Perhaps this site is a little graybearded to be discussing basic tools, but so it goes. I want to talk about site modeling, which is 90% mesh management, and I thought it better to break out the basics. Though site topics tend to intrude. Because the mesh tool is unchanged (+/-) since Archicad 6.5, we can also 'wish' for 'improvements'.
A mesh is a slab with a bumpy top. Without the bumpiness, it is indistinguishable from a slab. It's a polygon with three materials and one pen for plan and 3D. (Ignoring the other pen for the non-user ridges in plan. Why would I want to see those.) It doesn't interact with the floor plan cut plane and it doesn't have overhead pens. It has a cover fill, but unlike a roof, the fill won't distort along a slope. Archicad 6.5 really was a long time ago.
The construction methods of the mesh tool are surface-only, hollow, and solid. You'll hardly ever use anything but solid. The others are only surfaces and can't be used for solid operations. We wish for a thickened-surface road-type construction method.
A slab is placed by its top; the top is at the slab's elevation setting. Since a mesh's top is bumpy, that doesn't work and the mesh needs something for the elevation setting to refer to. This is the basis plane of the mesh.
The bottom of the mesh, which is flat, is positioned by the thickness setting, relative to the basis plane.
Originally, the bumps on the top were created by giving elevation distances for points, relative to the basis basis plane. Around version 8 or 9 we got the ability to set these heights relative to other, more meaningful planes: Project zero and sea level (assuming you have your levels set up that way). For most mesh work (i.e., site modeling), working to these planes is more productive.
This mesh has a thickness of 10'-0" and is placed at 5'-0" above the story, which is equal to project zero.
The thickness needs to great enough to accommodate the lowest elevation of any point on the top, or the mesh will go surface-only. If your lowest elevation is -4'-0", you need a thickness of at least 4'-0".
It is possible to do an upside-down mesh with the bumps facing down, by using a negative value for the thickness.
Mesh point elevations can be edited in plan with precision and efficiency, in 3D with precision and poor efficiency, and in section/elevation not at all.
In plan, the polygon palette associated with nodes has an extra button which will open the mesh point height dialog. Here you can enter the elevation relative to the basis plane, project zero, or sea level. (We don't use the 2nd reference level; it retains its generic name.)
You will also get the button on an edge. When you set the height of an edge, that height is used for the nodes at both ends of the line.
The 'Apply To All' checkbox controls whether the height edit will affect just the current point or all the points associated with that point. Associated means: All the points drawn at one time. So the main mesh polygon is an 'All', a polygon defining a hole is an 'All', a polyline of points is an 'All'. (Not sure what to call these 'All' things, and the reference guide is silent on the matter.)
Height editing is also available in the 3D window, but it's different. You can set the height of a point graphically by aligning it with another 3D element.
There is no mesh point height dialog, but you can type Z to enter a height relative to project zero, sea level, or the current user origin, but not the mesh basis plane. (In AC11, sea level is bug-crippled. In 12 it works.) The most frustrating limitation in 3D is the absence of 'Apply to All'. You can only edit one point at a time. This is a show-stopper for working with topographic contours.
Note that a vertical surface is not allowed, because that would require coincident contours at different heights.
Adding points to a mesh is just like editing any other polygon element, up to a point. Use the palette to add edge points. Draw a polygon within a selected mesh to create a hole.
Here the mesh becomes special. If you draw a new polygon within a mesh, you get a dialog box asking if you want to create a hole or add new points. The latter option can be used to create a plateau of a given elevation. And, such a new geometry does not have to be closed; it can be a polyline.
There is also a pulldown with options for how the default elevation of the new points should be set; always use 'Fit to User Ridges'. This places the new points 'on the ground' between your other elevations. The other options are not useful in my opinion, which makes the whole control rather silly. Fitting to 'All Ridges' doesn't makes sense; the non-user ridges are approximations that have nothing to do with the user's data. 'No Surface Fitting' places the new points at zero; not sure what that's supposed to accomplish. Contra this discussion (at 2.), of course I'm going to set the heights of the points, that's why I'm adding points.
Subpolygons can be selected independently from the mesh as a whole by clicking on the edge. If you select by a node you will get the whole mesh. To delete a contour or other subpolygon, just select it by an edge and strike delete.
Each additional set of new points is considered an 'All' for the purposes of elevation editing.
Kinds of All
So, to create topographic contours, draw a series of points, click on an node, select the Z-height button, set the elevation, and check the 'Apply to All' box.
What happens to a node at the intersection of an edge polygon and a contour? You get a weird node that is rather conflicted about its true nature. It should go along with the contour, since it was created at the same time. But it acts like part of the edge polygon. When you 'Apply to All' for the contour height, the node on the edge is unaffected. You have to set the height of that node separately. This is a error in the design of the tool; the contour should get priority. I consider this a real bug that wastes time and causes errors.
On the other hand, very large meshes can cause performance and integrity problems, and the recommended fix is to split the mesh into two (etc.) pieces. Contours meeting those shared edges might need to be moved, and there's your headache.
If you do have to move an edge/contour node, you need to know the trick: Move the edge out, move the contour end in, and put the edge back.
Naturally, you can magic-wand contours onto existing geometry. If the 2D contour meets the mesh boundary, you will get the psychotic compound node at that point. Actually the magic wand is the only way to get it. To avoid it, trace the contour manually or stop the 2D element before the edge. You would then need to add an edge node manually, and set its height to match the contour.
If you do use the magic wand, be aware of the Magic Wand Settings, which controls faceting of curves. This is key for keeping the mesh from becoming too complicated.
So, sorry, again, the obvious workflow is to get 2D contours and use those to magic-wand the mesh contours. But this leads directly to weird nodes. Normal work shouldn't lead to annoying tool behavior. Listen, Graphisoft mesh person: Topo goes to the edge. Deal with it!
In plan the mesh appears as a polygon, with lines at the 'User-Defined Ridges', as the subpolygons are known in the settings. If you show 'All Ridges' in plan, you get triangulation lines. I don't see the appeal of this, most days.
The mesh has limited plan attributes. There's one pen for the polygon and the 'Ridges', and another for the triangulation junk that I leave off anyway. There's only one linetype for everything. It can display a cover fill, but there's no foreshortening distortion due to slope.
In order for the mesh to meet site plan standards, we would need separate pens and linetypes for the perimeter and the contours. Further, the contours need to be allowed to curve and spline. (Whether they can truly curve in 3D is another question wish.)
There's another graphical issue in 3D and elevation. The 'Model Ridges' setting allows you to show the triangulation junk ('All Ridges Sharp'), hide the junk ('User Defined Sharp'), or hide everything ('All Ridges Smooth'). Oh wait, that doesn't work. 'All Ridges Smooth' is the same as 'User Defined Sharp'. (The only place you'll see a difference is in photorendering.) If your elevation is looking uphill, you will see horizontal contour lines. Again, if we had more attribute flexibility, we could set the contours to a non-printing pen.
• Independent plan attributes for the boundary and contours.
• Curved contours, in plan at the very least.
• Give contours priority at nodes shared with edges.
• 'Apply to All' for contour heights in 3D.
• Hide contours in elevation.
• Thick surface (road) construction method.
• Allow vertical surfaces.
Beyond the scope of the tool as designed, advanced features would include composite skins, more blobbiness (contours on all sides), and some sort of accessory capability. The mesh as it stands is primarily a (weak) terrain modeler, while Archicad completely lacks a truly free-form modeling tool. How much blob modeling should be natively available in Archicad is a topic of ongoing debate, but basic site elements such as roads should be a given.
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