How far is that by air? The derivation of an air: ground coefficient

The objective of this study was to derive a simple equation to convert distances between air miles traveled by a helicopter and ground miles traveled by an ambulance. We performed a retrospective analysis of a convenience sampling of 245 "lights and sirens" ground ambulance transports between 1993 and 1997. Ground distances were recorded from odometer miles for each transport. Air miles were calculated using the Global Positioning System (GPS), measuring the distance in a straight line from the scene to the hospital. Air and ground distances were entered into a computerized spreadsheet, and the correlation coefficient and regression equation were derived. A simple approximation equation was developed and compared against the derived regression equation. There was a strong linear correlation between ground miles and air miles at virtually all distances studied (R = 0.932, R(2) = 0.869). Regression analysis revealed the following relationship: Ground miles = 0.94 + 1.25 (air miles). This was simplified to an approximation of: Ground miles = 1.3 (air miles). The approximation equation yielded an answer within 1 mile of the regression equation for distances up to 40 air miles. It is concluded that in a mixed rural and urban EMS system, one may convert air and ground mileage estimates by using the simple relationship: Ground miles = 1.3 (air miles). This conversion coefficient may prove useful for EMS personnel in designing reasonable helicopter utilization policies, making accurate transport decisions and conducting research.