Line-of-sight

Line-of-sight refers to an unobstructed, straight path between an observer's eyes and a point in the environment. In other words, it's the clear and direct visual connection between the observer and the target.

When firing a missile weapon, combatants must be able to trace a direct line of sight between themselves and the target hex. Common obstructions to line of sight include buildings and structures, terrain features, vegetation, atmospheric conditions, curvature of the earth and differences in elevation.

Targeting in Combat

When firing missiles into an ongoing melee, hurled and fired weapons can be employed by drawing a straight line between the observer and target from one combat hex to another. Where the shot is obstructed by a friendly combatant, the shot cannot be taken. Otherwise, it's always assumed that the hit or missed shot will either cause damage to the target or fly past harmlessly ... unless a natural '2' is rolled, in which case the missile is considered friendly fire.

Cover

Objects that are partly but not wholly obscured by obstacles can be targeted. However, it must be determined what percentage of the whole target is visible; this is usually rated in 10% increments, simplifying the roll that must be made if a "hit" is achieved. For example, an archer fires at a target, of which 40% can be seen. If the archer hits, a second roll is made on a d10. If the d10 result is 4 or less, the hit counts; if not, then the missile has struck the obstacle and either stuck or bounded away harmlessly.

In extreme situations, players may wish to fire at targets that have as little as 3% or less that's visible. This is actionable, but keep in mind that most of the time, a hidden combatant won't show any visibility when hiding.

Concealed objects, those that are screened by vegetation, mist, rain or semi-transparent obstacles should also be rated as 1 to 10 in terms of how much cover they offer. In cases of the spells fog cloud and obscurement, rules there should apply. Otherwise, the DM should employ good judgment in determining how much "cover" a lack of visibility provides. A heavy rain, enough to obscure clear visibility, except in a storm so intense that any missile fire would be impractical anyway, should count as 10% cover. Light or moderate rain would provide none at all. Foliage may offer between 20% and 80%.

The cover for submerged objects, fired at from above, depends on the clarity of the water and depth. In most cases, a target five ft. below the surface of water filled with the normal amount of particulates would have a cover of 20%; thrice that at 10 ft., and at 15 ft., 100%.

Indirect Fire

Siege engines have the advantage of firing over obstacles and are able to target an enemy so long as the enemy is "sighted" — meaning, the enemy can be seen by a third party that is signalling to the artillerist. In such cases, the engine can fire a missile that hits in the vicinity of the target (or with great luck, the target itself), most likely causing damage with shrapnel. Indirect fire hits an area the size of a ship's hex, or the width of four combat hexes.

See also,
Combat
Naval Combat