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Old 06-02-2013, 04:27   #1
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Coordinate Math Question

Is there a recognized way to deduce one position from another?

That it, given a bouy, and the coordinates for it in degrees, minutes and seconds, is it possible to determine mathematically the coordinates for a point 200 yards west of it?
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Old 06-02-2013, 04:41   #2
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Ya. Too easy. 200 yards is 600 feet. A nautical mile and therefore a minute of longitude is about 6000 feet. So that's 0.1 minute of position west.

What? You are stuck in a cold latitude and not here on the equator where minutes of longitude are not 6000 feet. Multiply by the cosine of your unfortunate latitude.

It's all done for you with the MEASURE tool in OpenCPN.
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Old 06-02-2013, 04:42   #3
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Re: Coordinate Math Question

Err.. trick question or am I missing something here?

I don't recall the maths involved to convert distance into degrees longitude but is was simple when I did know it . It is a dependant on the latitude as in sin or cos or tan or something of the lat. times the distance in question (in nm).

So in the given example, the lat will remain the same and the long will be increased/decreased the required seconds.

Now hopefully someone with a better memory will chime in.
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Old 06-02-2013, 04:44   #4
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Re: Coordinate Math Question

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Originally Posted by daddle View Post
Ya. Too easy. 200 yards is 600 feet. A nautical mile and therefore a minute of longitude is about 6000 feet. So that's 0.1 minute of position west.

What? You are stuck in a cold latitude and not here on the equator where minutes of longitude are not 6000 feet. Multiply by the cosine of your unfortunate latitude.

It's all done for you with the MEASURE tool in OpenCPN.
Daddle is a minute faster (poor pun intended) and has better memory plus knows his cosines
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Old 06-02-2013, 04:51   #5
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Ya folks. Look at the time stamps! 2 minutes to answer that. And on an iPhone keyboard in a bar after many happy hour beers too!
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Old 06-02-2013, 05:13   #6
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Re: Coordinate Math Question

without google!!!!!!

Maybe....

New Lat = Old Lat +(cos(course*dist)

New Long= Old long + (sin(course*dist)*90/oldlat

Better hit send quick before someone else gets in, haven't checked that yet ....
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Old 06-02-2013, 06:34   #7
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Re: Coordinate Math Question

Or for we non-engineer types: Get in dink with GPS, paddle over to point in question, take reading, paddle back!
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Old 06-02-2013, 06:38   #8
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Re: Coordinate Math Question

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Originally Posted by conachair View Post
without google!!!!!!

Maybe....

New Lat = Old Lat +(cos(course*dist)

New Long= Old long + (sin(course*dist)*90/oldlat

Better hit send quick before someone else gets in, haven't checked that yet ....
should be (course)*distance...

all in decimal degrees , no so sure about the 90/oldlat - anyone actually know this stuff?
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Old 06-02-2013, 06:48   #9
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Re: Coordinate Math Question

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should be (course)*distance...

all in decimal degrees , no so sure about the 90/oldlat - anyone actually know this stuff?
Used to
Not now
I blame GPS
Time hasn't been kind to my little used maths brain cells

Thanks for the memories Jamer Six
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Old 06-02-2013, 07:02   #10
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Re: Coordinate Math Question

Sheeeeeeeeeeeeesh!
But I do know how to varnish...

Vic
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Old 06-02-2013, 07:02   #11
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Re: Coordinate Math Question

GO TO .........

The NGS Geodetic Tool Kit

theN download .........

Inverse/Forward/Invers3D/Forwrd3D

this is a program for inversing lats & longs
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Old 06-02-2013, 10:36   #12
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Re: Coordinate Math Question

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Originally Posted by capngeo View Post
Or for we non-engineer types: Get in dink with GPS, paddle over to point in question, take reading, paddle back!

cap'n... You forgot "grab drink" then dink.... You couldn't possibly pull this task off without refreshment....
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Old 06-02-2013, 17:47   #13
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Re: Coordinate Math Question

Oh, dear...
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Old 06-02-2013, 20:05   #14
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It has been a while...but the longitude of a known east west distance would depend on the latitude you are at. That is why you always get distance by measuring latitude when plotting with dividers...tell what I am missing...be gentle...long day
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Old 06-02-2013, 20:13   #15
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Re: Coordinate Math Question

I used to have a spreadsheet with the formulas. I will try to find it.

Did a google and here is one answer.
http://www.koordinaten.com/informations/formula.shtml

Here is a better one.
http://www.movable-type.co.uk/scripts/latlong.html
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