DRAFT 2/25/2005

GBACW 4.x Line of Sight Explained

Jim Dauphinais

February 2005

This is an unofficial explanation based on the rules as written and clarifications provided by Richard Berg and/or John Alsen on Consimworld.  Comments and Corrections are always welcome.  Please send them to JimDauphinais@aol.com

For some new players one of the most disconcerting sections of the GBACW Series Rules are those sections which address Line of Sight (LOS).  In part it is because the Series Rules do not really provide detailed LOS rules, but rather provide general guidance to players and rely on the players’ own structural visualization skills to resolve most LOS questions.  Specifically the Design Note preface to Section 10.2 of the Series Rules states:

We’ll try to keep this as simple and basic as possible, knowing full well that it is impossible to cover every angle. There are sure to be anomalies; try to solve them based on the underlying principles these rules portray.

In this article I will endeavor to walk through the GBACW LOS rules providing clarification and expansion in an effort to better define how LOS works within the GBACW world.

The Fundamental Concept

10.21 states:

Units firing at a target two or more hexes distant may fire only at targets to which they can trace a Line of Sight (LOS); they must be able to “see” the target. LOS is traced from the center of the firing hex, through the Front of the unit [7.11] to the center of the target hex.

In general, this rule as written has not been a source of confusion.  However, it is important to note that this rule establishes a convention that requires that LOS be traced from the firing hex to the target hex.  It is very important to keep this in mind when attempting to understand the GBACW rules.

Blocking Terrain

10.22 through 10.24 state:

10.22 Certain terrain hexes block LOS (see Terrain Effects Chart). If the LOS is blocked, fire is not possible. Blocking terrain hexes can always be fired into, but never through. (Consequently, you can always fire through Frontal hexsides into an adjacent hex.)

10.23 The following terrain hexes may block LOS, depending on their location:

•	Woods are treated as being one full level higher than the actual hex elevation. However, Orchards do not block LOS.

•	Infantry or cavalry units (but not artillery, which never blocks LOS) are treated as being one-half a level higher than the hex in which they are.

10.24 If both firing and target hex are on the same or different level, LOS is blocked by any intervening terrain with an elevation higher than both units.

In addition, 10.26 (introduced in GBACW 4.5) indicates:

LOS is always reciprocal—if the Firing unit can see the target, then the target can see (and fire on) the firing unit.

Furthermore, 10.27 and 10.28 (10.28 and 10.29 in GBACW 4.4) indicate:

10.27 Intervening infantry and cavalry blocks LOS for small-arms fire, and artillery fire at ranges of 1-3 hexes due to their elevation [10.23]. Given no intervening, blocking terrain, combat units block LOS for artillery firing at arange of four or more hexes, if such units are within two hexes of the target (and, of course, in the line of fire).

10.28 If LOS runs along the hexside of a hex that blocks LOS, the LOS is blocked. (Accurate? No sir. But it stops arguments.)

To understand how these rules work together, it is best to approach the concepts in discrete steps.  First, consider only small arms fire where the base elevation level of every hex is the same.   Under this example, any hex containing blocking terrain (including infantry and cavalry units) along the LOS from the Firing hex to the target hex will block LOS (10.22-10.24).  The important concept here is that the entire hex (and its associated hexsides (10.28)) blocks LOS in GBACW, not just the terrain depiction in the hex.

Now let us modify this example such that while the Firing hex and Target hex are still at the same base elevation level, but the base elevation level of other hexes along the LOS from the Firing hex to the Target hex are potentially different from each other.  Under this second example, the LOS from the firing hex to the target hex is blocked if any hexes intervening along the LOS from the firing hex to the target hex have a total elevation level greater than that of the firing and target hexes.  By “total elevation” I mean the sum of the base elevation of the hex and the additional elevation added by any blocking terrain in that hex.  Once again, it is important to recognize that the entire hex (and its associated hexsides) blocks LOS, not just the terrain depiction (e.g., the elevation contour lines) in the hex.

