California Surveyors Forums

Ok whiz kids I have a challenge.

Maybe so of you routinely descibe spiral curves within a description, however I never have and now I need to. Is there a standard format for describing the elements of a spiral curve? Please clue me in.

Marno, Ian, anybody

RAM

RAM,

ODOT routinely deals with spiral curves. You may find this link useful:

http://www.oregon.gov/ODOT/HWY/GEOMETRO ... pirals.pdf

Note that the only elements of the spiral curves that are being used in the ODOT legal description samples are the lengths of the spirals and the the long chord bearings and distances.

Might be helpful if you could post a drawing with additional info so we can see what you're dealing with.

Mark
Thank you, at this time I do not have a map except for a bad copy of the 1924 RR plat which is difficult at best to interpet. I have found many examples on the layout of a spiral but this is the first example I have seen of an acutal description. I had considered using the chord bearings and distance together with the degree of spiral curve.

Thanks again, still open to suggestion.

John,

That is how I was approaching it, area to the chord and describing the spiral. These description are for Certificates of Complaince prior to Lot Line adjustment, the map will be in the future once the "New" lines are established.

My biggest problem is I have never written a legal which describes a spiral with the elements so that someone esle can survey the spiral, and I am not finding many examples of those that have. I could just call to the line of the RR with an approx. distance, but then I can not calc area. Therefore I am still working on it and can be swayed by popular opinion.

The Southern Pacific Railroad used taper curves as an approximation of spiral curves. If this is what you have, it actually simplifies the description writing. If I remember right, the degree of curvature was changed for every 25' of centerline, so in a description you would go from a tangent curve to a compound curve with a different radius based upon a 50' chord definition in incremental changes in the degree of curvature, 5 degree to 4.5 degree and so on. If you find a deed adjoining the railroad outside of a curve and the radius changes every 30+/- feet, you are probably looking at a taper curve. If you can find the old SP curve tables, it will help to understand how this works, if you are looking at a railroad that was laid out with taper curves and not true spirals.

RAM,

As a reminder, I believe RR curves normally use the long chord definition for degree of curve instead of curve length as used in highway curves (at least for circular curves). As John pointed out, may be signifcant spiral differences as well.

Steve is close to correct. SPRR used a compound curve approximation based upon 30' cords through the spiral.

The definitive reference on these curves was written by Lee Perkins, a SPRR field engineer, in 1915. A PDF version of this book can be found here: http://www.archive.org/details/railroad ... 00perkrich

Although this method uses a series of compound curves, it is meant to be an approximation of a true spiral. One could write the description either as a series of circular curves or by describing the incoming spiral, the central curve, and then the outgoing spiral.

There is an example of a spiral described as a series of compound circulars on page 4.38 of Wattles. Like nearly every other element of a description, I don't think that there is one prescribed way to describe it. As long as you include sufficient info for the next surveyor to reconstruct it, you're good.

The surveyor retracing your work would presumably obtain the proper RW maps that show the basic alignment info, and knowing which RR (or road district) created the curve, work with the proper spiral definition (there seems to be almost as many as there were RR companies and highway departments).

The Perkins book has extensive tables in it that prescribe the number of 30' chords in the spiral depending upon the degree of curve of the central curve, and then also defines the degree of curve for each 30' segment of the spiral.

I'd email it to you but it's 20Mb. That doesn't work with most email servers.

Wow now I have to think some more. the Info on the record plat is station, delta, degree of curve and the total curve tangent, this info is for the centerline, however my boundary is the right-of-way as the RR was granted in fee. I have calc'd the C/L and the right-0f-way and have ties at the end to confirm my calc's. Thanks for the references, I will add them to my libary.

Would be interesting to see what you've got.

Thanks

I will try, may take a bit, today I have to move on to another pressing project.

John

Thank you, that is a great example, I did not like my wording in transition from spiral to circular to spiral and will revise to match this sample.

Once again thanks to all for your input

Russ