Hawth's Analysis Tools for ArcGIS

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 Summary

CREATE RANDOM SELECTION TOOL
Input: any feature layer (points, lines polygons)
Output: a random selection of features
Features:

• optionally marks the randomly selected features with a new field (RNDSEL)
• allows the selection of an absolute number of features, or a percentage of features

Limitations:

• any pre-existing selection of features is lost
• if the RNDSEL field already exists, it is overwritten
• note that the selection process is not weighted according to the size of the feature (every feature, no matter how large or small, has an equal chance of selection

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 Example

Figures A & B. In this example, a point feature layer has been used as the input layer, and 5% of the total number of points has been randomly selected.

 Help

Getting started. At least one feature layer must be loaded into ArcMap in order to use this tool. If your feature layer is an event theme (points) 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).

Add/update a RNDSEL field. The program will create a new field called RNDSEL and for all the randomly selected features will assign a value of 1. All unselected features have a value of 0. If the field already exists it is deleted and re-created. If you receive an error message regarding permissions or access to the table, it is likely that you do not have permission to modify this feature layer, or that it is already being edited elsewhere.

Saving the selection as a new feature layer. When features are selected you can easily save those selected features to a new feature layer by right clicking on the layer in the ArcMap Table of Contents, selecting Data, then Export Data.

Switching the selection. To switch the selection (all currently unselected features become selected and vice versa), open the attribute table, click on Options, and select Switch Selection.