Spiral Curves Blah

[chris-bouffard]

If the plans give you all of the information that you need, you should easily be able to calculate the spiral. I have come across this situation several times in NJDOT highway situations and NJ is good at setting up the monuments and giving you all the info that you need with baseline stationing and offsets, as well as the curve data.

[BStrand]

Right, but I guess my question is what element of a spiral curve is typically held?

I found a handy resource that I’m reading through at the moment and I think it’ll get me going the right direction, but I’m always curious to hear how the vets have handled them too. I think the last time I ran into a spiral curve at work was 3 years ago and then 4 years before that… so just long enough to forget everything I knew about them. 🙄😆

[chris-bouffard]

The elements most important are the length, radius, and delta that are given with a solid check in being the baseline station and offset. It took me hours sitting with my lead tech to explain the whole thing with him, and, at the end of the day, the light bulb went off in his head. Will he remember it three or four years down the line? I think so because we work along numerous county roads that use the DOT format, and they have lots of jug handles.

[BStrand]

How does a spiral curve have a radius with it being a parabola? The length and delta make sense, but my measured values won’t match those exactly since I’m holding found monuments. It seems like what I need is a way to proportion the record degree of curve to fit my measured spiral delta?

[murphy]

I’ve never had the nerve to do it, but it might be more realistic to calculate the R/W plan spiral (the one time it’s great to work at an engineering firm) then calculate a spiral that’ll fit the found monuments and show both or, more likely, represent it with an obnoxiously thick lineweight. I tend to favor the position that monuments hold over the intangible centerline, but I’m aware of instances where the opposite view prevailed.

[BStrand]

then calculate a spiral that’ll fit the found monuments and show both

That’s basically what I was planning to do, show record data and measured spiral + data. I’m not quite sure how to come up with the measured spiral at the moment though.

[john-putnam]

The given radius of a spiral is equal to the radius of the of full curve at one end and infinity at the tangent end. The spiral equation then creates curve (not a parabola) of varying radius beginning at infinity and ending tangent tot the full curve.

They are pretty easy to establish in 3CD but take a bit to do the math on paper. I normally hold the tangents, the radius (or degree of curve) of the full curve and the length of spiral.

• This reply was modified 1 month ago by  john-putnam.

[richard-germiller]

Dealt with spirals so much in AK. I would give the length of the spirals, sometimes not equal, the overall spiral tangent, the “a” value (rate of change, be careful, Civil3D gives some “A” value that is not the same thing, should be a number <1) and I’d also show the chord of the spirals on the centerline and the sides if the R/W needs to be defined, since a spiral can’t be offset, but it sounds like you are just dealing with a defined centerline and just showing a width, and I would include the usual circular curve data

Since a spiral can’t be offset I’m curious how others would define the R/W, on US Surveys BLM would define with equally divided short chords. I’d considered, but didn’t get the opportunity to try defining a separate spiral for the sides of the R/W.

Hope this helps.

[BStrand]

I normally hold the tangents, the radius (or degree of curve) of the full curve and the length of spiral.

I’m using Carlson + autocad so maybe you wouldn’t be familiar with the process, but any idea if there’s a way to hold the TS and the SC and have the software calculate a degree of curve and a spiral length based on equal chords between these points?

[thebionicman]

I would look to the deeds and ensure the take called for the plans or included a spiral. You may dodge the bullet (probably not).

Assuming the spiral is there, hold the elements given for calculations. It’s worth picking up the common texts in use at the time of road creation to determine which methods may have been employed.

In Idaho it is very common to find monuments on the right of way but not at station. It is unfortunate many surveyors will force their favorite spiral routine on the owners and push the curve several feet from clear evidence….

[BStrand]

Since a spiral can’t be offset I’m curious how others would define the R/W, on US Surveys BLM would define with equally divided short chords. I’d considered, but didn’t get the opportunity to try defining a separate spiral for the sides of the R/W.

Carlson apparently has a routine to offset a spiral. Whether it works or not I don’t know, but I guess that would be my first solution.

I would look to the deeds and ensure the take called for the plans or included a spiral. You may dodge the bullet (probably not).

I’m not seeing any sort of record of a take or an exception for the right of way in the deeds, so as far as I can tell it’s a prescriptive easement.

