battlecry said:
I wanted to know this as well, so I went and re-read the EU3 manual to see how it worked there. And yes, the percentages indicate the chances of a battle being fought on a particular terrain - so daemon's example above is accurate. However:
i) The EU3 system is calculated by "die rolls" - certain terrains, if attacked into will take away from the attackers "die roll" thus reducing the effectiveness of their attack. So in a 70% Forest/30% Mountain province, the attacker has a 70% chance of losing -1 per die roll (forest modifier) or a 30% chance of losing -2 per die roll (mountain modifier). The defender's 'die roll' is unchanged.
ii) It is possible, and might be more accurate for the scale of WWII battles, that the terrain % be recalculated for each 'die roll' rather than only once for the whole battle. This would simulate the progress of the stages of a battle through different terrains of a province, as each time it is the attacker's 'turn' his chance of receiving one or the other modifiers would be recalculated, all in the course of the same battle. So in the first "attack" stage he might be fighting in Forest, while in the second he might be fighting in Mountains, etc.
You are completely overlooking three important points in comparing EUIII to HOI3, even though they will use the same engine:
1. The size of the provinces
2. The shape of the provinces
3. The area of the armies
In EUIII the provinces are many times the size of those in HOI3. Compare the HOI3 Alpha Screenshots such as Dev Diary 6 South East UK to:
The East Anglia EUIII province covers the area of 11 provinces in HOI3.
East Anglia, and AFAIK other EUIII provinces are clearly based on historical administrative areas, there was no attempt to distinguish areas on the basis of terrain. It makes sense that such large areas might happen to encompass two or three different terrain types.
In Medieval times a battle would take place in an area smaller than most modern housing estates. In a province the size of East Anglia there clearly was a chance that the battle might take place in plains or marshlands.
I do not believe that Paradox will implement the EUIII system that you have described in HOI3. In a province measuring only around 30km across it makes no sense to suggest that a battle between several division sized units with a frontage of 10-30km, and with a depth of about 20km, that there is only a chance that the battle will take place in one terrain, but it might take place in another.
IMHO the HOI3 provinces are so small that it could be reasonably assumed that the whole province contains a single terrain, without it detracting from the game.
In HOI3, as Johan has explained, the provinces are not based on administrative areas, So they can be made to follow to some extent the real topography. Looking at the grey blobs and such on the HOI3 screenshots which are meant to indicate terrain I see nothing that would clearly indicate otherwise. The terrain features reasonably follow the province boundaries. Where they go over into an adjoining province, I would suggest this is just a measure of artistic licence to break up the map a little and make the topography look more realistic than it would if they stuck rigidly to the province boundaries. You can FE see the same effect with the rivers. A small part of the province of Slough can be seen north of the River Thames, but I would suggest that if you attack Bedford from Slough, there is not a chance that it would be considered that you might be attacking from north of the Thames, and therefore avoid the river crossing penalties. They just want the rivers to look less artificial than they would if they followed too closely the province borders.
o 
