Zone of Control (ZOC) is a very common wargame concept that refers to the ability of a force to prevent an enemy force from moving past it without engaging combat. In most wargames, a force will exert a ZOC effect over adjacent regions. In AACW the effect is limited to the region a force is in, so it only takes effect when units from both sides are in the same region.
I had a lot of difficulty understanding the manual’s description of how “Zone of Control” (ZOC) works in AACW (see the “Blocking Movement” section of the Movement chapter). I got much better info searching the forum, but still everything did not add up for me, especially how evasion values are computed, so I did extensive in-game testing.
Sorry for how long this post is, but it’s actually a pretty complicated subject. I don’t even want to say how many turns into a test game I got before I deciphered how evasion is calculated (hint: it was 1863). I have not been able to extensively test whether the ZOC formula actually works as advertised, since it's hard to set up tests in a game.
If anyone finds any errors in this, please correct me!
[SIZE="3"]Zone of Control (ZOC) – The Basics[/size]
Enemy presence in a land region inhibits movement out of the region. If you have a force in the same region as an enemy force, you may not be able to move out of the region and into a neighboring region that you do not have military control of. Regions that you cannot move into will appear in red on the map when the unit is selected. ZOC is most likely to prevent a force from penetrating deeper into enemy territory without engaging the enemy. A force can usually retreat back the way they came, but sometimes this is impossible and a force can become trapped. If a force is trapped by enemy ZOC and is forced to retreat in combat, the entire force can be destroyed, no matter how large.
The ZOC effect only happens when units from both sides are in the same region. This occurs most often after an inconclusive battle in which neither side was forced to retreat from the region. It can also happen if a cavalry unit in passive posture enters an enemy controlled region containing enemy units. (Only cavalry and partisan units can do this without automatically reverting to offensive posture, triggering a battle).
[SIZE="3"]Zone of Control (ZOC) – Intermediate Studies[/size]
When units from both sides are in the same region, there are three factors that determine which neighboring regions your force can move into:
- The enemy’s ZOC value (based on Patrol)
- Evasion value of your stack
- Your level of Military Control in the destination region.
Your Evasion value is compared to the enemy’s ZOC value in a complex formula to determine the minimum amount of Military Control (MC) you must have in an adjacent region to move into it.
The bullseye icon at the top of the unit panel provides detailed info regarding all of these values.
[SIZE="3"]Zone of Control (ZOC) – Advanced Studies[/size]
If you are in a region with an enemy force, you cannot enter a neighboring region if the enemy’s ZOC divided by your evasion value is greater than your military control of the region.
[INDENT](Z/E) > MC + 2*E = movement blocked[/INDENT]
Where Z is the enemy’s ZOC (modified patrol value), MC is your military control in the destination region, and E is the evasion value of your stack.
To calculate the MC you would need in a region to move into it:
[INDENT]MC = Z/E - 2*E[/INDENT]
This formula can yield negative values and values over 100%, but there is a hard upper limit to the ZOC effect. The value of MC+2*E is a maximum of 88% (90% MC, less 2 points at the lowest evasion value of 1). That is, you will always be able to retreat into a region in which you have at least 88% military control. There is no hard lower limit except when forced to retreat in battle. In a forced retreat, you must have at least 5% MC in a region to retreat into it. If you cannot retreat your force is destroyed. Outside of battle, there is no limit, so with a high enough evasion you can move into an area with zero.
These hard upper and lower limits are important to know because they come into play quite frequently, as there is a relatively narrow set of values for which the formula results will fall between them.
Each side’s ZOC in a region is determined by adding up the patrol values of all elements in all units within the region. Each element has a patrol value (listed in the element details screen) that ranges from 1 to 10 or more. Most infantry have a patrol value of 4, while cavalry have 9 or 10.
Patrol Value Modifiers
- Fort adds ZOC bonus = MC*fort level (I think all AACW forts are level 1 (?), so max bonus is +100 at 100% ZOC).
- Military Control: ZOC bonus = MC/3, capped at the amount of ZOC generated by units (so MC bonus can be as high as +33 to ZOC, but only if you have at least 33 ZOC generated by units). Currently a bug is preventing this bonus from showing in the tooltip for Friendly ZOC.
- Units in a structure generate 0 ZOC.
- Units in passive posture generate 0 ZOC.
- Fixed units add 0 ZOC.
- Entrenched units get a patrol bonus based on level of entrenchment
- Experience: +1 patrol for each odd-numbered * of experience an element has (i.e., +2 with three * of experience).
