Existing works on relay nodes placement in two-tiered wireless sensor networks (WSNs) assume two-dimensional terrains and ignore completely the impact of the topography on the communication quality. In contrast, this paper addresses constrained versions of the relay nodes placement problem while adopting more realistic assumptions. To ensure the connectivity of a two-tiered WSN deployed on a three-dimensional terrain, the proposed approach determines the relay nodes positions by analyzing the data extracted from the terrain topography. The main idea is to consider a concrete positions constraint, by requiring the placement of relay nodes only at the terrain crest points, which have a relatively wide visibility on the terrain and are well exposed to the sun. This allows minimizing the number of relay nodes required to ensure the connectivity, reduces the impact of the topography on the wireless communication quality, and offers the opportunity to equip the relay nodes with solar panels to extend their lifetimes. Based on this idea and inspired by the One-Step constrained Relay nodes Placement (OSRP) and Realistic constrained Relay nodes PLacement (RRPL) approaches, three algorithms, called 3D-OSRP (3D One-Step constrained Relay nodes Placement), 3D-RRPL-v1 (3D Realistic constrained Relay nodes PLacement version-1), and 3D-RRPL-v2 (3D Realistic constrained Relay nodes PLacement version-2), are designed, implemented, and evaluated by extensive simulations on realistic three-dimensional terrains. Obtained results demonstrate the relevance of considering positions constraint.Trans Emerging Tel Tech. 2019;30:e3558.wileyonlinelibrary.com/journal/ett network lifetime. Moreover, the limited communication range of the SNs requires the use of a multihop routing, in which the data passes through many intermediate nodes before reaching one of the CNs, which increases both packet loss probability and packet delivery delay. To overcome these challenges, recent works have suggested reinforcing this basic architecture, composed of SNs and CNs, by a third type of nodes, called relay nodes (RNs). [2][3][4] Having an extended energy autonomy 5,6 and a communication range often greater than that of SNs, 7,8 these RNs are designed to preserve or to restore the network connectivity by extensively participating in routing and driving data to the CNs, without contributing to the detection tasks assigned exclusively to the SNs. Nevertheless, the introduction of RNs generates new constraints that need to be taken into account. Indeed, the arrangement of these RNs in the RoI must be well examined according to several factors, which include, among others, the positions already occupied by the SNs, because the main role of the RNs is to reinforce or to restore network connectivity. In addition, because of their high-cost, 9 the number of RNs to be deployed must be minimized. Therefore, the deployment of RNs must be performed in a deterministic manner, 10 by precomputing their positions in the RoI, since the random deployment cannot respond ef...