On program startup, the unique, site-specific map is displayed on the screen with a small cross in the map center representing the user's site.  To create this map, the user must enter the site latitude and longitude in the cities.txt file to establish the map center.  Then the user may enter as many surrounding city sites as desired. These two sets of entries in cities.txt file need only be done once. State borders are drawn using the file states.txt, which is a modified version of the state borders file created by NWS Lead Forecaster Mark Mitchell, who has kindly allowed us to use it to improve the map display; this file contains the latitude and longitude coordinates of the US state borders.  
 

Upon receipt of a valid signal level from the Interface, the four items of stroke data, E1, E2, NS, and EW are acquired, stored with time and date to a hard disk archive (if desired), and analyzed for possible plotting.  Based on the sign of E1 and on the ratio of E2/E1, the stroke type and polarity are identified. Only if the stroke is of the negative return (NR) variety (identified by E1 positive and |E2/E1| smaller than a user-set threshold, typically 0.5), and only if it has sufficient magnitude does further analysis occur; otherwise the algorithm simply waits for the next valid signal pulse from the Interface. For the valid NR strokes, the bearing and true magnitude are found trigonometrically from the NS and EW signals. The new signal magnitude is then stored with the previous values in the appropriate bearing bin, one of 360, and a new average magnitude is computed using the updated values in this and adjacent bins. Based on a function of the new average magnitude, and provided that a sufficient number of strokes has occurred in the current bearing, a new yellow point is plotted along the bearing radial and at a distance, representing range, according to the reciprocal of the averaged signal magnitude. To ensure that the plotting algorithm uses only the most recent information, the time of each new NR stroke is also stored. After a user-selected time, such as 15 minutes, any plot exceeding this time changes color from yellow to red to denote a relatively old point. Similarly, all plotted red points are erased after twice this selected time. As a result, the newest plots are yellow and the older ones red before ultimately disappearing. This systematic color change allows one to easily monitor the advance of the plot patterns across the screen map as lightning and the associated thunderstorms move over the landscape.
 

To provide a sense of changes in thunderstorm coverage, a number of statistics about the current and recent strokes are also presented in graphical and text form on the screen. Finally, there is a menu displaying a number of parameters used by the algorithm to plot the points; these parameters can be adjusted while the program is running so that the user may optimize the system performance at his or her unique site.