We investigated self-assembled InAs islands grown on In0.52Al0.48As/InP(113)B by molecular beam epitaxy (MBE) using in-sim reflection high-energy electron diffraction (RHEED) and in-situ scanning tunnelling microscopy (STM). For a deposition of 3 monolayers (MLs) of InAs, the STM topography revealed a high density of well-organised hexagonal facetted dots. The dots are truncated (height similar to 3 nm) and elongated along the [1 1 0] direction. Their typical base dimensions are approximately 48 and 19 nm. The angles between facet orientation and crystal direction were accurately deduced from RHEED patterns. From these structural characterisations, the dot shape and formation mechanism are discussed using a standard scheme based on surface energy minimisation. A direct correlation is found between the dot shape and the surface reconstruction unit cell.