In addition to the network, the second fundamental challenge at the time SIMNET was conceived was the inability of graphics systems to handle large numbers of moving models. For example, most contemporary flight simulators used Binary Space Partitioning which is computationally effective for fixed environments since polygon display order (i.e., their depth coherence) can be pre-computed. While suitable for flight simulators, which largely have a point of view above the Earth's fixed surface, this technique is ineffective near the ground, where the order in which polygons overlay each other changes with the location of the point of view. It is also ineffective with a large number of moving models, since moving a model changes its depth coherence relative to the polygons representing the ground.