Navionics iPhone GPS Format

[durr]

Folks,

I don't know much about GPS, but I understand there are two common formats for GPS coordinates, decimal and degrees, minutes, seconds.

I am using the Navionics iPhone app, which seems to use degrees, minutes, and seconds. I have waypoints I'd like to enter, and they are in the form 35 33 61, 92 11 67. However, Navionics seems to want me to input 7 digits for latitude and seven digits for longitude.

Anybody know what I'm missing?

 

[jhebert]

Re: Navionics iPhone GPS Format

Your inquiry is not really related to global positioning systems, but to measurement of angles. In marine navigation, angular measurements are usually given in degrees measured from the equator or the prime meridian. There are three common systems of notation:

--decimal degrees

--whole degrees, decimal minutes

--whole degrees, whole minutes, decimal seconds

It is straightforward to convert among the various systems of notation. The units are related as follows:

60 seconds = 1 minute
60 minutes = 1 degree

In your narrative you mention an angle of "35 33 61" and give no unit designations. This requires that the units be deduced. Because the last two digits are greater than 60 this cannot be a notation in degrees, minutes, seconds. The inference is thus that this notation is likely in whole degrees, decimal minutes. This is written by convention as 35-degrees 33.61-minutes.

I will demonstrate the conversion process to other notations. This will show you how to make conversions:

Convert 35-degrees 33.61-minutes to decimal degrees:

Decimal degrees = 35 + ( 33.61 / 60)
Decimal degrees = 35.560166666666667

Convert 35-degrees 33.61 minutes to degrees, minutes, seconds (DMS) notation:

DMS = 35-degrees 33-minutes (.61 x 60) seconds
DMS = 35-degrees 33-minutes 36.6-seconds

It is a common format for decimal degrees to require expression of the angle to six decimal places, particularly when dealing with electronic navigation systems. The GPX format uses this notation. See

http://en.wikipedia.org/wiki/GPS_eXchange_Format#Units

Measurement of angles using Base-60 is an ancient convention. See

http://en.wikipedia.org/wiki/Sexagesimal

 

[sccatfish]

Re: Navionics iPhone GPS Format

The GPS units that I have had allow you to change between the two. I would imagine there should be a setting in that program to allow you to change it. If not, it's not very hard to convert as explained in the post above. Set up an Excel sheet if you have a lot of them to convert.

 

[jhebert]

Re: Navionics iPhone GPS Format

Any navigation application with any sort of decent design will provide a means for the user to select a preference for the notation used for angles that are going to be entered by the user. If your particular application does not provide this, you probably should look for a different application. The tedium of manually converting angle notation from one form to another is a task perfectly suited for execution by a computer, and you should let the computer do it for you.

 

[TerryMSU]

Re: Navionics iPhone GPS Format

Computer has two advantages...

1. It does not make mistakes (assuming the software is correct)
2. It is much faster that I am.
3. It is very consistent

TerryMSU

Any navigation application with any sort of decent design will provide a means for the user to select a preference for the notation used for angles that are going to be entered by the user. If your particular application does not provide this, you probably should look for a different application. The tedium of manually converting angle notation from one form to another is a task perfectly suited for execution by a computer, and you should let the computer do it for you.

 

[Pez Vela]

Re: Navionics iPhone GPS Format

Anybody know what I'm missing?

You're missing a zero in each of your numbers.

Because your waypoint is written with spacing in the form of "degrees-minutes-seconds" but has seconds exceeding 60 (which can't happen), I suspect that your waypoint is, in reality, in conformance with the Navionics method.

My version of Navionics for iPhone (v5.0.4) uses degrees and minutes only (with minutes carried out to three decimal places). That coincides with your observation of a 7 digit input for your version of Navionics as well, such as: (55? 55.555' N) read as "fifty-five degrees, fifty-five decimal five-five-five minutes North."

It seems likely that your waypoint is actually 35? 33.610' N and 92? 11.670' W, in the vicinity of the Greers Ferry Yacht Club in Arkansas.

 

[bruceb58]

Re: Navionics iPhone GPS Format

Just use a conversion program

Convert Latitude / Longitude in Degrees/Minutes/Seconds to/from Decimal (FCC) USA

Assuming your original numbers are in degrees minutes seconds. Like others say, the 61 can't be seconds though.