On the wiki for the PlanetaryCargoShips configuration file, it states that the PlayerSurfaceAltitude value is what altitude a player must be at in order for ships of this particular spawn type to be eligible to spawn. Not only is that untrue because the default value is around 6000 and they will spawn even if a player is standing directly on the surface, but setting this value to very low values will result in these encounters spawning within the ground, sometimes even 4km under the ground.