[alma-config] site survey notes

[Bryan Butler]

some notes regarding surveying of compact config and possible problem
pad locations from the trip of simon and myself to the ALMA site from 
2000dec11-2000dec17.    bjb - last amended 2001jan19.

note that all coordinates are SAm56 UTM.


 - first, issues of the array center, or the position of the most
   compact standard ALMA configuration (not the ACA).

   visited 6 sites on 2001dec13/14.  notes on the locations (in the 
   order that i [or roberto and i in some cases] initially visited 
   them on the 13th) are as follows:

   1 - "Chajnantor best" (first entry in table 1 of memo 338).

     position (straight from memo 338):
       627650E; 7454450N

     this is very near the site i surveyed in nov99 and called
     "Chajnantor North" (10's of meters).  the old "Chaj N" spot
     is somewhat better because of a ridge (roughly 10-15 m relief)
     to the NE (by about 100 m) of the new position.  "Chaj N"
     coordinates:
       627591E; 7454424N; 5020 m    [accurate GPS]
       627607E; 7454404N; 5065 m    [NRAO hand-held GPS from nov99]
       627612E; 7454405N            [NRAO hand-held GPS from dec00]
       627607E; 7454398N            [ESO hand-held GPS from dec00]

     info from digital topo map (at the "Chaj N" position):
       height:  5028 m.
       slope:  0.4 deg.    (in 200 m diameter circle centered here)
       rms:  0.7 m.        (in 200 m diameter circle centered here)

     this is still a very good place, as i noted from our last visit. i
     have a hand drawing in my log book from the nov99 visit, but it's 
     roughly ellipsoidal, 1 km E-W and 700 m N-S.  there is a thin 
     extension to the W (.5 km or so) which is narrower (N-S extent down
     to 100 m at 1.0 km from center, and 50 m at 1.2 km).  there is a 
     shallow gully that runs roughly N-S about 250 m W from the center.

   2 - "zoom" (last entry in table 1 of memo 338 - the center of
       conway's zoom configs).

     position (straight from memo 338):
       628590E; 7454000N

     info from digital topo map (at that position):
       height:  5013 m.
       slope:  2.7 deg.    (in 200 m diameter circle centered here)
       rms:  0.7 m.        (in 200 m diameter circle centered here)

     this place is relatively smooth, but it slopes pretty steeply from 
     the NW to the SE (steeper to the NW from this center spot).  

     steep quebrada is to the SW, but is not so visible from this
     place.  it's about 200 m from this center place.

     distance to quebrada to E is about 250 m (to its W edge), but it 
     is a relatively shallow one.

     distance to quebrada to W is about 200 m.  this is a deeper one
     than to the E.

     to the S, can go ~ 1 km - then hit a shallow quebrada and start
     into the foothills of Cordon Honar.

     to the N, a really shallow quebrada at about 200 m distance,
     another shallow one at 400 m, then two steeper ones at about
     550 and 850 m distance.

   3 - "Chajnantor Better [North]" (3rd entry in table 1 of memo 338).

     position (straight from memo 338):
       628050E; 7453650N

     info from digital topo map (at that position):
       height:  5025 m.
       slope:  1.3 deg.    (in 200 m diameter circle centered here)
       rms:  0.6 m.        (in 200 m diameter circle centered here)

     this place is relatively flat & smooth.  it's actually *very*
     close to the containers & especially the E STI antennas.

     steep quebrada to the S, only about 50 m away.  this quebrada is
     a split off one from the big one to the E.

     the quebrada just to the N of the containers is only about 100
     m away...

   4 - "donut" (2nd-to-last entry in table 1 of memo 338 - the center of
       yun & kogan's donut configs).

     position (straight from memo 338):
       628370E; 7453250N

     info from digital topo map (at that position):
       height:  5013 m.
       slope:  2.3 deg.    (in 200 m diameter circle centered here)
       rms:  0.7 m.        (in 200 m diameter circle centered here)

     this place is not relatively flat or smooth.  it's in the middle
     of a place where a bunch of quebradas come together, and there are
     many quebradas to all sides (shallow and steep).

     it slopes mostly W->E but then quickly goes into a quebrada.

     small quebrada only 50-100 m to N (10-15 m relief).

     merger of quebradas to W (and gets large) only about 100 m.

     big quebrada to S about 100-200 m.

   5 - "Chajnantor Better [South]" (2nd entry in table 1 of memo 338).

     position (straight from memo 338):
       627750E; 7453100N

     this is very near the site i surveyed in nov99 and called
     "Chajnantor South" ("Chaj S" is about 100 m to the NNE).  the old 
     "Chaj S" spot is somewhat better because of relief to the W.  
     "Chaj S" coordinates:
       627807E; 7453229N; 5017 m    [accurate GPS]
       627822E; 7453192N; 5020 m    [NRAO hand-held GPS from nov99]

     info from digital topo map (at that position):
       height:  5030 m.
       slope:  1.4 deg.    (in 200 m diameter circle centered here)
       rms:  0.6 m.        (in 200 m diameter circle centered here)

     this is also still a very good place, as i noted from our last 
     visit (similar to "Chaj N").  i have a hand drawing in my log book 
     from the nov99 visit, but roughly, it's roughly ellipsoidal, 700 m 
     E-W and 300 m N-S.  

