Randomly placing points with Hawth’s Tools
Problem: You need to position a specific number of points in random locations within a polygon
Randomization is sometimes needed for environmental sampling in the field. In my case I needed to produce maps of random soil sample locations within property boundaries. There is no easy way within ArcGIS to accomplish such a thing. Luckily, it is one of the many capabilities included with the fantastic Hawth’s Tools plugin. This plugin is available for free at http://www.spatialecology.com/htools/
Let’s put it to work in an ordinary neighborhood such as the one pictured below.
First, download Hawth’s Tools and run the setup package. Then, add it to your ArcMap toolbar by right clicking anywhere in the empty space, selecting Customize, and checking on Hawth’s Tools in the list of available toolbars. Before you run the tool, zoom in on your property and select it (indicated by the cyan outline). Once the parcel polygon is selected, chose Sampling Tools –> Generate Random Points from the HawthsTools drop-down menu.
A dialog box will come up in which you’ll need to specify:
- The Reference layer (layer within which you want to place the points). In this example that’s OwnerPLY, my parcel boundary layer. Check “Use selected features only” to limit the output to the single property you selected previously.
- Any layers where you don’t want points to show up. In this case, it isn’t feasible to take soil samples under the house, so I’ve told the tool to prevent points from occurring in BldgPly.
- Minimum distance between points, if you want that. I didn’t specify it here.
- The number of points to generate. Five in this example. Note: stratified sampling would create your specified number of random points in every parcel polygon.
- The name & location of your output shapefile.
The tool will generate five points following all those rules. It will add an ID column to the attribute table, filled with zeros. It’s an easy matter to change those zeros to numbered point IDs. Then, if you run your new shapefile through ArcToolbox’s Add XY Coordinates, and turn labels on for the ID field, you end up with this.
Give the field workers that map and table of coordinates, and they have what they need to go out and collect those samples. By the way, Hawth’s first Sampling tool: Create Random Selection, could be used to randomly select one of these 5 random points, for double duty.