Assuming the spiral is there, hold the elements given for calculations. It’s worth picking up the common texts in use at the time of road creation to determine which methods may have been employed.

Hold the elements of the spiral in the new description even if they don’t exactly fit the evidence on the ground? I’m cool with that as long as I don’t get a phone call 10 or 20 years from now from a surveyor going hey man what the hell…

In Idaho it is very common to find monuments on the right of way but not at station. It is unfortunate many surveyors will force their favorite spiral routine on the owners and push the curve several feet from clear evidence….

Right, which is what I’m trying to avoid doing by coming up with a “measured” spiral.

[thebionicman]

Nowhere did I say hold something in the face of evidence, with the exception of stationing. Any math solution must be tested against the other evidence. If there is a description with a spiral in the current deed it almost certainly came from a take deed or plans. Those creation document elements are your starting point. They can be elusive but are worth a deep dig.

Sidebar on stationing… Roads (here) were usually laid out center line first. It was a longstanding habit to set the monuments with a 90 glass (or worse), but the offset was chained.

[BStrand]

If there is a description with a spiral in the current deed it almost certainly came from a take deed or plans.

This was already explained in my first couple posts of the thread– the owner wants to split his parcel along the centerline of a highway that crosses it. The highway plans show a spiral on the centerline; this is the only place the spiral is mentioned.

[lurker]

I’m not seeing any sort of record of a take or an exception for the
right of way in the deeds, so as far as I can tell it’s a prescriptive
easement.

If it is indeed a prescriptive easement, then shouldn’t the easement be limited to the area necessary to maintain the ROW. Typically this is determined in the field by the existing conditions. Thus there would be no need to define a prescriptive ROW using a spiral curve.

[Norman_Oklahoma]

A spiral is a curve of constantly varying radius. Here is how the Oregon DOT describes spirals – this example is from their Right of Way Engineering Manual, a good source for certain formats. Pretty simple.

. . . which center line is described as follows:

Beginning at Engineer’s center line Station 1079+76.00, said station being
549.00 feet North and 723.28 feet East of the South quarter corner of Section 27,
Township 7 South, Range 11 West, W.M.; thence South 46° 59′ 38″ East
1,027.87 feet; thence on a spiral curve right (the long chord of which bears South
43° 51′ 22″ East 199.76 feet) 200.00 feet; thence on a 608.48 foot radius curve
right (the long chord of which bears South 30° 04′ 19″ East 158.97 feet) 159.42
feet; thence on a spiral curve right (the long chord of which bears South 1° 19′
41″ West 746.91 feet) 760.00 feet to Engineer’s center line Station 1101+23.29
Back equals 1101+16.80 Ahead; thence South 13° 12′ 56″ West 119.42 feet to
Engineer’s center line Station 1102+36.22.

• This reply was modified 1 month ago by  Norman_Oklahoma.

[BStrand]

@Norman_Oklahoma Nice, they totally ignore the elements of the spiral and just do the long chord. I would love to do that but I was concerned about creating a gap…

[richard-germiller]

Since a spiral can’t be offset I’m curious how others would define the R/W, on US Surveys BLM would define with equally divided short chords. I’d considered, but didn’t get the opportunity to try defining a separate spiral for the sides of the R/W.

Carlson apparently has a routine to offset a spiral. Whether it works or not I don’t know, but I guess that would be my first solution.

I guess I should have said that there’s no mathematical way to offset a spiral, to my knowledge anyway,

Civill3D and other programs that I’ve used can offset them, but the result is a polyline with a whole bunch of teeny-weeny segments.

[Norman_Oklahoma]

“…they totally ignore the elements of the spiral and just do the long chord. “

Not ignoring. The bearing of the connecting tangents, the length, and the ending radius fully defines the spiral. The chord bearing and distance provide redundancy. That is (more than) all you need.

[BStrand]

I guess I should have said that there’s no mathematical way to offset a spiral, to my knowledge anyway,

Civill3D and other programs that I’ve used can offset them, but the result is a polyline with a whole bunch of teeny-weeny segments.

Oh, I see.

[thebionicman]

Idaho roadways created by prescription can be limited to improvements or a set statutory width. The law here is not simple and few have bothered to read (much less understand) it. They simply echo the last incorrect width shown by a surveyor or asserted by a receptionist at the local highway department.

One of the Boise law offices maintains an excellent road handbook and posts it free online.