Evasion is much more complicated than Patrol because different rules apply to units and stacks. A unit’s evasion value is the average evasion of all elements in the unit, including the leader (if any). A stack’s evasion value is equal to the lowest (worst) evasion value among the units in a stack.
Unit Evasion Modifiers:
- Rough terrain: +5
- Leader Bonus: +2.5 for a leader in the unit (this bonus was observed in testing but not confirmed by Pocus - - he thinks it does not exist).
- Weather – possibly (No effect observed in tests, but it should give a bonus equal to the “hide” bonus based on weather+terrain. I never observed any bonus other than +5 for rough terrain. Either testing was incomplete or there is a bug)
- Experience: +1 patrol for each even-numbered * of experience an element has (i.e., +2 with four * of experience).
- Small Stack: +3 (less than 5 units)
- Large Stack: -3 (more than 12 units)
- Weather -- possibly (see above).
- Leader inactivity.
- Leader rank or strategic rating.
- Command point penalties.
- Military Control.
- Rough terrain = all except clear, prairie, desert, woods.
- Unassigned Leaders do not count as units for stack size bonus.
- Leaders give a +2.5 evasion bonus only as part of the unit (i.e., in a division or embedded with a brigade).
- Leaders’ biggest impact on evasion is that they have a value of 30, and this is included when averaging the evasions of the unit’s elements. (NOTE: this was not intended, and will possibly be removed in a future patch).
- Note that stack size is NOT based on CP as usual, but the number of units in the stack. From a practical standpoint, this means you will almost never see the large stack penalty. (NOTE: this will possibly be addressed in a future patch).
- Adding a wagon to a stack will drop the stack’s evasion down to 1 in open terrain – support units count as a separate unit in the stack, so a wagon’s low evasion applies to the whole stack.
- Evasion and Patrol values also play a role to avoiding combat, if your force is on passive posture or if you have the ‘evade fight’ special order enabled.
[SIZE="3"]How the Numbers Add Up[/size]
A single division operating independently will get the small stack bonus of +3. A normal division made up mostly of infantry and artillery will have an evasion of around 10 in open terrain or 15 in rough terrain. If the division is carrying a supply wagon, that would drop to 4 in open terrain and 9 in rough.
A corps or army stack can qualify for the small stack bonus (see note above – this may change). A corps with three divisions and a wagon is still only 4 units, so it’s a small stack. It would get 4 evasion in open terrain. Without the wagon it would get 10 (or whatever the worst division’s evasion is). So, even a big stack can travel with a fairly high evasion value if it leaves the wagons behind.
A large corps stack made up of more than 4 units including a wagon would have the minimum evasion value of 1 in clear terrain and 6 in rough terrain. This stack will have a hard time retreating into a region it does not control, and can be held in place by a relatively much smaller enemy unit – even a single division.
An 18-element cavalry division will have a clear terrain evasion of about 16. This includes the small stack bonus of +3 and the leader bonus of +2.5. The smaller the division is, the greater the impact of the leader’s 30 evasion rating in raising the average evasion for the unit. A cavalry division with just 4 cav or horse artillery elements and a leader would have a clear terrain evasion of around 19. At this evasion level, the unit is virtually unstoppable by enemy ZOC, but would still need the minimum 5% MC for a forced retreat into an adjacent region.
Total patrol value (ZOC) for a typical division will be around 60-70. A 4-division corps would have around 250-275. A super-sized corps of 5 divisions would have as much as 350 ZOC. Multiple corps stacks in a region can put the total ZOC up over 800, especially if there is a fort present (+100). A full cavalry division will have around 170 ZOC.
The table below shows the level of Military Control you will need to move into a region, based on your Evasion and your Enemy’s ZOC value.
EDIT: The "hard mimimum" of 5% applies ONLY to forced retreat in battle. Outside of battle, if your evasion is high enough you can move out of a contested region into one with zero MC (generally, one row higher on the table above the first red highlighted row).
You can see that a large corps with a wagon and evasion of 1 would need at least 48% MC in the destination region to move out of a region where the enemy has 50 ZOC (less than the ZOC generated by a typical division). You can also see that even with an extraordinarily high ZOC, the enemy will have a hard time pinning down a cavalry division.
Keeping a High Evasion
A large corps traveling with a wagon train is going to have a real problem retreating after combat, especially if the enemy can get around behind and block you from retreating back the way you came. You can greatly increase your evasion value by not traveling with wagons or any unassigned, corps-level artillery. If you leave these units with the Army HQ stack, and keep the HQ behind you in an adjacent region, you’ll still be able to draw supplies, and the HQ can still march to the sound of the guns and bring along his support artillery. After the battle, the HQ would revert back to his original location, which would be open for your corps to retreat through if necessary.