When viewing a map on a computer screen, however, the scale becomes less easier to calculate. A computer screen is made up of pixels (picture elements) that are used to display the computer application, and in the case of Terrain Navigator, the map. Because screens vary widely in size and resolution, it is nearly impossible to correlate a scale (such as one inch = 2000 feet) to the display (unless the display has been specifically calibrated to do so.) Thus, maps displayed within Terrain Navigator Pro may be scaled to your computer display, but will be printed at the desired scale.
Finally, the Aerial Photos included with Terrain Navigator Pro are not taken from raw printed base maps. As such, they have no predefined scale. However, to display them in a manner similar to 1:24,000 USGS topographic maps, we have chosen to scale them at 1:12,000.
Terrain Navigator Pro guru Dick Blust has an excellent explanation of map scales on his JustTrails blog: