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View Diary: Daily Kos Gulf Watchers ROV #185 - BP's Gulf Catastrophe (314 comments)

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  •  Problem with ROV distance calculator (18+ / 0-)

    After lots of head-scratching, I've concluded that the "Calculator for distance from BOP" linked in every ROV diary was giving me bad directions.

    I think the author assumed that your "easting" number decreases as you move from west to east, but that's contradicted by the site survey map that BP submitted to the MMS (see this PDF). MC252_A is at 1,202,803, and the edge of MC253 (the block to the east), is at 1,203,840.

    A simple way to test the calculator's misbehavior is to enter 0 in every field except "ROV Location" East. Enter 1000 in that field, and the calculator will tell you that easting 1000 is west of easting 0.

    I'd suggest adding a warning next to the calculator link to let people know it's not reliable. Also, there's a link to an outdated seafloor map that seems to be based on the calculator. The easting numbers along the top go from 1,206,000 at the left down to 1,200,000 at the right.

    •  Thanks for checking that out. (14+ / 0-)

      After getting bad info that contradicted observations and other resources I stopped using it.

      Peraspera; would you add the disclaimer suggested with a link to wdz's comment embedded in it?

      I'm no Nate Silver, TomTech, or VoteforAmerica ("WineRev" Eeman, Recounting Minnesota)

      by Tomtech on Fri Jul 09, 2010 at 12:01:33 AM PDT

      [ Parent ]

    •  Shameless self promotion (8+ / 0-)

      Most people who are using the calculator are trying to ask, "where is this ROV and what might it be looking at?" Might I submit that my sketch map might be better suited to the purpose? Easting and northing are marked in the borders of the map, with brown crosses in the body of the map at the 1000 foot points.

    •  The outdated seafloor map (3+ / 0-)
      Recommended by:
      peraspera, Yasuragi, wdz

      is a view from the reservoir.


    •  It's just UTM coordinates with wrong units (6+ / 0-)

      The coordinates you see most of the ROVs use are just UTM coordinates with units of feet, instead of meters.  And yes that means easting numbers should increase as you go east.   If you want to see where something is, open Google Earth, change your preferences to display UTM instead of degrees lat/lon.
      Then take the numbers you see on the ROV display, convert from feet to meters with a units calc, and move your mouse around in Google Earth over the area until they match up with the converted ROV #s.   I know that sounds clumsy (and it is) but we can't do anything about the fact that they're using units of feet instead of meters.  
      It gets a bit easier once you've made some placemarks for reference.

      All the ROV operations are inside one UTM grid so distance calculations are pretty straight forward trig (length of hypotenuse of right triangle).
      distance = sqrt((E2 - E1)^2 + (N2 - N1)^2) in units of feet.  

      BTW, I was watching Skandi ROV 1 last night while it was attached to one of the manifolds and the E & N coordinates were jumping all over the place, by hundreds of feet within seconds.  Be careful making maps off numbers you see only once, by one ROV.

      •  Not quite UTM (4+ / 0-)
        Recommended by:
        i m bobo, sullivanst, Yasuragi, wdz

        because it's the Clarke 1866 ellipsoid and not the WGS-72 or WGS-84 one.  It's done that way because of history; US geologic maps used the Clarke system before the International Geophysical Year worked out the currently-used geoid. (Actually, they used the "state datum planes" but let's not go there.)

        It may matter, also, that the units are "survey feet" rather than civil feet. One civil foot = 0.3048 metres, one survey foot = 3.2808333... metres (3937/1200).  That will make a fair difference in converting the northings, which are in the millions of feet.

        •  Ouch (3+ / 0-)
          Recommended by:
          i m bobo, Yasuragi, wdz

          Thanks for the clarification.
          What a mess.  So for the most part one just has to stay fairly local, and work from relative positions.

          •  Difference actually not that great. (3+ / 0-)
            Recommended by:
            i m bobo, sullivanst, Yasuragi

            Per the definition above
            1200/3937 = 0.304800609601219 meters/survey feet
            versus 0.3048 meters/civil feet
            Means with a northing of say 10,431,552 (from one of our ROVs)
            10,431,552 * 0.3048 = 3179537.0496 meters
            10.431.552 * 0.304800609601219 = 3179543.4087 meters
            Or a difference of roughly 6.35 meters.

            That's certainly noticeable but not significant if just trying to find your way around in Google Earth.  

            Can't do much about the ellipsoid, but again probably not a huge difference.  

        •  Bleah, mistake in the copypasta (3+ / 0-)
          Recommended by:
          i m bobo, sullivanst, Yasuragi

          One survey foot is 0.30480061... (1200/3937) metres.

          •  NP. I knew what you meant (2+ / 0-)
            Recommended by:
            i m bobo, Yasuragi
          •  That's a big difference. (2+ / 0-)
            Recommended by:
            i m bobo, Yasuragi

            You had me brain fuzzled there for a moment.

            For our purposes the ROV feeds are in feet (or meters). And that remains true on both axis'?

            With no correlation to degrees, I assume there is no problem at higher latitudes.

            I'm no Nate Silver, TomTech, or VoteforAmerica ("WineRev" Eeman, Recounting Minnesota)

            by Tomtech on Fri Jul 09, 2010 at 12:41:24 PM PDT

            [ Parent ]

            •  Yep (5+ / 0-)
              Recommended by:
              Tomtech, i m bobo, sullivanst, Yasuragi, wdz

              same on x/y axes.  
              Eastings are measured from the edge of a UTM grid line, and Northings are measured, for all practical purposes, from the equator, both in the same units.  UTM (or this variation of it) is used because it makes it extremely easy to calculate distances and directions at relatively small scales using simple trig (as long as you don't cross from one grid area to the next).  

              Northern latitudes don't change relative distances or angles in the coordinate system at all.   The globe is sliced up like orange peels and laid flat on rectangular UTM grids, so all that happens at northern latitudes is that the slice itself takes up a smaller percentage of each grid (left edge of slice starts at a higher easting value, and ends at a lower easting value).  It breaks down near the poles because the slices are so thin that you can't go any significant distance without running off your slice and having to switch to the next UTM grid area.  

              Of course these ROVs like to mix it up by sometimes displaying the z axis (depth) in meters, even though they're using feet on x and y.  ;)
              We've seen em do ROV ops around 1400-1500 feet depth before, so I was thrown off when the BOA ROVs working on the 1200' jumper, were showing a depth of 1524, but then I saw the sea bottom.  Turns out the BOAs show depth in meters, while most of the other ROVs show it in feet.

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