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Nav exercise #3


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It's close to that time of year when the "bus" is parked and charts/trip planning dance in my head. 

For this exercise I've taken a screen shot of  popular kayaking destination Stonington.  I hope some may find this a useful exercise, though no substitute for one-on-one chart-and-compass work.  Feel free to submit your own practice questions for this "chartlet", perhaps adding answers as I have. Likely easiest to work through by printing out (choose "grayscale printing" to save your color inks).

1.    1.   An early morning start from Duck Harbor, IAH, has put your group in good position to meet another paddling group at Green I. quarry, for lunch and a swim on this hot July day.  After leaving Harbor I.  in a northerly direction, an unexpected fog bank rolls in, and your group finds itself “at sea” as to your location.  Your non-mapping, primitive Garmin hand-held displays “68.38.367W 44.07.153N”.  Approximate the position of your group on the chart, then pinpoint your precise location after doing the math. 



2.     2.  You have not loaded waypoints for the quarry into your GPS, so you will calculate the waypoints, using the tip of the cove on Green Island’s SE shore.  

3.     3.  After a restful and playful afternoon at the quarry, pre-sunset, it’s time to make for home base.  Russ, a relatively green navigator, thinks he might have a devil of a time locating our camp on Hell’s Half Acre. He’s asked you for help finding the shortest route back “home”. Walk him through the process, including bearings, features, aids to navigation, and ETA.

4.     4.  The next day, a few in the group decide to visit MITA’s recently-added George Head I.. As you round the southern shore of Coot I., someone claims to make out Wreck I. behind Bare-Yes or No?  From here, one should be able (good visibility/?binoculars) to point out (Yes, No Maybe)?:  Sprout, John, Buckle/Spruce, near Potato, far Potato, George Head, Little George Head. Round, St Helena (duh-look for the smoking volcano).

5.     5.  Triangulation exercise:  E of John I. you take bearings of 84 and 05 degrees, magnetic to RN “4” and “2”, respectively.  Where are you on chart?  What is your GPS waypoint?  Radio your location to Coast Guard, using back bearings. 

6.     6.  What distinguishing intrinsic feature(s) might separate Devil and Camp from your position in exercise 5 above? 


Answers:  Nav exercise 3 answers.docx




Edited by gyork
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Bonus question #7: Gary's coordinates could lead you to four different locations depending on how you interpreted them. (I'm going to change his example slightly to make the second issue more ambiguous: “68.38.37W 44.07.15N” ). List them, and explain the two things you can do differently when writing or radioing latitudes and longitudes to avoid these errors. :)

Edited by Dan Foster
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Seems no one else wants to play Dan, so I'll chime in for #7.  Yes, I'll admit to being not a little sloppy :wacko:with the OP.  This is the type of notation that I use and understand for my charts, but recognize the ambiguity of the OP. 

 -LATITUDE FIRST, LONGITUDE LAST!,  unlike the UTM method of reporting/designating eastings first and northings second (go right[east] up[north]). 

The standard format for reporting GPS positions, according to NSARC ( United States National Search And Rescue Committee ) is  Degrees, Decimal Minutes (DD° MM.mm’).  Also, from the same link kindly referenced by Leon [thank you!] many months ago (see attachment):  

Latitude is always read and written first noting “North” since the U.S. is North of the Equator. Longitude is always read and written last noting “West” since the U.S. is west of the Prime Meridian. When speaking or reporting Latitude and Longitude use the following: For example, 39° 36.06’N by 76° 51.42’W, should be stated as per the following: “Three nine degrees, three six decimal zero six minutes North by seven six degrees, five one decimal four two minutes West.” The words, “degrees,” “minutes,” and “decimal” must to be spoken.



How might one interpret “68.38.37W 44.07.15N”?: 

Location 1.  My original post should have been written 44° 07.153'N 68° 38.367’W (or 44° 07.15'N 68° 38.37’W).  The proper superscripts are present, and the thousandths of a minute eliminate the ambiguity/possibility of confusing the 3rd set of numbers as seconds.

Location 2.  By habit (hopefully), if read quickly, one might see the first part of the waypoint N and the last E, leading them to pinpoint the location as smack dab on the Greenland (receding) ice cap! Likely CG dispatch would see the error of the report.

Without superscripts (where is "degrees" on my Mac?), and listed as 2, rather than 3 decimal places, might lead the reader to interpret this 2 ways, assuming the designation is degrees.minutes.seconds:

Location 3.  If the reported error is recognized (W is distinguished from N): 68degrees.38minutes.37secondsW; 44degrees.07minutes.15secondsN.  Converting to DD° MM.mm’:  68° 38.62’W 44° 07.25N’, placing you in the water 313degrees, 0.2M from Location #1.

Location 4. As in #2 above, (quick interpretation as lat first, long last) 68degrees.38minutes.37secondsN; 44degrees.07minutes.15secondsW.  Converting to DD° MM.mm’:  68° 38.62’N 44° 07.25W’, still on the Greenland slush cap.


CG call.doc




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The other ambiguity in the original coordinates was the lack of datum. If someone had plotted those coordinates on a USGS topo map, they might have plotted them assuming you used the NAD27 datum, which was the the standard on all US maps up until 1983, and still shows up a lot in current maps and charts that were originally produced before 1983. The most notorious local example is National Geographic 2013 edition of their waterproof "chart" of the Boston Harbor Islands, which touts being "regularly updated" with GPS coordinates and a UTM grid, but you have to read the fine print on the back of the map to realize they are still using NAD27 (the North American Datum of 1927).

The lat/lon values reported by everyone's phones, and by most GPS receivers and software (assuming you haven't selected a different datum), reference the WGS84 datum. There's a significant offset between the two datums, which means if someone gives you coordinates in NAD27 and you interpret them as WGS84 (or vice versa), you'll be off by about 150 feet in New England. In several other coordinate systems (Massachusetts state plane, for example), they added in a huge fixed number to any coordinates expressed in the new datum, so that if you mixed up datums, you'd end up with results that were hundreds of miles away, making the error immediately obvious.

So, for clarity, I'd do everything that Gary suggests above, and then tack on "WGS84 datum" at the end, or "all coordinates in degrees and minutes of latitude and longitude, WGS84 datum" somewhere near the list of coordinates.

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I came up with mostly the same answers as Gary.

For #3, I took a different route back to Hell's Half Acre: cove to N of Potato, to S of Camp, threading the needle (high tide!) straight back to Hell's Half Acre.

I use a different technique for determining GPS waypoints off of a chart:

Find two chart lines that lie on either side of the point. Let's assume they are horizontal lines of latitude N44deg 08' and N44deg 09'. Take a plastic ruler and place the 0cm mark on the lower line, and rotate the ruler until the 10cm mark hits the upper line. (cm are unimportant, you can use inches or anything with 10 equal spaces). Now slide the ruler until it hits the point. Let's say it hits at 7.2 centimeters. That means that the point is 7.2 tenths of the distance from the lower line to the upper line, or 72%. 72% of the distance from N44deg 08' to N44deg 09' is a latitude of N44deg 08.72'

Repeat for longitude. Works best with UTM coordinates, since everything is in base 10.

Here's the view from #5. Blue line points to Camp I, red points to Devil I.

Devil Camp.jpg



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