6.1 CHECKING LOS: The ability of a unit to "see" and "fire directly" at another unit in a different hex is dependent on having a LOS between those units. Whether a unit has a LOS to a given hex is usually determined by stretching a sewing thread taut between the center of the firing hex and the center of the target hex. If the thread does not cross a terrain depiction capable of obstructing the LOS between the target and firing hexes (with the obstruction visible on both sides of the thread) [EXC: Inherent Terrain hexes (B.6) block or hinder LOS if traced exactly along their hexside even though the terrain depiction is not visible on both sides of the thread], there is a clear LOS between the two hexes. If players still cannot agree whether a LOS is blocked or not, the matter is resolved by a dr: 1-3 LOS is not blocked; 4-6 it is blocked. It is usually (but not always; see B.6 & B9.1) the terrain depiction which can potentially block or hinder LOS—not the hex containing that terrain type.
6.11 LOS CHECKS: Before setup, LOS may be checked only by the side setting up first, and only if that side is the Scenario Defender, or has Pre-Registered Fire (C1.73) or Bore-Sighted weapons (C6.41-.42). During play, neither player may make potential LOS checks to determine if a LOS exists for an attack until after that attack has been declared [EXC: Concealment Removal (12.14), Road entry cost (4.132)]. Should the LOS check for an attack reveal a blocked LOS, the units which were to have made the attack are still considered to have fired for all purposes (they thought they saw something); that fire would not generate DM status nor affect units in the obstacle that blocked the LOS, although a DR must still be made to check for possible Random Events and the retention of any Multiple ROF.
6.12 ATYPICAL LOS: Occasionally, the rules will specify that an entire hexside, part of a hexside, or a vertex be used for tracing LOS to/from a firer/target instead of the hex center dot. This occurs during Road use (4.132), Bypass (4.34, D2.32), Climbing (B11.42), Underbelly Hits (D4.3), Snap Shot (8.15), and movement between Rowhouses (B23.71).
6.2 OBSTACLES: Each terrain type [EXC: Depressions 6.3] is defined as to whether or not it presents an obstacle or Hindrance to LOS and, if an obstacle, the height of that obstacle. LOS extends into or out of obstacles, but not through them into hexes beyond the obstacle except in certain cases wherein the target/firer are at an elevation ≥ the height of the obstacle or are adjacent to a hexside obstacle. The terrain in a firer's hex never blocks LOS traced from its hex center dot, although it may block LOS traced from a vertex of its hex across the interior of its own hex.
EX: Woods are a level 1 obstacle to LOS. If the firing unit is beneath level 1, it cannot see through ground-level woods to a target hex at level 1 or lower. The target unit would have to be at an elevation > the top of the obstacle to possibly establish a LOS to a hex at ground level beyond that obstacle.
6.21 HALF-LEVEL OBSTACLES: Any terrain feature defined as a half-level obstacle blocks all same-level LOS through (not into or out of) that terrain hex. Half-level obstacles never form Blind Hexes nor block LOS to/from a higher level Location.
6.3 DEPRESSIONS: Certain terrain types are defined as being relatively narrow slits carved into the surface below ground level. Although they present no obstacle to LOS between units at or above ground level, units IN Depressions are often out of LOS of even relatively nearby higher level units. A unit must be at least one level higher for every hex of range to units IN a Depression to have a LOS to them [EXC: Units with a clear LOS between them through other continuous Depression hexsides (exclusive of vertices) need not count those intervening Depression hexes in determining the necessary elevation advantage]. A unit in a ground level hex always has a LOS INTO an adjacent level -1 Depression hex, but a unit two hexes away must be at level 1 or higher to have a LOS INTO that hex, and a unit three hexes away must be at level 2 or higher. Boards 24 and 25 contain Depressions that change elevation along their length, thus creating several additional LOS possibilities; see B19.5.
EX: A unit in 5FF6 cannot see INTO EE8 because it is two hexes away with only a 1 one level elevation advantage. However, a unit in GG7 can see INTO EE8 because it has a LOS INTO Depression hex FF7 and then along the Depression depiction INTO EE8, whereas FF6 cannot trace a straight LOS INTO an adjacent Depression hex, and then on INTO EE8.
6.4 BLIND HEXES: Assuming an otherwise clear LOS, even if a firing (or target) unit is at an elevation > the height equivalent of any intervening full level obstacle, a number of potential target hexes that are both directly behind that obstacle and also equal to the full level height equivalent (i.e., ignoring any half-level) of that obstacle are considered Blind Hexes to the firer. Blind Hexes cannot be seen by the firer unless the Blind Hex is at an elevation ≥ the full-level height of the obstacle (in which case it is not a "Blind" Hex after all). This "blind" area can increase as the range to the obstacle increases (6.41), and/or decrease according to the extent of elevation advantage over the obstacle (6.42).
6.42 For every full-level elevation advantage > one level over an obstacle, decrease the number of Blind Hexes created by that obstacle by one, to a minimum of one [EXC: non-Cliff Crest Lines may have their Blind Hexes reduced to none by sufficient elevation advantage; B10.23].
