Tangent or Non Tangent - RS related

[LS 4722]

I was taking a look at the 2010 PM (Sunday Comics post) and noticed some really dumb notes on the map.

PDF attached..

Every one of those monuments he is calling off are original..I imagine it's because the pipes dont fit the math... They never will, so why do surveyors call them off so many times?

Does a curve on an RS have to be geometrically pure?

What's wrong with non tangent curves?

[Stephen Johnson]

No. 13 is just ABSOLUTELY STUPID. 0.05' is still inside the pipe and the other two you show aren't much better. The only way you should call a pipe off is if it is NOT original or it is obviously disturbed. We have way too many button pushers with a certificate hanging on their walls.

[Steve Martin]

If you hold found original monuments at the BC and EC then you will in all likely end up with non-tangent curves.

My preference is to hold the found original monuments for the tangent line only and fit the record radius curve in tangent to the back and ahead tangents. The implied intent of the original map is to create tangent curves and a tangent curve solution honors that intent. The monuments are held for line and the BC or EC slides up or down the tangent by a few hundredths or a few tenths.

[Stephen Johnson]

If you hold found original monuments at the BC and EC then you will in all likely end up with non-tangent curves.

My preference is to hold the found original monuments for the tangent line only and fit the record radius curve in tangent to the back and ahead tangents. The implied intent of the original map is to create tangent curves and a tangent curve solution honors that intent. The monuments are held for line and the BC or EC slides up or down the tangent by a few hundredths or a few tenths.

Works most of the time.

I have found situations where that is not feasible and you end up with non-tangent curves, often to both tangent lines.

[LS 4722]

I have found situations where that is not feasible and you end up with non-tangent curves, often to both tangent lines.

Same here. My feeling is that an RS is a document that allows a following surveyor to recover the monuments I have on my RS or CR, so if non tangent does the job better so be it.

Monuments have a habit of disappearing over time so the more confidence I can build in the retracing surveyor then I did my job. It the follower decides to make my curves tangent, that's fine..whatever works for the situation at hand.

[bruce hall]

"Does a curve on an RS have to be geometrically pure?

What's wrong with non tangent curves?"

No Paul, it doesn't. Nothing wrong with a non tangent curve if that's what the monument positions indicate.

[btaylor]

Mark me down as another who holds the monuments and makes curves nontangent. So many surveyors hate to see the pretty math not work out and call monuments off. Simply cannot relate to this.

[JMS8070]

I, like Mr. Martin, hold the monuments for tangency and then fit the curve between the tangents. I would have never fathomed that someone would insert a non-tangent curve in a centerline where one was never there before…original monuments or no. I was taught that tangency and radii hold…period. There was never an option of making a tangent curve non-tangent.

I would like to add that it isn’t just a matter of keeping the math pretty for all of us. If for years you have been taught, told, and practiced that tangency holds…well…then...tangency holds.

But like I said above…you are definitely making me think about it…

So how about the lucky surveyor who finds original monuments at the CL BC/EC and also on the R/W BC/EC…and they don’t match up? Do you hold the monuments and the radii and then have a variable street R/W in this area?

Great post!

[land butcher]

IMHO original monuments hold regardless of position, unless obviously disturbed. I did work in one area of the central coast where we found many CL to PL monuments equaling less than shown street width. Oh well, your street is only 29.85 instead of 30 half width. Don't like it, talk to the surveyor that set the monuments 50 years ago. :)

[Steve Martin]

I do hold the original monuments - I hold them for line.

The issue is that every measurement has error and every retracement will come up with slightly different numbers, so something has to give.
Either the concept of tangent curves gives or the tangent lengths give.

You could just as easily say that the radius of the curve has to give to hold the BC and EC and fit a curve in between. It is by convention that we hold the record radius. We say that clearly the plat indicates an even number radius and that was the intent. Well clearly the plat indicates tangent curves too and that is the intent.

My preference is to hold the monuments for line only and preserve as much of the intent as possible. I have also found situations where non-tangent curves were the best course of action.

[btaylor]

Generally we are dealing with a tenth or so, so a lot of this comes down to differences that is laughable to the general public. My problem with holding them for line is that, when talking about going from the centerline to the right of way, the obvious way one follows in the footsteps is to occupy the actual monument, sight the tangent, and then turn 90. If one is treating the monument as not being the point of tangency, one is going to be off the original survey.

[pls7809]

Great point Bryan.

[Steve Martin]

Forgive me, I have not set up and turned 90 in a long time.

The way I look at it the monument will either be exactly on the line being retraced, or it will define the tangent and the retracement curve will be within a few hundredths of the monument, while preserving the intent of the map to place a curve between two tangents with a record radius.

As you said the differences can be laughably small.

[btaylor]

Steve, yes it can be mere hundredths, but I have come across retracements where a fillet of the record radius would force me to call off the monuments 0.2-0.3' or so. It is great if we are talking mere hundredths, and it could very well be that your area has some very good measurers compared to some up here in the Bay Area.

