Question about converting Latitude/Longitude coordinates in decimal to deg/min/seconds

[DavidFL]

OK, the reason I'm asking about "accuracy of the sonar" is that, in all likelihood, if you actually get within a 45' circle of where you wanted to be on the planet in open water you're doing really well. There are errors from the GPS system itself, there are positional errors in the NOAA charts; there are errors in the presentation and rendering of the information on your sonar display; there are errors in the sonar sensor: they all have +/- 'tolerances' that are combinations of accuracy, resolution, and rendering.

In summary, the coordinate transformation using the conversion you described is good enough....more likely more than good enough...for what you are trying to do.

 

[LI-er]

In summary, the coordinate transformation using the conversion you described is good enough....more likely more than good enough...for what you are trying to do.

And I don't mind circling around a bit on a nice warm summer day with calm seas which is when I would be diving. As long as I know I'm in the ballpark.

Thanks

 

[tursiops]

Hello, I hope someone familiar with boating and navigation can answer this question, I cannot find answers following a rather extensive internet search.

I am taking GPS coordinates of potential dive sites from NOAA charts and inputing them into an App called Navionics which I use while boating. The problem is this- NOAA maps give decimal coordinates while Navionics app only uses deg/min/seconds- and Navionics only has 3 places for seconds with no decimal places.

So for example I have the following GPS coordinates from NOAA:

40.948704
-73.202179

Run through a decimal to degrees converter I get:

40° 56' 55.334"
73° 12' 7.844"

So do I input the seconds into Navionics as 055 and 079 respectively? If so, how much accuracy do I lose, will I still be close enough to the location to pick it up on sonar without circling around for an hour?

Thanks

All those answers about 1 sec being 90 ft are incorrect. You must use 1 nautical mile for one degree, not one statute mile. The former is 6076.13 ft. The latter is 5280 ft. Divide by 60, that is one sec.

Edit: corrected typo....69 not 69

 

[Cirdan]

Decimal seconds are not an option with Navionics App.

I mentioned decimal seconds because you showed that format of position as an output from your "decimal to degrees" converter.

If you want to standardize on position formats for your own use with the various devices and applications that you use, and this is a subjective choice,

• The DDD.ddddd format exists for mathematical convenience, but is not user friendly as you can't get a quick idea of positions by looking at something that is expressed in fractions of degrees, which are essentially a 60 nautical mile grid.
• Some "old school" recreational marine navigators are still on DDD MM SS(.s) format.
• Many if not most "new school" recreational marine navigators are using DDD MM.mmm format. This is the default format in the Navionics boating app.
• The latter format will get you within 6 foot position resolution worldwide.
• Most commonly used digital navigational instruments and applications will accept or display positions to some resolution in any of these formats. Navionics, iNavX, etc.

Hope this is helpful

 

[Cirdan]

All those answers about 1 sec being 90 ft are incorrect. You must use 1 nautical mile for one degree, not one statute mile. The former is 6076.13 ft. The latter is 5280 ft. Divide by 69, that is one sec.

Divide by 69???

I think you are confusing the conversion from minutes to seconds with the conversion of minutes to decimal minutes ... and/or mixing nautical and statute miles.

Stay in nautical miles. Divide one minute by 60 to get one second. Works every time.

 

[LI-er]

Many if not most "new school" recreational marine navigators are using DDD MM.mmm format. This is the default format in the Navionics boating app

ok- you are correct- and that makes sense. I am confusing seconds for minutes decimal places in Navionics.

So my original post was wrong- and Navionics will get me within 6'.

That works!

 

[lowviz]

I struggled with mapping a square grid over an area and then turning those points into Lat/Lon. Came close, but not exact. Gave up and asked Dr. Math:

(you can use the exact equations to determine error bounds in feet)

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Hi, Dennis.

As Dennis wrote to Dr. Math
On 01/14/2016 at 12:00:36 (Eastern Time),
>[Question]
>Hi, I'm trying to depth survey a small lake. The intent is to create
>a square grid for the survey points. I've read the archives
>concerning latitude and longitude problems. I now have the problem
>solved to my satisfaction, but remain intrigued. I'm looking for an
>analytic answer. Grid points are 50 feet apart, so a flat projection
>is easily supported. I'd like to be able to calculate how many
>degrees(a very small number) per foot there are in both the latitude
>and longitude directions.
>
>[Difficulty]
>Instead of a "distance between" problem, I'm looking at the limiting
>case of a single latitude, longitude point. At that point, how many
>degrees per foot are there in either direction?
>
>[Thoughts]
>For 41 degrees latitude, I get 2.75E-6 degrees per foot N-S
>direction.
>
>For -75 degrees longitude, I get 3.64E-6 degrees per foot E-W
>direction.
>
>These are approximations, would really appreciate the analytic
>approach to this for more precision and general interest.

ANSWER:

The mathematical basis for the calculations you want to do is found here:

Transformation between (x,y) and (longitude, latitude)
http://mathforum.org/library/drmath/view/51833.html

There you'll find the formulas:

y = R*(b2-b1)*pi/180
x = R*(a2-a1)*(pi/180)*cos(b1)

where x is the east-west separation, and y the north-south separation,
of two nearby points; b1 and b2 are the latitudes of the points; a1
and a2 are the longitudes of the points; and R is the radius of the
earth in the desired units (feet in your case).

You want to know the ratio of the separation (x and y) in feet to the
difference in angle (b2-a1 and a2-a1) in degrees. From the formulas
above we get:

North-south: (b2-b1)/y = 180/(pi*R)
East-west: (a2-a1)/y = 180/(pi*R*cos(b1))

Using R = 3956 miles = 20 887 680 feet, we have

North-south: 2.743*10^-6 degrees/foot
East-west: 2.743*10^-6/cos(latitude) degrees/foot

For your example, at latitude 41 degrees, cos(41) = 0.7547096, so the
east-west factor is 3.635*10^-6 degrees/foot. The north-south factor
is always 2.743*10^-6 degrees/foot. These match your calculations. I
don't know whether all four digits in these numbers are valid, as the
earth is not a perfect sphere, so the radius I used is an average number.

I'm curious how you arrived at your numbers without an "analytic
approach".

- Doctor Rick, The Math Forum
<http://mathforum.org/dr.math/>

 

[rjack321]

Navionics doesn't appear to have that option. I guess I could try a different one.

Here's the converter I used. If you plug 40.948704 and -73.202179 into the boxes you get seconds which are actually produced to the thousands of a decimal place.

Lat Long to DMS Converter (Decimal to Degrees Minutes Seconds)

There is a glitch in the app if you can enter (eg) 100 seconds with no decimal place. That doesn't exist.

The reason you are getting so many decimal places in all these conversions is because the various sources of old numbers are being divided by 60 and then rounded and then multipled by 60 and rounded back.
e.g. 20 seconds into decimal minutes is 0.3333333333333333333 (it keeps going)
0.3333 minutes back into seconds is 19.998"

Just try selecting a different format, DDD MM.mmm is very common and more than enough to get the transducer cone from your depth sounder over a target.

 

[LI-er]

There is a glitch in the app if you can enter (eg) 100 seconds with no decimal place. That doesn't exist.

The poster above made me realize my error- I edited my first post.

I thought decimal minutes in Navionics was the seconds. There is no place to enter seconds in Navionics but as pointed out by @Cirdan I'm good to within 6' using minutes to the thousands place.

 

[Wookie]

Navionics is an excellent program but a piss poor app.

I highly recommend iNavX. It successfully brought my 100footer from Panama City to Key west when my PC gave up the ghost, and it speaks all position formats including northings and eastings.