I'm working on an amendment to a 12 year old subdivision. I was having a problem plotting the lines and having them close. The curves looked kinda weird on the plat. I figured out that the curves are almost all non tangent. Plot the chords and it mostly works out, but out of the 20 or so curves on the plat they are almost all non tangent. No mention of it either. So you drive down the road and where the curve starts there is 2-4 degree angle point.
I’ve seen a few non tangent curves to make things fit here and there, but almost all of them? For me it's harder to draft (design) with non tangent curves than tangent ones. The weirdness never ends in this business!
Is this very common?
Sadly, yes, because too many surveyors don't step back and ask themselves, could anyone close this lot? The worst I've ever seen was three non-tangent curves in a row, each with just the Radius, Delta, and Arc. In other words, an infinite number of solutions.
A few local surveyors would only put the curve length and a general direction like "southwesterly" in their descriptions and all we could do was scale their drawings if we had one.
They are the worst to retrace because they only set monuments at each end of the route.
It is more common for the radius bearing to be called out if the curve is non-tangent. Any surveyor should know to do this.
That varies widely with geography and period. Our current local practice is to give the chord bearing and distance and leave it to others to figure it out. I personally think such maps and descriptions are trash. We don't survey to create tests for our peers. We do it to preserve evidence and present it for use by land-owners...
> Sadly, yes, because too many surveyors don't step back and ask themselves, could anyone close this lot? The worst I've ever seen was three non-tangent curves in a row, each with just the Radius, Delta, and Arc. In other words, an infinite number of solutions.
If it's a subdivision chances are it probably wasn't drawn or designed by a surveyor, more than likely and engineer designed it and an engineering type drafter drafted it. In those instances it is not uncommon to find non-tangency to the extreme that has been identified. With that said...there are certainly exceptions.
Someone also mentioned to note the bearing from the PC to the radius. In WA this is somewhat prevalent while in ID it is unheard of....at least in my neck of the woods.
I prefer chord brg and distance and dont even worry about the actual curve unless I need to accomplish something based on it.
Non tangent curves- on a tangent
So, for those who may be in the dark or just like to pick up new tricks, how do you draft your tangent curves?
I was taught to use the 'fillet' command in autocad. Bring your centerline tangents to the p.i. and commandline 'fillet'. This will give you the option to set the radius and, as long as cad sees the radius as accommodating the two tangents, it will create the curve tangent to both.
This can be iterative (and maddening) if the lot layout is fully baked and includes improper geometry. Potential to leave you with road sections not centered in the right-of-way.
If the lots can be tweaked, reset their frontage, dimensions and acreage after getting the centerline and rights-of-way dressed up with better geometry.
