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Intro:

    The p50 laser scanner was used to scan the sides of the towers to see if they are still vertical after the platform collapse.
Timeline:

•  10aug20: Tower 4auxNorth pulls out of socket
• -6nov20: tower 4 MainCable 4 (southern main) breaks
• 01dec20: platform collapses.l
  

The tower configuration was/is:

• Prior to the collapse: 
  
• T12,T4 had 4 step backs
  
• T8 had 6 step backs
  
• After the collapse
  
• T12,T4 had 3 step backs (one stepback  broke off)
  
• T8 had 4 step backs (2 stepbacks broke off)
  

 Labeling the faces of the towers:
    When the p50 looks perpendicular to a tower face, it will see 3 flat faces.

The image below labels these faces:

Each color shows the 3 faces that are visible when looking perpendicular to a face.

• face 1 always points toward the center of the dish
• When looking at the central face, the other two faces are labeled Left and Right
• The face number increases clockwise from the face pointing inwards.
• As an example, take tower 12
• face 1 points south,
  
• face 2 points north.
  

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Processing the point cloud.

    The steps taken to process the point cloud were:

•  Scan a particular set of tower faces.
• For each face there will be N stepbacks
  

• project the points onto   the xy, yz, and xz planes.
  
• remove points that don't belong to the faces
• do  a robust linear fit (throwing out 3sigma outliers) to the tower faces projected on the xy plane
• If there are N stepbacks in the tower, there will be N projected lines and N linear fits.
• Use the points within 3sigma of the fit as points belonging to a face/stepback
  
• rotate the N step back datasets to the x,z plane using the rotation angle computed for the bottom most step back face.
  
• eg: after rotation T12 southern face:, x-east, y-north , z up.
  
• For each face/step back generate a histogram in z (with either .1 or .2 meters bin size).
  
• Include a reverse_index option so we have ptrs to all the points in a bin
• compute the average y value for each zbin to see how the tilt changes as a function of z
• compare the values computed with p50 scans prior to the  platform collapse (if the data is available).
  

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Table of tower scans:

tower	date	face	.las file	link to writeup	p5 location	Notes	plots
12	201209	1	.las	201209	visitor center balcony	After collapse	.ps  .pdf
	200904	1	.las	201209	eastSide rim roade	after aux4North failure	.ps  .pdf
	201223	3	.las	201223	road next to Visitor center	after collapse	.ps  .pdf
	201124	3	.las	201223	road next to visitor center	after aux4N and T4M4 failure	.ps  .pdf
4	201228	2 2L 2R	.las	201228	southeast side of helipad	after collapse	.ps  .pdf .ps  .pdf .ps  .pdf
	200915	2 2L	.las	201228	southeast side of helipad	pre collapse	.ps  .pdf .ps  .pdf
8	201111	1 1l	.las	201111	west side of helipad	pre collapse	.ps  .pdf .ps  .pdf

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Tower 12

09dec20: T12 (face 1) scan south looking face from vc balcony

    The scanner was located on the visitor center balcony looking north to the southern face of T12.

• The scanner was about 10 meters from the base of tower 12 (bottom step back). 
  
• At the step backs, there was some shadowing from the scanner being so close to the tower.
  
• There were 163 million pnts before excluding outliers

 We compared these results with data taken 04sep20 before the collapse

• The scanner was located on the east edge of the rim road (about 250 meters away)
• there were a lot fewer points on the tower since the longer distance was done at at lower resolution.

    After rotation the coordinate system was:

• X - east
• Y - north
• Z - up
• + angle is CCW looking down the Z axis.
  

The first plots shows the scanned data results for 9dec20 (.ps) (.pdf)

• The y value is averaged over z bins of .1 meter.
  
• The scanner was about 10meters from the base of the bottom step back.
  
• Page 1: face rotation relative to bottom step back
• Each step back face was rotated to the xz axis. The plots shows the angle difference relative to the bottom step back face.
• Positive angle is CCW looking down from the top
  
• Page 2: y  value vs  z. 0 is defined by the mean value of the lower face.
• top: Y value vs z . + y is northward.
• you can see the 3rd step is bowed a bit.
• it starts out south, bends northward (inward) and then comes back out.
  
• Bottom: y value vs z, with face mean removed.
• This lets you see the deviation with finer detail (x axis is now cm)
• Since the mean is removed from each step back face, the transitions between stepbacks won't line up.
• the 3rd step back has a variation of 7 cm.

The 2nd plot shows the scan of the T12 southern face on 04sep20 (pre collapse) (.ps) (.pdf)

• the y value is averaged over .2 meters.
• the scanner was 250 meters from the base of the bottom step back.
  
• T4 auxNorth cable had already fallen, but the T4 M4 cable was still attached.
  
• Page 1 face rotation relative to bottom face.
• The face angles are rotated CCW relative to the 9dec20 measurements.
• The 2 aux main cables from tower 12 had a large tension difference.
  
• The west cable tension was much higher than the east aux cable tension (about 400 kips vs 720 kips)
• This rotated the tower CCW. especially the upper section
• Page 2:
• The data was binned to .2 meters in z (there were fewer points).
• Top: Average y position vs z
•  Bottom: average y value vs z with face mean value removed.

09dec20 Summary:

• The faces are rotated by less than a degree relative to the bottom face
• sep20: the rotation goes 0 to -.3 degrees and then jumps up to .85 degrees (CCW)
  
• The tower was being rotated by the aux main cable tension difference (from t4auxN failure)
• the largest rotation occurred at the upper step back
• I wonder if the rotation was part of the reason why the 2nd to 3rd transition had cement failure during the collapse
• dec20:
• It looks like the entire tower unwound after the cables were released and the top section fell off.
  
• Y deviation vs z
• The 3rd step back has a y variation of 7 cm
• This was seen before and after the collapse. This may have been present from the start.

processing: x101/p50/towers/t12/f1south/getdata_20xxxx.pro, plotit_20xxxx.pro

23dec20:T12 North looking face (face 3)

    Data was taken from the road next to the visitor center. The same location was used on 24nov20 prior to the collapse (the 24nov20  spot was about 20 feet farther away).
The plots show the results of the measurements:

• Scan of north face 23dec20 (.ps) (.pdf)
• scan of north face 24nov20 (.ps) (.pdf)

The results are similar to the south looking face. There is an unwinding of the tower after the collapse.

processing:x101/p50/towers/t12/f3north/getdata_20xxxx.pro, plotit_20xxxx.pro

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Tower 4

28dec20: T4 face 2 from SE side of helipad

    Data was taken from the se side of the helipad looking toward tower 4 (face 2 facing NE).

•  The 201228 position had a little better view of the tower than the 200915 spot.
  

• 201228 T4 post collapse
• Face 2 post collapse (.ps) (.pdf)
• Face 2L post collapse (.ps) (.pdf)
• Face 2R post collapse (.ps) (.pdf)
  
• 200915 T4 pre collapse
  
• Face 2  pre collapse. (.ps) (.pdf)
• Face 2L pre collapse (.ps) (.pdf)
• angle didn't give enough points for face 2R
• The face 2L had large rotation angle for upper steps backs before and after collapse.
  

• 
  

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