Adding the reciprocity provision to this discussion, as the rule states in general if a firing hex has a LOS to a target hex, the target hex has a LOS to the firing hex (10.26).  However, there is an exception.  Specifically, as indicated by 10.27, when artillery is firing at a target hex that is four or more hexes away, infantry and cavalry units are only considered blocking terrain if they are in a hex along the LOS from the firing hex to the target hex that is within two hexes of the target hex.  In this particular situation the target hex may not have a LOS to the firing hex even though artillery in the firing hex may have a LOS to the target hex.  It should be noted that this special one-way artillery LOS represents artillery fire arcing over intervening units and the “within two hexes of the target hex” blocking effect represents such artillery withholding fire to avoid endangering friendly units that are too close to the target hex.

When the Firing Hex and Target Hex Are At Different Elevations

The greatest source of confusion with the GBACW LOS rules has been when the firing hex and target hex are at different elevations.  The reason for this is three fold:

•	Blind (or shadow) areas can be created behind intervening terrain when the firing hex and target hex are at different elevation levels;

•	The GBACW rules that address these blind areas are incomplete and rely on the structural visualization skills of the players;

•	Recent attempts to clarify the rules in this area have been partially derailed due to an accidental rules deletion during the layout process during the publication of the GBACW 4.5 Series Rules.

As before, it is best to start with the simplest case first.  Specifically, putting blind area concerns aside for now, the LOS from the firing hex to the target hex is blocked if any hexes intervening along the LOS from the firing hex to the target hex have a total elevation level greater than both the firing hex and target hex. If this is not the case, LOS may still be blocked depending on whether the lower of the firing hex or the target hex is in a blind area along the path of the potential LOS.

Blind (or Shadow) Areas

A blind (or shadow) area is an area behind intervening terrain that is blind to an observer located at an elevation greater than or equal to the total elevation of the hex the intervening terrain is located within.  This blind area can also be thought of as the shadow cast by the intervening terrain.

GBACW 4.4 contained two provisions (which were accidentally deleted from the GBACW 4.5 Series Rules) that dealt with blind areas associated with gradual elevation changes:

10.25 If the target is lower than the firing hex, and the intervening terrain is at the same elevation as the firing hex, then LOS is blocked if the intervening terrain is closer to the target than it is to the firing hex. (Exactly halfway is not considered to be closer.)

10.26 If the target is higher than the firing hex, and the intervening terrain is at the same elevation as target hex, then LOS is blocked only if intervening terrain is closer to the firing unit than to the target. (Exactly halfway is not considered to be closer.)

These two provisions deal with the generally plateau nature of gradual elevation level changes.  Specifically, due to the fact that most elevation rises are on a fairly convex curve, as a unit moves further up the rise, blind areas develop at lower levels behind higher areas further down the rise.  Under these two provisions blind hexes can form behind intervening hexes of the same total elevation as the higher of the firing or target hex.  If the intervening hex that is of the same total elevation as the higher of the firing or target hex is closer to the lower of the firing or target hex, the LOS is blocked between the firing and target hexes because the lower of the two hexes is in the blind area behind the intervening hex.

There are four factors to be aware of in regard to these two provisions:

•	The provisions are a design for effect achieved through a modification of similar provisions used to address more significant elevation changes;

•	The provisions do not adequately deal with the geographic crest (i.e., summit or high point) of an elevation feature;

•	The provisions do not adequately deal with hexes that are at the same level as the higher of the firing or target hex, but which are not directly connected to the elevation feature the higher of the firing or target hex is located upon.

•	The provisions do not adequately deal with the significant drop off in elevation that occurs behind full-level blocking terrain such as woods.

The first of these factors does not present a problem.  In fact, the rule is an elegant design solution for dealing with gradual elevation changes along a given elevation feature.  The provisions only breakdown when the higher of the firing or target hex is located behind the geographic crest of the elevation feature.  For example,   [gettysburg map].  The solution to this problem is to look past the LOS behind the higher of the firing or target hex and determine whether the elevation continues to rise.  If the elevation continues to rise, apply the two provisions normally.  If it does not continue to rise, LOS will be blocked if the distance in hexes from the higher of the firing or target hex to the intervening same level hex is greater than the number of same level hexes immediately behind the higher of the firing or target hex.  For example, [Gettysburg map].

The third factor concern intervening hexes of the same elevation that are separated from the higher of the firing or target hex by lower elevations also presents a problem that must be resolved.  These same level elevation hexes are not part of the same elevation rise as the higher of the firing or target hex.  Therefore, they cannot be seen over to a lower elevation.  LOS is always blocked when a hex with intervening terrain of the same elevation level as the higher of the firing or target hex is separated from the higher of the firing or target hex by lower elevation levels.  For example, [Gettysburg map].

