Tactical Movement in GITM

In GITM, when units move on the tactical map, players do not specify the exact path in which the unit will march. They specify only the start point and ending point, and optionally a waypoint to pass through. When all other units on the tactical map are allied, then moves automatically succeed. However, when neutral or enemy units are present, units must move in a straight line from the start point to the ending point, or if they use a waypoint, in a straight line from the starting point to the waypoint, and a second straight line from the waypoint to the ending point. This document explains how to tell whether movement in a straight line from one square to another will be blocked by terrain or by a neutral or enemy unit.

When units move in a straight line on the tactical map, the line on which they move is defined as a series of one-square steps. If a unit is moving more columns than rows, then the number of steps is the number of columns it moves. For example, if a unit is moving from 3-1 to 7-3, then it is moving 4 columns and 2 rows, hence its move requires four steps. If it is moving more rows than columns, then the number of steps is the number of rows. For example, if a unit is moving from 11-4 to 8-13, then it is moving 3 columns and 9 rows, so the move requires nine steps.

If the unit is moving more columns than rows, then at each step it must move one column and less than one row. Let C be the number of columns it moves and R be the number of rows it moves (with C bigger than R); then the fraction of a row it must move is R/C. Example: If the unit is moving from 3-1 to 7-3, which is 4 columns and 2 rows, then in each step it must move 1 column and 0.5 (2/4) rows. Its path will then be: 3-1, 4-1.5, 5-2, 6-2.5, 7-3. If the unit is moving more columns than rows, then each step must be 1 row and C/R columns. Example: if the unit is moving from 11-4 to 8-13, then each step is 1 row and 0.33 (3/9) columns. The path will be 11-4, 10.67-5, 10.33-6, 10-7, 9.67-8, 9.33-9, 9-10, 8.67-11, 8.33-12, 8-13.

To determine if a path is blocked by terrain, round each row or column to the nearest integer (round 0.5 down; thus, round 2.5 to 2). In the first example above, the rounded path would be 3-1, 4-1, 5-2, 6-2, 7-3. In the second example, the rounded path would be 11-4, 11-5, 10-6, 10-7, 10-8, 9-9, 9-10, 9-11, 8-12, 8-13. A unit's move is blocked by terrain if:

Example 1: A unit moving from 11-4 to 8-13 is blocked by terrain if there is impassible terrain at any of the squares 11-5 (rounding 10.67 to 11), 10-6 (rounding 10.33 to 10), 10-7, 10-8, 9-9, 9-10, 9-11, 8-12, or the destination square 8-13.
Example 2: If there are river or mountain squares at 4-8 and 5-9, then a unit moving from 4-9 to 5-8 or vice versa is blocked by terrain. (Note that if there are lakes or other impassible terrain at 4-8 and 5-9, movement between 4-9 and 5-8 is not blocked; the two lakes do not join diagonally.)
Example 3: If 7-3 is flat terrain and 8-3 is high hills, then movement between 7-3 and 8-3 is blocked by terrain.

Whether a path is blocked by an enemy unit depends on whether the move is due N,S,E, or W, due NW, SW, SE, or NE, or some other direction.

Case 1: A move due N, S, E, or W. In this the case the move is entirely in one row or one column, for example, a move from 5-2 to 5-7 is due south along the path 5-2, 5-3, 5-4, 5-5, 5-6, 5-7. This move is blocked by an enemy or neutral unit if any square of the path contains an enemy or neutral unit. Otherwise it is not blocked.

Case 2: A move due NW, SW, SE, or NE. In this case the move is on a 45-degree diagonal, and the steps are 1 row and 1 column each. Example; A move from 5-10 to 8-7 is due northeast along the path 5-10, 6-9, 7-8, 8-7. In this case the path is blocked if any square in the path or adjacent to it contains an enemy or neutral unit. This is because the path from (say) 6-9 to 7-8 passes diagonally between 6-8 and 7-9. An enemy unit in either of those two squares can block the movement. An enemy unit at 5-8 (which touches 6-9 only diagonally) cannot block the movement.

Case 3: Any other move. In this case the move will necessarily contain fractions in some steps along the path. Example; a unit moving from 3-1 to 7-3 passes through 4-1.5, 5-2, 6-2.5, and 7-3. In this case, an enemy or neutral unit blocks the path if it is in a square on the path, rounding fractions in either direction. A unit in 4-1 can block the path because 4-1.5 rounds down to 4-1. A unit in 4-2 can also block the path because 4-1.5 rounds up to 4.2. An enemy unit in 5-2 can block this path, but an enemy unit in 5-1 or 5-3 cannot, because 5-2 involves no fraction and cannot be rounded, up or down, to either 5-1 or 5-3.

Graphical examples

T/U means blocked by an enemy unit or impassible terrain at this square. U means blocked by enemy unit, but not terrain, at this square.
 
Case 1: N,S,E,W movement Case 2: NW,SW,SE,NE movement Case 3: Other movement
Movement from 1-1 to 5-1:
Path is 2-1, 3-1, 4-1, 5-1
 

1 2 3 4 5 6
1            
2            
3 FR1IN T/U T/U T/U T/U  
4            
5            
6            
Movement from 1-6 to 5-2:
Path is 2-5, 3-4, 4-3, 5-2
 

1 2 3 4 5 6
1            
2       U T/U  
3     U T/U U  
4   U T/U U    
5 U T/U U      
6 FR1IN U        
Movement from 1-2 to 6-5:
Path is 2-2.6, 3-3.2, 4-3.8, 5-4.4, 6-5
 

1 2 3 4 5 6
1            
2 FR1IN U        
3   T/U T/U U    
4     U T/U T/U  
5         U T/U
6            

Maintained by Stephen Schmidt. Last updated 12-30-11.