EX: The distance to a level 1½ obstacle is 12 hexes. Three blind hexes (1 [normal Blind Hex; 6.4] +2 [extra Blind Hexes; 6.41]) are created by that obstacle for a unit at level 2; two blind hexes are created by that obstacle for units at level 3 at the same range; and only one blind hex is created by that obstacle for a unit at level 4 or higher at that range.
6.43 The number of Blind Hexes created by an obstacle can be changed by the height of the hexes directly behind the obstacle along the LOS. If a hex behind an obstacle is at a lower level than the elevation of the hex containing the obstacle (not the obstacle itself), the difference in elevation of those two hexes is added to the number of Blind Hexes which are created by that obstacle to determine if that hex behind the obstacle is actually a Blind Hex. However, if the Blind Hex is created solely by a Crest Line (B10.23), only any difference > one is added since a difference of one level is necessary for a Crest Line to even exist. If a hex behind an obstacle is at a higher level than the elevation of the hex containing the obstacle, the difference in elevation of those two hexes is subtracted from the number of Blind Hexes which are created by that obstacle to determine if that hex behind the obstacle is actually a Blind Hex.
EX: There is no LOS from Level 3 hill hex 2J4 to Level 1 hill hex H2 because the intervening woods in I3 makes H2 a Blind Hex (6.4). The LOS to H1 is also blocked, because the difference in elevation of I3 and H1 is added to its Blind Hex creation. Had H1 been a Level 1 hex instead, a LOS to it would exist; alternatively, had J4 been a level 4 hex instead, a LOS to H1 would exist (6.42).
6.6 UNITS: The presence of Personnel units in a hex does not block LOS through that hex (although the presence of an AFV/wreck may hinder [and thereby modify] fire through the hex; D9.4). Direct Fire attacks can be traced through units in intervening hexes without affecting them unless specified otherwise.
6.7 LOS HINDRANCE: Some terrain types are not uniformly high or substantial enough to be considered a complete obstacle to same-level LOS, and are listed as LOS Hindrance hexes because each one hinders such fire traced through its hex to another hex but not sufficiently to prevent it entirely (see Terrain Chart/Desert Terrain Chart/PTO Terrain Chart). All same-level Direct Fire and spotting attempts traced through (not just into or out of) an effective LOS Hindrance hex are modified by a +1 DRM to either the IFT or To Hit DR, or the Artillery Initial Accuracy dr. The presence of such a Hindrance always negates Interdiction and FFMO. Being in a LOS Hindrance hex [EXC: SMOKE (24.2) and FFE Hindrance (C1.57)] does not hinder the LOS of a firing or target unit; it is only the presence of a LOS Hindrance hex between the same-level firing and target units (regardless of whether either/both are Personnel or vehicles) that forms a LOS Hindrance [EXC: SMOKE and FFE Hindrances are effective Hindrances to LOS even if they are in the viewer or target hexes rather than between them; these Hindrances (24.4 & C1.57), as well as bridge (B6.2), orchard (B14.2), and tower (B34.2) LOS Hindrances and Fog (E3.311) may affect units at different levels]. Unlike range, which is always the least number of hexes from firer to target regardless of LOS, LOS Hindrances are incurred whenever the LOS crosses a Hindrance—regardless of how small a portion of that hex it may be. Whenever a LOS crosses (or goes along the shared hexside of) two hexes that have the same range to the firer, however, only the LOS Hindrance of the hex with the highest applicable LOS Hindrance DRM is counted. Therefore, the number of hexes in which a LOS Hindrance applies will not exceed the range.
EX: A 4-6-7 squad in 3T1 using Defensive First Fire vs a Non-Assault moving squad in V2 traces his LOS through a Hindrance hex (the grainfield in U2). The 4-6-7's attack is modified by +1 for the LOS Hindrance, +1 for TEM (hedge), and -1 for FFNAM for a total modification of+1. Now, assume that T1 is a building with an upper level and the firing 4-6-7 is at level 1. The same attack would now be resolved without any DRM (-1 [FFNAM] +1 [hedge TEM] = 0).
EX: Even though the LOS from 4V3 to W10 encounters seven LOS Hindrance hexes in an area equivalent to a range of only five hexes, the Hindrance DRM is +5, not +7. Hexes V6 and W6 are both three hexes from V3, so only one +1 LOS Hindrance DRM is counted for those two hexes. Similarly, hexes V7 and W7 cause only one +1 Hindrance DRM between them. If there were a wreck in V7, however, the total LOS Hindrance DRM would be +6.
6.8 Even units in adjacent hexes do not always have a clear LOS to each other. This is especially true of units using Bypass to transit an obstacle hex with the obstacle between the mover and the firer.
EX: Even though in adjacent hexes, units IN gully hexes 12BB4 and AA5 cannot see each other because they are not connected by a Depression hexside.
EX: A unit on the ground level of 12U5 cannot see a unit on level 2 of U5 or level 1 of V4 because of the intervening building levels.
EX: A unit in 12P2 does not have a LOS to a unit in O3 Bypassing the O3-N3 hexside.