The situation I have is a plat boundary that follows part of a highway right-of-way line that's 200' wide, 100' on each side of a centerline which is described in tangent-spiral-curve fashion. How do you write a legal description along a line that's an offset of a spiral? For whatever reason, I've never had to do this before, and the old-time plats I've seen this done on don't really have a consistent mathematical format, other than just calling out the chord from one end of the spiral to the other.
Are you writing the description for the first time? Make sure this is what they want, if you are.
Otherwise.
What does the deed say that conveyed the property for the Right-of-Way take?
Most of the time; the Right-of-Way line isn't on a spiral...
If you search this site; I remember @mightymoe had a pretty good take on this subject.
The proper language may exist in the highway take deed. If not it's fairly simple.
Thence along the right of way descriped in deed instrumrnt number x on a spiral curve lying 100 feet left of the following described center line...... ...the long chord of which bears x, a distance of x...
It's a highway? If it has an alignment (I would think so), the portion of the description along the ROW could also be described by station and offset.
It's a mixed bag with highways, unfortunately they often took ROW's using a spiral curve spiral. DOT heard enough complaints about that procedure that they stopped using them for ROW. Clearly, it's needed for pavement but unless the highway has to have a perfectly aligned pavement section, it's not great for ROW.
I will call out the elements of the curve centerline, apply stationing as needed for jogs and define the offset. It's always messy, you might try and follow the original taking, but those can be so wordy, adjusting that language can be very helpful.
Most descriptions I see in this case simply state "South of Hwy" or "NE along the margin of Hwy" or simply except out the Hwy (and you are lucky if you have an instrument number).
But, I agree with bionic's approach.
@dougie I'm not writing a new description in this case. The r/w deed legal description is written something like: "That part of Section 1, lying within a strip 200' wide, 100' on each side of this centerline..." then it describes the tangent line, then a distance along a spiral of a 2 degree curve for a distance, then along a 2 degree curve for a distance, etc.
I should also mention that my project is a re-plat, and the previous plat boundary described this line as something like: "thence northerly along highway r/w line to the section line, etc.".
I'm dealing with something similar, although I don't need to write a description. Is there enough information for you to determine which spiral curve definition was used for the centerline?
Here is a Professional Surveyor (xyHt) article from 2015 that discusses setting offsets from a spiral curve.
@gene-kooper I'm not sure how I would determine the spiral definition if it's not specifically called out (which I've seen sometimes in the description). All I'm given on the plat is the delta, degree of curve, Ic, Sc, Yc, Xc, L, & Ts.
The elements given are often a clue to the spiral type. The era and source of the alignment are also clues. Any found monuments with evidence of reliance should help you stick a fork in it...
@allen-wrench I hear you. I called an expert about it and am waiting to hear back. Maybe Loyal will be along shortly to provide his wisdom on spiral curves.
Here is an 2020 post of his in a thread on railroad curves that ends with some info on highway spirals.
Railroad ROW w/ Searles Spirals
Loyal expanded on the topic with several follow-up posts in that thread.
ETA: Fixed link and attribution to Loyal for the thread