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

[LI-er]

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"

[edited- it was pointed out that I am confusing decimal minutes with seconds. It was explained I need to convert from degrees to decimal minutes to be compatible with the Navionics App which will provide high accuracy (within 6')]


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

 

[hammet]

1 second of latitude is about 100ft.

( 1 min of lat = 1 nautical mile
1 second of lat = 1/60 nm and 1 nm = 6076ft
1 second = 6076ft/60=101ft)

ETA: longitude will vary. It will approach 100ft per second as you get close to the equator and lessen as you go towards the poles

 

[DavidFL]

1 second of latitude is about 90 feet. 1 second of longitude is about 90 feet at the equator; less as you travel away from the equator. If you round off the decimals you are probably getting within .5 seconds of latitude/longitude....so about 45 feet in each direction. BTW....seconds are never going to be bigger than 59, so your question about 079 indicates a decimal point slip....you would be entering 008 for that longitude. How accurate is your sonar?

 

[Storker]

how much accuracy do I lose

One minute latitude is one nautical mile (1852m).
One second latitude is 1/60 of that, or 31m.
One second longitude is 31m at the equator, less the higher your latitude is.

So, if you can't input decimal seconds into your GPS, worst case you'll be at most ~40m (130'), probably less than 30m (100') from your destination when your GPS tells you that you've arrived.

 

[rjack321]

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

#1 You can change the most navionics software to accept other lat/long formats (have not used that exact app) so you don't need to convert

#2 You can only have 60 seconds before you create a new minute and start over with zero seconds (+1 minute more). So something is not right about potentially having up to hundreds of seconds. Are you sure the last digit isn't preceded by decimal point? If it is you have about 9ft accuracy which is as good as any GPS can get anyway.

 

[Cirdan]

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

Accuracy of received positions is one thing. Resolution of specified positions is another. To determine how precise you want to specify positions, you want to think about resolution.

Assuming you set your device or application to use degrees - minutes - decimal minutes, here is a summary:

Position resolution - applies to latitude globally:
One degree of latitude = 60 nautical miles
One minute of latitude = 1 nautical mile = 6080 feet
One decimal place of one minute of latitude = 0.1 nautical mile = 608 feet
Two decimal places of one minute of latitude = 0.01 nautical mile = 61 feet
Three decimal places of one minute of latitude = 0.001 nautical mile = 6 feet
For longitude, it depends on what latitude the position is at. Same as latitude at the equator, smaller than latitude as you move away from the equator towards the poles.

In practical terms for recreational navigation, there is no point in going beyond three decimal places of one minute of latitude or longitude. If you are using the degrees - minutes - seconds - decimal seconds format, you can use seconds with one decimal place for roughly equivalent resolution.

 

[LI-er]

so your question about 079 indicates a decimal point slip....you would be entering 008 for that longitude.

Yes, my mistake, thank you.

How accurate is your sonar?

As far as sonar accuracy?.. Don't really know, it tends to be pretty good at picking up unusual bottom contours and wrecks. To be honest I didn't know accuracy was an issue with sonar- if it sees something on the bottom it displays it on the screen. I'm pretty much dropping the anchor dead on top of the wrecks lately - or is it more complicated than that?

As you can tell I'm a relatively new boater but I've gotten pretty good at reading the sonar and finding stuff, which is why I'm taking it to the next level and looking for sites that are not commonly published.

you can use seconds with one decimal place for roughly equivalent resolution.

Decimal seconds are not an option with Navionics App.

 

[nolatom]

I'm an English major with a dusty 100-ton license, so bear with me.
Okay, one second is a sixtieth of a nautical mile, meaning 100 feet, at least on the latitude scale. So if you can go to one decimal place for a second (a tenth of a second), that's 10 feet, and the second decimal place (a hundredth of a second) would be one foot.
10 feet is probably doable. But you need one of your only 3 Navionics "places" to put a zero in front of the minutes (eg 055 seconds rather than 55.3 seconds)??. That's a problem, since 100 feet makes a bigger "box" to search than 10 feet.

 

[LI-er]

#1 You can change the most navionics software to accept other lat/long formats (have not used that exact app) so you don't need to convert

#2 You can only have 60 seconds before you create a new minute and start over with zero seconds (+1 minute more). So something is not right about potentially having up to hundreds of seconds. Are you sure the last digit isn't preceded by decimal point? If it is you have about 9ft accuracy which is as good as any GPS can get anyway.

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)

 

[LI-er]

But you need one of your only 3 Navionics "places" to put a zero in front of the minutes (eg 055 seconds rather than 55.3 seconds)??. That's a problem, since 100 feet makes a bigger "box" to search than 10 feet.

That appears to be precisely the issue. Although 100 ft or less isn't all that bad when you're looking for wreckage on sonar.