September 18, 2006

WWWTP - Story and Bonus Question

CRSo, lots of people were able to answer the question posed in the last post… “What’s wrong with this picture.”

That’s great!

Here’s the (short version) story behind the picture.

While geocaching, we came upon this compass rose. As K’s shadow passed over the tiles, I noticed her shadow was almost exactly north to south. Because she was vertical, that would have to mean that the Sun was directly south. The Sun is only directly south in Massachusetts at noon (EST), and the clock was reading 3PM. That implied that the compass rose was not indicating north.

I got out my magnetic compass and put it on the ground. Sure enough, it told me that north was actually about 60 degrees west of what the compass rose was indicating. (See detailed image) 60 degrees is quite a bit to be off by.

I had my GPSr, of course, and so I double-checked with the satellite data. A good GPSr can tell you not only where you are, but also where the Sun should be in the sky. Since the GPSr doesn’t have a built-in compass (mine doesn’t, anyhow) this feature can be helpful in finding north. You point the Sun icon on the display toward the actual Sun, and it gives you a rough idea of north. Here is another picture showing the face of the GPS, and showing that the satellites agree with the magnetic compass. If the Sun is directly to our left then north is definitely not directly to our right.

So the bonus question is, why do you think they got it wrong? Can you guess what my theory is, or perhaps come up with an even better one?

Here’s some help. A google map indicating the area where the compass rose exists:

Here’s a non-interactive map indicating the approximate shape of the park, and the orientation. The compass rose is roughly at the indicated point. It’s more or less centered in the park.

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North is “up” on this map. And you should be able to figure out what “compass rose north” is based on the other images.

Any guesses why the compass is off by as much as it is? I don’t have the definitive answer, but I can give you my theory after I see some of your theories.

Posted by James at September 18, 2006 11:10 AM
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Don't know if this comment will post, I'm using (of all things), the 'Voyager' web browser that comes standard with QNX, a Posix OS.

My theory as to how it got so messed up is a simple one. I think the park compass was laid out correctly, and then whoever installed the letters simply put them on the wrong points. If you move all the letters one point to the left (so that NW is N, and SW is W, etc.) then the compass will be correct or a lot closer to correct.

That's my guess... I can't see your google map, Voyager is an extremely weak browser. I mean it's better than Lynx I suppose, but not much!!!

Posted by: Chuck S. at September 18, 2006 12:55 PM

From what I can tell, it seems like Water St is "parallel" to route 79;

The people putting the compass place could have really believed that 79 runs directly north and south.

That, or it's the fault of an ancient Wizard race that hate compasses, and just so happen to reside in Fall River.

Theory two seems to be the most sound, and sane.

Posted by: Derek at September 18, 2006 12:59 PM

OK - at the end of this comment I'm giving my theory, so don't read the comment unless you don't care about my stupid theory. First, comments on Chuck and Derek's theories.

Chuck: It's true, that would be a lot closer to correct, but it would still be off by roughly 30 degrees, which is quite a lot. Being off by 30 degrees would, in my mind, need its own additional explanation.

Derek: Good eye, I didn't even notice that. It looks like 79 (to the south) is roughly 60 degrees from north, east northeast. It is possible that someone thought that 79 N actually ran north there. Its a plausible explanation.

Of course, either of these theories could be correct. Being off by 30 degrees doesn't really need an additional explanation because people do screw up randomly. But I like to look for plausible explanations because errors of this type often show up in code.

In Chuck's one problem (possibly) compounds another. Someone randomly miscalculates north, and then another person comes by and has trouble interpreting the miscalculation, and so puts north even farther off.

In Derek's a misleading and incorrect reference is used. In code, this would be equivalent to something that happened to me once way back at We-Know-Energy. I was told to get some variable values from an array that looked like it had accurate data. Turned out it was only valid a subset of the time, so it resulted in code that produced bad reports.

Here's my theory.

Note that the park lines up with "Water Street." The frontage is parallel to the street and the length of the park is perpendicular. And Water Street appears to be about 30 degrees off from north. The squared-off features of the park all line up with the street. They're all 30 degrees or so off from north.

So, if you were going to tell a person how to place the compass rose correctly, you might say: "It's 30 degrees off from the street" or "It's 30 degrees off from the width line of the park." There is a line of 3 flags right next to the compass rose. You might say "It's 30 degrees off from the line of the flags."

If you assumed the person knew which direction you meant, and they didn't know, they might measure 30 degrees off from the street and go east (clockwise)instead of west (counter-clockwise). 30 plus an extra 30 is 60. Way off!

Well, it's just a theory. Again, this kind of thing happens in code with either ambiguous explanations of how things are supposed to work or bad assumptions. Or both.

But, anyhow, I am holding out some hope that there's some GOOD explanation as to why this compass rose is 60 degrees (or so) off from actual north. The Portuguese have historically been known for their navigation and seafaring skills. It doesn't seem right for this park honoring the Azorean city of Ponta Delgada to have a compass rose in it that doesn't point north.

So if someone has the inside scoop, I'm all ears.

Posted by: James at September 18, 2006 2:02 PM

A WAG would be they used a map to set the compass and didn't realize that "up" on the map wasn't north.

Posted by: B.O.B. (bob) at September 18, 2006 2:15 PM

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