Reference Information: Explanation of LOS rules
BattleTech Master Rules (BMR Revised page 26):
In all cases, 'Mech units are considered to be 1 Level high (at their
firing/target position). Terrain along the LOS between units is intervening
(blocking) if:
 The terrain or terrain feature is higher than both units, or
 The terrain or terrain feature is adjacent to the attacker and higher
than the attacker, or
 The terrain or terrain feature is adjacent to the target and higher than
the target.
Maximum Tech Dead Zone Rule (Max Tech Revised page 21):
Players must check for dead zones if there is no intervening terrain higher
than both the attacker and the target, but there is still some question as to
whether LOS exists. Note which of the two units is at the higher elevation (2 in
the diagram above) and which is the lower (1 in the diagram above). Then find
the highestlevel hex between the two units, designating it "Hex X." The
elevation of Hex X (H in the diagram above) may or may not create a dead zone.
To find out, follow the steps shown below.
 Subtract the elevation level of the lower unit from that of Hex X. This
number is "A." (In the diagram above, A = H  L1  1; remember, the lower
'Mech is 1 level above its terrain L1.)
 Subtract the level of Hex X from that of the higher unit. This number is
"B." (In the diagram above, B = L2 + 1  H; remember, the higher 'Mech is 1
level above its terrain L2.)
 Subtract B from A and multiply the result by 2. This number is "C."
(Using the above values for A and B, this gives C = ((H  L1  1)  (L2 + 1
 H)) * 2 = 4*H  2*L1  2*L2  4.)
 Count the range to Hex X from both units. Subtract the range to the
lower unit (D1 in the diagram above) from the range to the higher (D2 in the
diagram above). This number will be negative if the higher unit is closer to
Hex X than the lower unit. Add this number to C. (In the diagram above, (D2
 D1) is added to C, giving a total formula of Result = 4*H  2*L1  2*L2 
4 + D2  D1.)
If the final result is greater than 0, the lower unit is in the dead zone and
LOS is blocked. If the result is 0 or less (negative), then LOS exists between the two
units. This equation can be put into another form:
H = (L1 + L2) / 2 + (D1  D2) / 4 + 1
This gives the maximum obstacle height, H, which does not block line of
sight. Anything higher, does.
Note: Rules 2 and 3 from the BMR rules, concerning higher adjacent terrain,
still apply, regardless of the LOS calculation method.
Important Note: IMHO the above calculation, while official Level 3, is
wrong. Unless L1 and L2 are fairly close to each other, it can
give answers that are absolutely incorrect. You may want to check out this
Line of Sight Comparison and
form your own opinion.
Maximum Tech Diagramming LOS Rule (MaxTech Revised page 22):
This alternate method uses a diagram, on page 22, but this diagram can also
be put in terms of an equation. This equation, resulting from the geometry of
the diagram in MaxTech, and using the variables in the diagram above, is:
 Result = H  (L1 + (L2  L1) * D1 / (D1 + D2) + 1)
 Again, if the Result is greater than 0, LOS is blocked;. if the Result
is 0 or less (negative), LOS is not blocked.
 Rules 2 and 3 from the BMR rules, concerning higher adjacent terrain,
still apply.
If the final result is greater than 0, the lower unit is in the dead zone and
LOS is blocked. If the result is 0 or less (negative), then LOS exists between the two
units.
As with the previous example, the above formula can be put into the form:
H = (D1*L2 + D2*L1) / (D1 + D2) + 1
Again, this gives the maximum obstacle height, H, which does not block line
of sight.
As noted above, it is my opinion that the official Level 3 formula from
Maximum Tech is in error when D1 and D2 are different, greatly in error when
they are greatly different, and I suggest using this Diagramming method and
formula instead.
Please report any errors or questions concerning the code or results this
page produces to
Rick Raisley.
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