More info on how you should plan your land objects:
The game engine relies heavily on calls to a section of code called the collision modeler, or collision traverser. The collision traverser is essentially a continuous check on where you're at and if you're going to collide with something. And frame rate is impacted enormously by the ability of the traverser to keep up with detecting collisions of your plane or vehicle with the terrain and objects. And that speed, and your frame rate, is super critical when it comes to large land objects and how they are set up.
In layman's terms, each land object is actually a hierarchy of groups, and each group header is a node. So a simple, square, 1 mile, single poly, 4-vertex object could in theory have a hierarchy of the following:
world
group 'model'
group 'ground'
group 'low'
group 'g01'
poly 'tex0'
Each line above that starts with 'group' represents a node. To detect a collision, the traverser is constantly looking at where you are at and what is close. The nearest nodes have to be instantly processed, and then the polys inside it have to be assessed by the traverser to tell the game that a collision happened. With just one poly, the collision traverser would only have to process that single poly.
Now consider the same 4 mile object, with 18,340 polys, called 'ttown2x2m'. This is the land object we see at the center of Ndilses in AH3. If you kept all 18,340 polys in the one node "group 'g01'", the traverser has to process all 18,340 polys in one step, at every moment. That can impact frame rate. It would take the traverser 100 times longer - every instant - to traverse all 18,340 polys at once, than it would if the traverser only had to traverse 183 polys in the one node out of 100 that the game was interested in, had you broken up the hierarchy into 100 nodes of 183 polys.
And it would be even more efficient for the traverser if you broke up those 100 nodes of polys into 25 nodes of 4 nodes, and those 25 nodes into 7 groups of 1 to 4 nodes, and those 7 groups into groups of 1 to 4 nodes. That reduces the traverser's work load. So game execution becomes more efficient the more hierarchical the terrains and objects become. You don't want it to be ragged, or unbalanced. You want it hierarchical.
So, when planning a land object, you want to think in terms of hierarchy and balance. Break it up into quadrant nodes (nw, ne, sw, se), then grid nodes (g01, g02, ...) within the quadrant, all the way down to nodes that are 1/4 mile squares. This 'breaking up' can include areas of highly detailed meshes: if you really want a detailed mesh, like the high ground at the very center of the Ndisles tank town, or a series of berms, create those as meshes that drape over the rough-mesh terrain, and make those draped meshes as their own nodes. The traverser won't look at the hidden mesh, just the stuff that is on top and visible.
Two important notes:
1. Object count matters a great deal to the game overall. The current limit is roughly 50,000. However...
2. A single object can have many surfaces and vertices. The current limit for a single object is roughly 60,000 verts.
The templates I am creating will have a library for use inside of AC3D, with this hierarchy already baked in. The library will also include various objects like road sections, railroad sections, bridge abutments, berms and families of berms, small hills, cliffs, that you can pull from the library and drop onto the terrain. And I almost hesitate to add this, but caves and railroad tunnel entrances are possible. But in all cases, you need to constantly check the Tools / Model Info to make sure your vert count isn't getting too high.
