Sunday, March 18, 2012

Degrees, minutes.... Hours? Weeks? Months?

What's up Dick? You can't enter the waypoint position? Okay, I understand it's in a degrees minutes, and seconds format, and you changed your system format to input it I assume? Good. You say it won't accept the seconds, that's odd, what did you do then? You inputted it again, and just dropped the seconds, and that didn't work well either? Then you changed the latitude from 23 minutes to 24 minutes, and it was near the mouth of the channel? What was next? You just gave up? There was no place called Little Palm Island on your charts, and you even called and asked the dock master to verify that the website lat/long numbers on the website were the correct ones? Tell you what Dick, send me the link for the marina where you got the numbers, and I will take a look. A little while later I do take a look, at the link, and I immediately see what's going on. Boy are the directions screwed up. This marina would be lucky if anyone could find them based on their published directions.

The Little Palm Island Resort seems to be a very nice place to go to. My only gripe is that the directions on how to get there via water must have been written by a someone who has never seen a boat, and was from some place like Death Valley. Let's start with the fact the Little Palm Island's name was changed to Munson Island, and or Little Munson Island in 1923 when the Munson family bought it and the three adjacent islands. Since then the more recent nautical charts, have always shown it as Munson Island. I guess it's more quaint to call it Palm Island, rather than the Palm Island Resort on Munson Island. I get it, sort of.

Now for the position issues. The latitude and longitude appears to be in the degrees, minutes, and seconds format, but unless you own a time piece custom made by an eccentric watch maker from Pluto, the last time I looked there are only 60 seconds in a minute, so where did the 78, and 63 seconds come from? (Post publication note, read the comments below.) I did some playing around with my SeaClear package at home to see what was up with the numbers. The first thing I tried, was adding a zero to the 78, and 63 "seconds" and feed it in as degrees, minutes, decimal minutes, and lo the location was exactly the flashing red marker "50" just south of Big Munson Island.

While we're on the subject, I abhor placing waypoints exactly on top of a marker. If you do so, on a dark and stormy night, or during other various inclement conditions when you arrive at the waypoint, it could be with your bow, along with all of the subsequent fiberglass repairs. So if  red "50" was what they meant, I would have, and did in the picture, put the waypoint away from the marker a bit. Oh, and don't get me started on the LORAN position information since LORAN passed away after a long lingering illness over two years ago. I'm not going to take the time to do the conversion to see where the waypoint actually was. The point has now become somewhat moot to say the least.

The point of this exercise is that GPS position formats matter a lot, and the directions to this marina missed by furlongs. I like the format DDD.MM.MMM (degrees.decimal minutes). It is very commonly used, and provides a little more charting precision than DDD.MM.SS. So here is an example. In my revised marina directions the latitude of the near marker "2" location is 24.37.040N. If we convert this to DDD.MM.SS you end up with 24.37.02N. This is close, but what if you try to convert it back? The answer is you get pretty close to the original number, but not quite because of rounding up, or down in the conversion. Converting back you end up with 24.37.033N.

Now what happens if you just add a zero to DDD.MM.SS  (24.37.02o), and enter it into the DDD.MM.MMM format in your chart plotter. You have just moved the original point about 100'. If you entered it as 24.37.200, the original point has now moved about 1200'. So you have to pay attention. Most chart plotters allow you the change the input formats. So if you want to enter a DD.MM.SS position, change to that format, enter it, and them change back to your preferred format. The system will do the conversion automatically for you. The conversion calculations are simple, but there are a lot of online converters you can use. That's the end of this. My tedious pedantic droning is even boring me to death. Just get it right, it's important.

I sent an e-mail to the property management about a month ago, I never heard back from them. It will be interesting to see if they ever change their wonky directions. Dick however does know how to get there, now!

5 comments:

  1. Odd, when I was learning to navigate, that WAS the method for specifying deg-min-decimal. (using * for degree as I don't have a degree symbol on my keyboard):
    23*36.78N is the decimal (hundredths) minutes,
    23*36'47"N is in d-m-s
    Those formats are awkward with computer character sets, so the more confusing ways seem to have become common...
    23.36.078N (decimal)
    23.36.47N (d-m-s)

    ReplyDelete
  2. Scott, you're right. I suspected immediately that's what they were using, but that format is not in common use today, and hasn't been for quite a while. The three predominate formats are DDD.MM.SS (degrees, minutes, seconds, and not used often today because of the lack of accuracy). DD.MMM.MMM (degrees,decimal minutes, the most common), and DDD.DDDDD (decimal degrees, not used often in marine applications, but it is common in other cartographic disciplines due to the ease of programming the math calculations). Because of the much improved accuracy of the GPS, DD.MM.SS, and DD*MM.SS have long since fallen by the wayside, along with DD.MM.SS.S (degrees, minutes, seconds, tenths of seconds), the most insidious of all of them. Had the ad used DD.MM.MMM, there would have been no issues because the format can't easily be confused with any of the others.

    ReplyDelete
  3. Capt. Ken E. BeckMarch 19, 2012 at 4:25 PM

    I'd go for 24-37.033N or 24 37.033N.

    ReplyDelete
  4. Bill - thank you again for the always informative, frequently entertaining blog.

    There is an excellent article in Good Old Boat about a sailor that was led astray by this very issue. If you interested in how this could have been a contributing factor in getting lost at sea, read this; http://www.goodoldboat.com/pdfs/SailingInAFog.pdf

    ReplyDelete
  5. Thanks for the story link, it was a very good read and advice. At various time in my life, I have had pieces of those kinds of events occur to me. In Georgian Bay I had a freighter come out of the fog passing in front of me. The good news was that he had seen me on radar long before I had seen him visually. The bad news was it took a few minutes to slow my heart rate down. Thanks for the link. Bill

    ReplyDelete