A simple map overlay can be created which represents the range capability of the glider. It's use will reduce the pilot workload and help him to make swift decisions based on his current energy condition and his location relative to nearby airports. They can be visualized as inverted cones centered over each landing site. Determining the sailplane's location on the map is not difficult since the airplane is usually fairly high.
High altitude allows easy identification of landmarks.
The altitude of the sailplane can be read directly from the altimeter. What is needed is a device that can be used in conjunction with the map to show the pilot how far he can travel in any direction from his current location and altitude. This distance will be affected by the wind speed and direction. Let's start by assuming that there is no wind. So for every 1000 feet of altitude lost the glider will move forward 30,000 feet, or 5.68 miles.