The current best such set of spherical harmonic coefficients is EGM96 ( Earth Gravity Model 1996 ), determined in an international collaborative project led by NIMA . The mathematical description of the non-rotating part of the potential function in this model is:
42.
Evaluation of several model forms in the 1960s concluded that " the gravity model and intervening opportunity model proved of about equal reliability and utility in simulating the 1948 and 1955 trip distribution for Washington, D . C . " ( Heanue and Pyers 1966 ).
43.
The application of these models differs in concept in that the gravity model uses impedance by travel time, perhaps stratified by socioeconomic variables, in determining the probability of trip making, while a discrete choice approach brings those variables inside the utility or impedance function.
44.
He wrote " A General Theory of Traffic Movement, " ( Voorhees, 1956 ) which applied the gravity model to trip distribution, which translates trips generated in an area to a matrix that identifies the number of trips from each origin to each destination, which can then be loaded onto the network.
45.
Other LRV models were built : a static model to assist with human factors design; an engineering model to design and integrate the subsystems; two one-sixth gravity models for testing the deployment mechanism; a one-gravity trainer to give the astronauts instruction in the operation of the rover and allow them to practice driving it; a mass model to test the effect of the rover on the LM structure, balance, and handling; a vibration test unit to study the LRV's durability and handling of launch stresses; and a qualification test unit to study integration of all LRV subsystems.
