I have the spiral curve offset problem and how to put it in a description.
I found reference this book on the subject. Has anybody bought the book.
I suppose in a description you could describe the chord and just say the boundary is offset from the centerline spiral. This is first for me, how is it done?
Mr. Crume has posted on this board.
Leon:
IT DEPENDS on the type of "spiral" that you are talking about.
Searles Spirals are a piece of cake, Talbots, 10-Chord(s), Euler/clothoid, etc, are another issue.
Loyal
A real simple single 200 foot long, 2 degree spiral into and out of a highway curve. CAD knows how to do and I've done the real math exercise in the past. The chord is only about 0.8 feet away from the spiral near the center. Break it into a couple straight sections and wouldn't be far off. I'm not going to stake it, I just need to describe it.
Then describe it as being parallel to and at a 50' right of the described C/L. It is not mathematically possible to describe parallel spirals. There is a good reason you don't find C/L and parallel ROW spiral descriptions written by those who used spirals. They the C/L spiral description and call for a line being such and such distance from and parallel to the following describe C/L spiral. Your computer will do it's best to draw such a line but it won't be right, at least along the spiral segments of the ROW line.
jud
I megaloathe spirals (hate is just not strong enough of a word). IF you've done the work, and IF you feel it's right, THEN, why don't you break it into a few chords along the spiral, and at the end, say they chords were along the spiral and give all the data to back it up.
KISS principle
Most discussions in which I've read about so termed "parallel spirals" refer to them as "evolutes." The form of the equation for a spiral evolute differs from that of the spiral equation. Generally, because it is not a spiral of the same equation form, many do not use the term "parallel spiral." Nit-picky semantics!
Spiral evolute offsets on the concave side of the spiral should be less than the smallest spiral radius of curvature. If not, the points generated on the evolute can be or are in reverse sequence sense to those of the spiral.
I suppose in a description you could describe the chord and just say the boundary is offset from the centerline spiral
Of course, there should never have been a boundary or right-of-way along a spiral, the old railroad guys knew this simple idea, the right-of-ways for railroads are always simple curves (except when that aren't) even if the centerlines are shown as spiral-curve-spiral on a valuation map-those are wrong by the way.
However, sometimes they were done that way; in my opinion not a very professional thing to do.
And when they are, its best to describe the elements of the spiral-curve-spiral along the centerline, tell what your offset is and direction of it, then a chord along the right of way line so someone can come behind you and retrace.
Your computer will give you the line and probably a number of short chords making a poly line representing the spiral segment and you can get an acreage that way.
It's awful if anyone is still doing them. I think I have shamed most designers I work with to STOP!!!!!:-@
I think jud is on the right track, so to speak.
When right-of-way offsets to spiral curves are referenced here, it is ALWAYS to the centerline, e.g., "...thence parallel with and XX feet NESW'ly from a clothoid spiral curve of length XXX feet beginning at Highway Engineer's Station XXX+XX.XX, entry bearing X XX-XX-XX X, entry degree of curve zero, exit degree of curve XX,..." and so on and so forth.
I have found there is really no practical way to condense a spiral curve legal description as far as clothoids go, they do seem to be long-winded if you actually include the info necessary to build it out from scratch.
As was mentioned earlier, the Searles is quite easy in comparison since it is in reality a multi-centered compound curve.
I assume this thread was about -legal- descriptions of spiral offsets...
> Then describe it as being parallel to and at a 50' right of the described C/L. It is not mathematically possible to describe parallel spirals. There is a good reason you don't find C/L and parallel ROW spiral descriptions written by those who used spirals. They the C/L spiral description and call for a line being such and such distance from and parallel to the following describe C/L spiral. Your computer will do it's best to draw such a line but it won't be right, at least along the spiral segments of the ROW line.
> jud
Nailed it!:good:
Thanks for the example. Probably what I'll do.
I have sort of an interesting one here. I need to describe both sides of a state highway for about a mile. The road was first upgraded to a state highway from a county road in 1937. They took a 100 foot wide ROW. It had all 3 degree simple curves. In 1964 they rebuilt and improved the highway. They realigned the center line and widened it for slopes. In this stretch the widening was mostly on one side. They switched to 2 degree curves with spirals. The road commission didn't release the "extra" ROW on the old side. So on one side of the highway the ROW remains the old simple curves and the other side has new and improved spiral curves. There are two engineer's center lines that cross in a few places. Most of the ROW marks remain from both projects, just on the one side from the 1937 road plan.
The adjacent property descriptions never have been upgraded other than to except that portion in the state highway. I've seen surveyors in the past "stake" the corners in the ROW from the original descriptions.
Chucking this in for good measure.
Not sure that it will give a descriptive solution but certainly gives a mathematical one!
https://dl.dropboxusercontent.com/u/3449356/Line-Spiral.pdf
Yeah, I'll just give the chord and add this as a note to the description for those that really want to know. The CAD guru's should program that into their software.
