Broken back curve or not?

March 16, 2009
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Posted By paul plutae on 3/14/2009 at 9:05 PM

Today we did a nice survey in the Laurel Canyon area on a nice residential street called Lookout Mountain. This street was particularly nice because it had a width of 50 feet and not the normal one of 30 feet, which is common in the hillside areas of Los Angeles.

A huge benefit of todays survey was the abundance of monumentation of both the city engineer and private surveyors. The subdivision is one of those tracts that was a table top survey, that is, the closest the original surveyor got to the lands was on his drafting table, but at least his calculations were decent.

The question I have is, should curves be concentric just to have a nice math fit, or should they be broken back to reflect actual field measured conditions?

Below is a listing and description of the monuments that were found.


PT NO DESCRIPTION

19 FOUND L/T IN TC ON CHORD PROD PER CEFB
20 FOUND L/T AT BC - C/L INTERSECTION
LOOKOUT MOUNTAIN / OAKSTONE PER CEFB
21 FOUND SPIKE ON C/L CURVE PER CEFB
22 FOUND (4) PUNCHES IN MH RIM PER CEFB ON CHORD
26 FOUND L/TAG IN TC LS 3435
28 FOUND SPIKE IN LEAD AT C/L PRC CEFB
31 FOUND L/T IN TC ON PRC LINE PER CEFB
33 FOUND L/T ON C/L PER CEFB
34 FOUND L/TAG IN TC LS 3125
35 FOUND L/T ON C/L PER CEFB
36 FOUND LEAD IN TC- NO NAIL
37 FOUND L/T ON C/L PER CEFB
38 FOUND L/T IN TC
79 FOUND CONC NAIL/TAG IN TW LS 5173

COORDINATES

N E Z
19,-427.408290,2031.273775,-1.030000
20,-450.287403,2174.081816,-26.610000
21,-433.106300,2066.839916,-12.180000
22,-429.614591,2045.045185,-9.150000
26,-388.557907,1988.013150,5.320000
28,-362.357449,1984.345643,3.150000
31,-369.473697,1973.633353,7.780000
33,-313.303375,1741.585790,39.030000
34,-309.328195,1900.588016,20.310000
35,-302.379872,1916.385179,14.110000
36,-297.713710,1853.805542,22.960000
37,-284.416712,1827.546452,26.930000
38,-300.323822,1805.848439,30.220000
79,-431.487444,1880.275058,0.000000


Here are two snapshots of the record map. It was done in 1908. The map is in two parts because the area of the survey is on two different map pages. The survey was to re-establish lots 195 and 197.

Lot 195 - Lookout Mountain Park Tract

Photobucket

Lot 197 - Lookout Mountain Park Tract

Photobucket

Point 20 is the intersection of Lookout Mountain and the 40 foot wide street coming in from screen south, which is now called Oakstone, lying between lots 177 and 179.

Point 21 is a spike in lead on the c/l curve.

Point 28 is also a spike in leade at the PRC of Lookout Mountain.

Points 33, 35 and 37 are lead and nails that are on the c/l of Lookout Mountain.

The remainder of the points, save for points 19 and 22, which are ties to point 21 on the chord produced from point 20 , are all private engineer / surveyor monuments.

You have two choices here. Math fit and fudge it all, or best fit the curves using the centerline points and break the back of the centerline curves where the PRC is, you cannot have both.

Point 79 is a found concrete nail and tag. The RS, partially shown below, states that an iron pipe was set at 79's location, but it does show the monument to be set on the PRC line.

RS NEAR LOTS 195 AND 197

Photobucket

Some of the abbreviations I have used are as follows:

L/T lead and tack (or nail)
CEFB city engineer field book

The other abbreviations are pretty common and you guys know what those mean.

CEFB 21009 PAGE 7 (partial)

Photobucket

CEFB 21009 Page 8 - BLOWUP

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CEFB 21009 PAGE 8

Photobucket

Points 33, 37 and 35 are the found L/T's shown on the above field book.

With this information, what would you guys do to :

1. Establish lots 195 and 197
2. Show the curve data etc on a filed record?




PS The delta of 204° 00' 30" shown for the 50 foot radius was just a drafting error. The draftsman arrowed to a 180° line, and not to the EC that lies common with lots 219 and 221.

PPS The notation on FB 21009 Pg 8 that says " Survey 9515-54" just refers to the found monuments that were used to establish the centerlines of the streets and are not relevant to this post.


To read the rest of this thread go to www.i-boards.com/bnp/pob/messages.asp?MsgID=1381001&ThreadID=131059&IsResponse=False#1381001

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