Spatial Analyst Viewshed function produces incorrect results

Description

The result from the Viewshed function is incorrect, it shows that many areas that should be visible are not visible.

Cause

This is a known issue with ArcGIS 8.1. The problem is related to the way input fields are handled when either the decimal delimiter is changed, or the regional settings are other than English (US) in the Regional Setting section of the system's Control Panel.

Solution or Workaround

1. Right-click the Observer point layer in the table of contents and click Open Attribute Table.
2. Click the Options button and click Select All.
3. Run the Viewshed function.
   
   The output should display those areas that are visible from each observer point.