[land butcher]

Cad is a tool and it can be made to do what you want it to, not what it wants to do. There are ways to force it give the pic/data you want, might take some manual work but there are workarounds.
Having a 8lb sledge next to the computer sometimes helps.

[bruce hall]

also measure the chord between the found monuments at the bc-ec, measure the delta at the pi, and then calculate a new radius, instead of 450 feet one might get 450.22(or whatever number it turns out to be) and pop that sucker in.

I would be holding the measured delta, and measured chord, and "vuala", there is a pretty good looking curve and the radials to the new raduis point, based on the found monuments, are probably real close to perpendicular to the tangents(but that is just a guess on my part)

You guys put it wherever you want, but I love stuff I can touch and call "good". And that probably isn't the correct way to spell "vuala" either.

[btaylor]

Bruce that is also a good idea. I do that too if it enables me to be close to holding those monuments while preserving tangency.

[LS 4722]

It is by convention that we hold the record radius..

In my world, tangents define the measured radius.

We say that clearly the plat indicates an even number radius and that was the intent..

I have to disagree. A map radius is just part of the design.. Map intent is the lot layout. Parallel lines for example..or...even 50 foot lots and the excess in the last lot...that's map intent as far as I am concerned.

[Dave Karoly, PLS]

A map is an inanimate object; it can't have an intent.

The intentions of the subdivider are most accurately shown by the physical evidence. If the pipe was set at the PC or PT then showing it on-line but not at the PC or PT is not correct.

Paul's method of showing the measured radius is most correct.

We don't live in a perfect world. If a non-tangent curve fit the physical evidence better then the original lines follow a non-tangent curve.

It isn't about what pleases me or Brown or Wattles, it is about what it actually is.

[Dave Karoly, PLS]

My opinion is, do whatever holds the pipe intended to be the PC or PT. If this means showing a measured radius then do so. If a non-tangent curve fits the actual physical evidence better then do so. The intent is to retrace the lines where they are, not where they should be.

[LS 4722]

The intent is to retrace the lines where they are, not where they should be.

BINGO! Give that man a ceegar! (and a Gin Martini)

[bruce hall]

curves shown on are maps are tangent at the bc's and ec's probably has more to due with the definition of the word "curve" than anything that Brown or Wattles has to say.

If you think this tangency thing is their idea, your wrong. Maybe I should look it up in an old math book. I mean really old. But I'm not gonna. That concept has been around for a long, long time. I guessing at this, but it makes sense to me.

I prefer having the bc's and ec's at the point of tangency simply cause the math is so easy for the sidelines. And this is "normaly" how I bust out the curve. Hopefully the monument(if it exists) is close by.

It can get messy on the edge of the street using non-tangent curves.

I have a tough time calling an original spike and washer not at the bc where the guy said he set it, just cause it doesn't fit the pot using the calcs.

[LS 4722]

One of the best methods of avoiding a negligence charge is to reference an acceptable treatise or written manual to establish having met the standard of care.

Thats a good funny!

How many LS's have been chastened by the board as being negligent because they did not use a tangent curve???

[btaylor]

I cannot see how one would be negligent by holding the original monument at the position it was set in, when the alternative is to call it off -- say what you want, but you are calling it out of position by holding the math.

[LS 4722]

I cannot see how one would be negligent by holding the original monument at the position it was set in, when the alternative is to call it off -- say what you want, but you are calling it out of position by holding the math.

Is that the correct spelling of your name?

Anyway, I am 100% with you. What *MANY* LS fail to realize is that a formal map that notes monuments placed is the very same thing as a written deed with calls to monuments.

If a formal map says that 2 inch pipes were set at the BC/EC of a map curve, and both pipes are found during the survey then the preceding course will terminate at the found pipe and the next map element, a curve, will start at that pipe, adjust to the EC pipe, and so on and so forth..

It is the same as a deed that says " Thence North, 123.00 feet to a 2 inch Iron pipe tagged LS xxx, being the beginning of a tangent curve to the left having a radius of 100 feet. Thence Northeasterly along said curve through a central angle of DD° MM' SS", an arc length of 80.00 feet to a 2 inch pipe, thence tangent to last said curve, 89.45 feet to a 2 inch iron pipe..."

All the elements in the deed between the two found pipes adjust to those two pipes. It does not matter if the curve radius works out to be 102.26 feet and the arc length becomes 79.18 feet, the pipes define the beginning and end of the curve.

The values on the map and in the deed, the math values, are junior elements and just mathematical guidelines to the controlling elements of the map or the deed, the 2 inch pipes.

So what we have are not those that regard tangent over non tangent, but two camps. One holds senior elements, the other camp prefers to hold junior elements.

Im still on my 1st cup of coffee so the post may have a few typos etc..

A curve is just a line leading from one pipe to the next. Would the tangent only camp hold a deflection angle over a called and found pipe, be it a map or deed over the pipe itself when both the beginning, angle point and end of the angled map or deed line has recovered originals pipes marking those positions? I sure hope not. It's the same with a curve, you adjust the elements, be it radius and arc length to the pipes, just as you would hold the measured distances and angle for an angled line.

Just senior/junior stuff is the way it appears to me.