in order to use this tool. If your point layer is an event theme and you are experiencing problems, try making it a shapefile and trying again (right click on the event theme in the Table of Contents, select Data, then Export Data).

Referencing unique values. When you select a point layer from the drop-down list, the fields from that layer are added to the lower drop-down list that allows you to select a unique value field (to generate multiple MCP's). Only fields that can contain categorical data are added to this list (e.g. no fields with floating point numbers are included). A field of the same type and size is added to the MCP polygon layer and the unique values are copied for each MCP. This enables you to Join the original point table and the new polygon table together if you need to.

Using only selected features. When you select a point layer from the drop down list the program checks to see if there are 3 or more selected features in that layer, and enables the "Use selected features only" option. If there are less than three features selected, the option remains disabled (note that you need at least three points to create a polygon).

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