   6 - "Chajnantor Good" (4th entry in table 1 of memo 338).

     position (straight from memo 338):
       630400E; 7453550N

     info from digital topo map (at that position):
       height:  4912 m.
       slope:  1.0 deg.    (in 200 m diameter circle centered here)
       rms:  1.3 m.        (in 200 m diameter circle centered here)

     this place is pretty close to both the pipeline and Cerro Agua
     Amarga.

     the view of the bolivian andes is very nice, but otherwise the
     place is not so good:

       - slope is relatively steep to NW and N.
       - ridge to SE, only about 30 m away, maybe 20 m relief.
       - pipeline is so close.
       - agua amarga and chascon so close.
       - steep quebrada nearby to W.


   so, in summary, if i had to order them strictly in terms of how they 
   look qualitatively, folding in what i know quantitatively about the
   slope and roughness, i would say that the order is (best to worst):

        1 - "Chajnantor best"
        2 - "Chajnantor Better [South]"
        3 - "zoom" 
        4 - "Chajnantor Better [North]"
        5 - "donut"
        6 - "Chajnantor Good"

   where 3 & 4 are roughly equivalent (3 only slightly better), and
   5 & 6 are roughly equivalent (5 only slightly better).  now, there 
   are still certainly issues with numbers 1 & 2 - namely, whether 
   arrays (of either the zoom or donut type) can be designed around 
   these central locations.  but that is a challenge to the "array 
   designers"...


 - second, issues of possible problem pad locations in the current
   strawman donut and zoom configurations.

   al tabulated 53 possible problem pad locations for the current
   configs.  turns out that 2 of these were duplicated locations, 
   leaving 51 unique places - 25 donut, 26 zoom.  from al's maps, you 
   can see that most of them (39 of the 51) are concentrated in 2 or 3
   quebradas which are to the SE of the containers.  

   i didn't have time to go to all of the locations, but did get to the 
   12 locations which were in the other 3 quadrants (as defined with
   the containers at the origin) on 16dec.  i surveyed these 12 
   locations in 1 hour - roughly 5 mins each.  this might not be
   entirely representative though, as they were much more spread out
   than the ones which are clustered to the SE of the containers.
   also, while some locations only took a couple of minutes, there were
   some which were pretty tough to get to (notably the ones to the SW).

   anyway, first off, here is the full list provided by al (condensed
   by me - see al's web pages for the full descriptions and pictures):

   donut:

    1 - 628668  7453299
    2 - 628608  7453346
    3 - 628593  7453530
    4 - 628546  7453611
    5 - 628513  7453663
    6 - 628359  7453569
    7 - 628198  7452857
    8 - 628202  7453213
    9 - 628170  7453180
   10 - 628204  7453142
   11 - 628228  7453132
   12 - 628407  7453137
   13 - 628484  7453120
   14 - 629094  7453312
   15 - 628305  7454084
   16 - 628668  7453299
   17 - 628608  7453346
   18 - 628198  7452857
   19 - 628253  7452919
   20 - 629688  7452987
   21 - 626802  7453565
   22 - 627008  7452664
   23 - 629189  7454534
   24 - 628446  7452481
   25 - 629064  7453014
   26 - 628446  7454092


   zoom:

   27 - 628162  7453278
   28 - 628180  7453222
   29 - 628192  7453116
   30 - 628235  7453055
   31 - 628696  7453348
   32 - 629345  7454045
   33 - 628218  7453231
   34 - 628225  7453178
   35 - 628273  7453126
   36 - 628406  7453115
   37 - 628469  7453121
   38 - 628550  7453359
   39 - 628421  7453446
   40 - 628350  7453517
   41 - 628261  7453552
   42 - 628508  7452484
   43 - 628869  7452508
   44 - 629345  7454045
   45 - 628172  7452840
   46 - 627923  7454241
   47 - 627038  7452703
   48 - 626892  7453936
   49 - 626810  7453595
   50 - 627005  7452664
   51 - 629688  7452987
   52 - 629796  7453329
   53 - 629195  7454540

   note that positions 7 & 18 are duplicates as are 32 & 44...


   and here are my notes as transcribed & paraphrased from my log
   book from 16dec:

   notes on pad positions to the NE of containers:

   32/44  this is a questionable place - in a flat place in the middle 
          of a relatively steep quebrada

   23  this is an OK place.  it's on the E edge of a shallow 
       quebrada, but seems OK.

   53  very close to 23 - same story.

   26  completely OK - don't know why al put it in his list...

   15  at the head of a shallow quebrada, but seems OK.

   46  this is on the edge of the steep quebrada which is just to the
       north of the containers - too close to the quebrada edge, so
       probably bad.


   notes on pad positions to the NW of containers:

   48  this is on a ridge between two quebradas, but seems accessible
       and should be OK.


   notes on pad positions to the SW of containers:

   21  on the north edge of an island/ridge between 2 quebradas.
       it's not as bad as some of the others, but i would call it
       questionable.

   49  this is in the middle of the northern of the 2 quebradas
       which position 21 sits between.  again, not terrible, but
       questionable.

   the next 3 locations (to the SW) were particularly tough to get to.  
   they are in the area where the quebradas start to get more
   concentrated and steeper.  you have to pick your path to these
   places pretty carefully.  had to backtrack a few times.  you can get
   access to them along the ridges between quebradas, but it's tough
   going now, because you don't know which ridge will get you to the
   right place in the end.  saw a few sinkholes in this area.

   22  in quebrada to the south of the CBI (not the one directly to the
       S, but the next one over).  very steep quebrada.  probably not
       acceptable.

   50  in same quebrada as position 22 and probably also bad.

   47  on the N edge of the quebrada which position 22 sits in.  also
       probably bad.



   so, of the 12 places i surveyed, roughly half of them seemed OK,
   and half seemed questionable to almost certainly unacceptable.