The fourth factor concerning the aburpt drop off in elevation behind full-level blocking terrain can partially be dealt with in a similar manner.  Specifically, intervening hexes that contain blocking terrain and have the same total elevation as the higher of the firing or target hex will always block LOS.  For example, [Gettysburg map].  The other blind area impacts caused by the abrupt elevation change behind full-level blocking terrain are discussed below.

Abrupt Elevation Changes Along The LOS

When elevation abruptly drops there is a great potential for blind areas to form behind those drop offs.  In GBACW there are two forms of such sudden drop offs:

•	Behind Minor, Steep and Sheer Slope hexsides;

•	Behind full level blocking terrain (e.g., woods).

In effort to provide clarification in version 4.5 of the Series Rules, Section 10.27 of the GBACW 4.4 Series Rules was replaced with a new Section 10.25:

10.25 SLOPES AND LOS: Gradual Slope hexsides have no effect on LOS (except when it applies to 10.24). A Minor or Steep Slope hexside, will block LOS under the following conditions:

•	If the firing unit is higher than the target, the Minor or Steep Slope hexside will block LOS if it is closer to the target than the Firing unit.

•	If the firing unit is lower than the target unit, the Minor or Steep Slope hexside will block LOS if it is further from the target unit than the firing unit.

•	If the Minor or Steep Slope hexside is exactly halfway between, it does not block LOS.

CLARIFICATION: The above might be clarified by putting it in different words: Firing downhill? LOS  is blocked if the blocking item is closer to the Target.  Firing uphill? LOS is blocked if the blocking item is closer to the Firing unit.

Note that Minor Slope hexsides only appear in the 1st and 2nd Editions of Three Days of Gettysburg.  To reduce LOS complexity, Minor Slope hexsides were not used in the 2004 Edition of Three days of Gettysburg.  Also, note that 10.25 also applies to Sheer Slope hexsides, which are a steeper version of Steep Slope hexsides, and the hexsides of full-level blocking terrain hexes.  The latter is not readily deduced from the rules, but 10.25 does in fact apply because of the abrupt dropoff behind full-level blocking terrain.  Furthermore, Richard Berg has confirmed as much through responses to specific hex-to-hex LOS queries.

Under 10.25, even if the higher of the firing or target hex is higher than the total elevation level of intervening hexes, LOS will still be blocked if the lower of the firing or target hex is behind and lower than an abrupt elevation change hexside that is closer to the lower of the firing or target hex than the higher of the firing or target hex.  For example, ________.

There are two factors to be aware of in regard to 10.25:

•	The “closer to the lower” rule is a simplification that assumes that higher of firing or target hex is at an elevation twice as higher as the total elevation level of the higher elevation level of the intervening abrupt elevation change hexside relative to the elevation of the lower of the firing or target hex;

•	The rule fails to consider valid LOS across continuous abrupt elevation changes.

In regard to the first factor, elevation advantages over abrupt elevation change hexsides can on occasion exceed two.  However, for simplicity this is neglected under the rules much in the way blocking terrain is assumed to fill its entire hex including the hex’s hexsides.  The simplification is adequate for the existing games in the series, but the simplicity of the rule may need to be reconsidered if future games in the series involve more substantially greater height advantage over abrupt elevation change hexsides.

The second factor is not addressed in the rule as written.  A continuous abrupt elevation change is an exception to 10.25 that involves a valid LOS crossing several contiguous abrupt elevation change hexsides in which each abrupt elevation change hexside crossed looking down to the lower of the firing or target hex is no steeper than the last such hexside crossed.  In order of steepness, abrupt elevation change hexsides are ranked as follows: full-level blocking terrain hexsides (steepest), Sheer Slope hexsides, Steep Slope hexsides and Minor Slope hexsides (least steep).  Gradual Slope hexsides (i.e., elevation contour lines) are always considered less steep than any abrupt elevation change hexside.  It is important to note that while there may a valid LOS across a continuous abrupt elevation change, artillery may not be able to fire across this LOS due to gun depression issues as artillery may not fire to an adjacent hex two elevation levels lower or higher than the hex it is located within (10.67).  It is also important to realize this also means that artillery cannot fire two hexes away to an elevation level four levels higher or lower, three hexes away to an elevation levels six levels higher or lower, etc.