Hi Dave,
For the vast majority of road design (based on the ease of doing manual
computations) horizontal geometry has been based on a combination of
tangents, arcs and spirals (normally clothoid), while vertical geometry has
been based on the parabola. I'm sure the cubic parabola would be a better
theoretical model for the vertical alignment, but without getting into that
discussion, it seems there is a limitation of human capability of designing
the combined curve as one object.
Until someone puts together an "object" which modifies the 3D behaviour
while reporting:
Cost - construction
Cost - annual operating for the community as a whole
Safe travel speed in dry weather, in wet weather, in snow conditions to
suite much of North America, Europe and north Asia and only an area of
Australia about the size of Switzerland.
and allows the user to drag to it around in 3D, I think it is likely we will
continue to do design the two more or less independently.
The B-spline type curves may be viable for safe speed determination -
possibly someone has a PhD in analysing this. If so, it hasn't come to my
attention. However, unless there is a viable theoretical model, you
probably need to stay with an object as described in the first paragraph.
In terms of appearance in a drawing, there is a real limitation on using a
tessellated 3D polyline. That is the real inability to grip edit it in a
meaningful fashion. The alignment editing tools seem to handle this well in
2D, but for time reasons I haven't investigated the 3D dragging
characteristics.
--
Laurie Comerford
CADApps
www.cadapps.com.au
"Dave Simeone" wrote in message
news:4050ffc8$1_2@newsprd01...
> Excellent feedback.
>
> Q - Historically we've generally "accepted" the tessalation of 3D vertical
> curves (ie, break the geometry into short segments) when working from a
> grading feature line. I'm guessing that ya'll - (I'm trying to learn to
> speak in Wedding's language - I've got "Ya'll" and "All y'all" down pretty
> nicely) - would like a smooth 3D spline. What is the requirement? The
> smoother the better? Engineers go to great lengths to have accurate
vertical
> (profile) geometry. I'm guessing that the true 3D geometry should be
carried
> through the grading of the EOP, Kerb (how's that all y'all Aussies?), etc
> geometry.
>
> Thanks
> DAS
>
>
> "Steve Cannon" wrote in message
> news:40508db2$1_2@newsprd01...
> > Hi Dave,
> >
> > > Does the need to grade off a profile become less
> > > important if there is dynamic corridor modeling
> > > functionality in the product? Is there something else
> > > that you are doing that may require a different solution?
> >
> >
> > I do not wish to speak for Brian, I am sure he will offer his own take.
> But
> > as a site designer, the marriage of a grading object to a profile (and
> > alignment) is the most important grading feature that has never been
> > addressed in either LDT nor C3D. You guys have always treated corridor
> > design and site design as two distinctly different disciplines, when in
> > fact, the designer often needs to use common methods. The approach would
> be
> > to use 'quick' alignments for much more than traditional roads and
> channels.
> >
> > As a site designer I may want to layout the swale or berm in plan view,
> but
> > input or modify the vertical data for that swale in profile. Then attach
> > various grading objects that describe the ditch/berm and any associated
> > features, then control the whole grading by the profile(and alignment).
> > Stationing may be arbitrary as opposed to absolute. Having to go through
> the
> > whole corridor process by defining templates would be an exorbitant
waste
> of
> > time.
> >
> > As I design the back of a cul-de-sac or a return, I may want to layout
the
> > curb in plan view (arc), add some elevations to the end points, and then
> > view that arc in profile to add a pvi add the back of curb with a
vertical
> > curve. A grading group could be made to represent the sidewalk and row.
> and
> > I could see my grading change as I change the profile.
> >
> > In fact, for local residential roads in subdivisions, forget corridor
> design
> > altogether (at least for preliminary design) Layout a CL alignment,
define
> a
> > FG profile, attach several dependent grading objects to the alignment
that
> > define the road section, and play with profile while querying the
grading
> > group. Now you are designing a road in 2d and 3d at the same time. I
> > suppose you could even take it a step farther and attached pad grading
to
> > the grading group, since most pad grading is controlled by roadway top
of
> > curb elevations.
> >
> > In LDT 3d polylines have limitations - no arcs in 3d. Designers really
> need
> > to work in profile and plan at the same time. We get around this by
> > converting 3d polys back and forth to alignments and profiles using our
> own
> > routines. But we are restricted by the definition of the 3d poly. In
> C3D,
> > you could have a feature lines, made up of arcs and tangents, and
> hopefully
> > controlled by alignments and profiles. So we can have curves that have
> > elevations. By attaching alignments and profiles to feature lines to
> grading
> > objects, you provide a very simple process for grouping simple grading
> > objects to define a more complicated design, that is controlled by the
> > alignment and profile. You also need to supplement features lines with
> many
> > more interactive ways for the designer to edit and manipulate the
> elevation
> > directly from plan view, that result in the profile being changed, that
> > result in changes to the grading group.
> >
> > sc
> >
> >
>
>