Retracing the centerline of a 111 year old highway
I am researching the highway CL/bounds shown on the attached picture. Generally speaking, the portion I am interested in is between "N" and the triangular intersection at "N" and "M". Designed by the finest Harvard educated engineers of the time, this highway was laid out and opened in 1913.
With no other information available to me, I have found myself becoming comfortable with the assumption that curve radius values like 573.7' (10° curve), 319.6' (18° curve) and 1998.9' (2°52' curve) are indicative that the curves on this highway are laid out by chord definition and should not be mistakenly input as arc definition. I came to this conclusion because most of the radius values shown on the map can be directly related to specific chord definition degree of curve values on the table in my 2nd edition copy of Route Surveying by Carl F. Meyer. Some other curve data, not so much though.
Any thoughts one way or the other as I continue to do some research and desk surveying? Any advice is welcome. I'll get out there eventually 🙂
I think if the if the curves were laid out by chord then they would have put the degree of curve info on the map.
In my area, old curvilinear alignments usually don't show radius, they show delta, degree of curve, curve length and tangent length so you need to figure out if they are arc (highway) or chord (railroad) definition to calc the radius. The ones that turn out to be Chord Definition don't show the ARC length of the curve, but the curve length in 100 foot chords.
• The formula for calculating Curve Length in Chords is: 100(Δ/DR)
• The formula for calculating True Arc Length is: (pi x R x Δ)/180
(EDIT: I just noticed that in the pdf attachment, every occurrence of the "divided by" symbol got corrupted into some sort of indecipherable smudge. Sorry about that chief)
They may have laid them out using chords, but it's irrelevant since the plat is giving out the radius and the arc length, the two things that you really need to know. Plus, they give the delta, everything is there.
Thanks for the thoughts. Perhaps I am overthinking this....I know none of us were there, but why not use even numbers for curve radius' if that was the case?
I know none of us were there, but why not use even numbers for curve radius’ then?
Maybe some terrain they were trying to build around wasn't conducive to that.
When the angles are to a minute and the distances a tenth I don't get too caught up on the dimensions.
I was chatting with a senior enginerd about to retire. He was lamenting about the new enginerds and said; "do you know what they're doing"?
"What?"; I ask.
"They are designing curves with even foot radius!!!"
I was aghast, I don't even design ranch road easements with even foot radius, it's always using a degree of curve. The reason is obvious for old-timers.
I just got a bit of time to look at the drawing in autocad, what I looked at checked out perfectly. The radius is chord defined,
Even degree of curve, as it should be!!
Frankly, for an old plan set it's awesome.
I wish all of mine would be like that one.
Imagine laying out a road with curves in 1910, how would you do it with a transit and chain. The degree of curve defined using chords is the simple way to accomplish the task. No one would ever think of an even foot radius. That would be engineering malpractice.
They might have you flogged.
