We've decided to use the vehicule rules from Ultimate Combat in an upcoming game, but we've ended up having a debate as to whether or not the 5 foot squares which make up a vehicle should or shouldn't have to match 5 foot squares on the battle mat.
My argument in favor of making them match is that it would probably ease play. Some arguments against making them match might be that it creates a weird looking situation where a 5x10 light chariot or cart being drawn by a horse would be off center compared to the horse or that a player might prefer to have her 10'x20' vehicle have a 5 foot wide lane of open squares down the center with furniture and other objects in the 2.5' spaces to the sides (as opposed to having a 2x4 space where all of the squares are halfway filled with objects).
Perhaps some pictures would help explain what I'm talking about. Imagine that each letter below is 2.5 feet wide and tall so that a 4x4 square of letters is a 5 foot square. By the snap to grid method the horse (H) and chariot or cart would be arranged in one of these two configurations:
HHHHCCCC
HHHHCCCC
HHHH
HHHH
HHHH
HHHH
HHHHCCCC
HHHHCCCC
If the vehicle doesn't have to match squares on the battle mat then the arrangement could be a more elegant one like this (note that since the Large horse always has a grid vertex at his center the vehicle would "straddle" 4 different squares):
HHHH
HHHHCCCC
HHHHCCCC
HHHH
While writing this up I realized that a reasonable compromise for this particular case might be to assume that the chariot or cart is wider than it is long (which seems pretty odd for the cart). That would look like:
HHHHCC
HHHHCC
HHHHCC
HHHHCC
The last solution is probably the cleanest looking, but even it doesn't answer another question we have, which is what happens if a vehicle tries to assume a diagonal position. Imagine that a 50 foot long keelboat decides to turn and face NW on a map where the squares run north to south. Does the ship have its own special grid which moves with it and needs to be adjudicated on the fly by the DM to judge where its squares are in relation to those on the battle, or can a ship only face in whatever 4 directions would allow its squares to still map onto the battle grid? Certainly the rules for the "Turn" action indicate that the driver changes the vehicle's facing by 90 degrees (so there are only 4 directions). Should you be able to turn less?
If vehicles should have their own "special squares" should it be legal to design them such that they occupy half squares on the battle mat (such as the chariot in my second diagram)? If so what squares would a rider in the chariot who wields a melee weapon threaten? Would it be just the 4 squares the chariot straddles or those squares and the 10 squares around them (as if he were riding a large mount)? I suppose that diagonal facings would probably require off the cuff ruling by the DM in any event.
I'm mostly in favor of a "snap to grid" method and only having 4 facings even if it looks clumsy since it seems easiest to adjudicate. On the other hand, I'll admit that my method could cause some pretty weird looking situations where a wagon following a SE to NW road on a north/south grid would be facing north or west despite the fact it is moving NW. What do other people think though?
I agree with you, but snap to grid is where it's at.
It's the simplest and most workable way to adjudicate the rules, even if it might look 'odd'.