In this paper we present the development of a variable structure interacting multiple model estimator for tracking groups of ground targets on constrained paths using moving target indicator reports obtained from an airborne sensor. The targets are moving along a highway, with varying inter- visibility due to changing terrain conditions. In addition,the roads can branch, merge or cross. Some of the targets may also move in an open field. This constrained motion estimation problem is handled using an IMM estimator with varying mode sets depending on the topography. The number of models in the IMM estimator, their types and their parameters are modified adaptively, in real-time, based on the estimated position of the target and the corresponding road/visibility conditions. This topography-based variable structure mechanism eliminates the need for carrying all the possible models throughout the entire tracking period as in the standard IMM estimator, improving performance and reducing computational load.