Tangent or Non Tangent - RS related

[btaylor]

Is that the correct spelling of your name?

Correctamundo

[Anthony Maffia]

Okay, here's a real world example I'm working on. Which solution would you hold?

Eighty year old urban hill subdivision, 50 foot wide roads, PTs monumented. One tangent is record 100 foot length & bearing, other is 270 feet, long by 0.05' but bearing is off by almost 2 minutes.

C1: A tangent 86 foot radius curve puts the BC 0.19' & the EC 0.25' from the monuments. The curve lengthens beyond the monuments.

C2: A non-tangent 86 foot radius curve, holding the monuments.

The curves are substantially the same: the midpoints are 0.0009 feet apart.

I prefer C2 over C1. Why call monuments off by 2 tenths to fit a tangent curve, when both curves are in the same position?

On other projects, I've held the tangency when retracing surveys that did the same, so I'm not firmly in one camp or the other. But I dislike surveys that hold 2 monuments and call 10 others off.

------

Dave, I think your Wattles quote is taken out of context. He's talking about interpreting curves in a deed description, not a map.

[btaylor]

Great example Anthony. That is my experience on some surveys as well. At some point you are calling off monuments too much for my liking when holding the geometry. Additionally, as you mathematically show, the changes are not visible whatsoever. It is just "ugly" in terms of math.

[LS 4722]

Okay, here's a real world example I'm working on. Which solution would you hold?

Eighty year old urban hill subdivision, 50 foot wide roads, PTs monumented. One tangent is record 100 foot length & bearing, other is 270 feet, long by 0.05' but bearing is off by almost 2 minutes.

C1: A tangent 86 foot radius curve puts the BC 0.19' & the EC 0.25' from the monuments. The curve lengthens beyond the monuments.

C2: A non-tangent 86 foot radius curve, holding the monuments.

The curves are substantially the same: the midpoints are 0.0009 feet apart.

I prefer C2 over C1. Why call monuments off by 2 tenths to fit a tangent curve, when both curves are in the same position?

On other projects, I've held the tangency when retracing surveys that did the same, so I'm not firmly in one camp or the other. But I dislike surveys that hold 2 monuments and call 10 others off.

------

Dave, I think your Wattles quote is taken out of context. He's talking about interpreting curves in a deed description, not a map.

A bearing out by 2 minutes is not a consideration. The pipes lock both ends of the curve in, so adjust accordingly. If record radius works, great. If a new radius is calc'd, just as good..as long as the curve is contained by the pipes, the math floats.

[Dave Karoly, PLS]

If the two answers (tangent/non-tangent) are substantially different that might show up in Lot corner monuments on the R/W in the curve although sometimes there is a systematic shift between the lot corner monuments and the centerline monuments.

I think they ran centerlines through during construction, probably set hubs for the centerline points then staked the lot corners off of that. Later after construction was completed they ran in the centerline pipes with slightly different errors which can be up to a few tenths. I am just speculating, though.

[Dave Karoly, PLS]

I think the Board has an automatic claxon alarm that goes off whenever some Surveyor dares to call a curve non-tangent.

AWOOGA AWOOGA SOUND COLLISION DIVE! DIVE! DIVE!

FIRE ONE TORPEDO! WOOSH! ONE'S AWAY SIR!

[LS 4722]

If the two answers (tangent/non-tangent) are substantially different that might show up in Lot corner monuments on the R/W in the curve although sometimes there is a systematic shift between the lot corner monuments and the centerline monuments.

I think they ran centerlines through during construction, probably set hubs for the centerline points then staked the lot corners off of that. Later after construction was completed they ran in the centerline pipes with slightly different errors which can be up to a few tenths. I am just speculating, though.

I bet that the arc length for a new calc'd radius would be pretty close to record with that 2 minutes being applied. What we don't know is, is the difference 0.25' - 0.19' OR 0.25' + 0.19'?

It still wont matter..my experience has been that even with non tangent curves, you still get a reasonable meas/record value, block corner to block corner and that's pretty important..

Case by case situation, there is no pat answer..definately not in a book anyway.

[Anthony Maffia]

0.25+0.19

prorating lot widths through the curve either way results in the same results, different by maybe 0.01 iirc.

I definitely prefer saying the curve is non-tangent by a few minutes than the monument misses the BC by 2 tenths.

Perhaps this is the same "perception" thing whereby I tend to hold bearings over distances in a retracement, when neither is supposed to be superior to the other.

[btaylor]

FWIW it's worth Anthony, I think you are proving that the curve is in fact not tangent as monumented. It is not that you are creating the situation.

It is the same with any retracement of a rectangular block when you find the four original block corners (This happens all the time right?). You do not spin the 90 degree rectangle over a best fit and call all four off 0.2' in different directions, and create perfect lot lines 90 degrees off the right of way.