ONE IS STATED AS DECIMAL DEGREES THE OTHER IN DEGREES MINUTES AN
~ Captain Paul's response to: ~ Kruser ~
In the Latitude ~ longitude format or Position or Location choices, the name varies on the brand and model of your GPS unit, there are THREE options available in most GPS units to state a latitude~longitude position.
They are Decimal Degrees stated in the setup as H,DDD.ddddd, this is the way the Google positions you indicated are stated, and Degrees, Minutes, Seconds and tenths of seconds, expressed as H,DDD,MM,SS.s, which is the way your GPS unit is now set up and Degrees, Minutes and thousandths of minutes, expressed as H,DDD,MM.mmm.
A position can be stated in either of these settings. The numbers will be different but the position will be the same.
Remember your high school math? There are sixty seconds in a minute, sixty minutes in a degree and 360 degrees in a circle. Latitude and Longitude values are not stated in a full circle 360° format as latitude and longitude positions are split to a northern, southern, east and west hemisphere. This hemisphere is the H in the aforementioned position format choices.
The U.S. is in the Northern hemisphere for Latitude and the Western hemisphere for Longitude. All positions should have both the N designation for North and the W designation for the Western hemispheres.
The Google positions of latitude N 30.251616° ~ - 91.72465°,(the means West), converts as follows, to 30° plus .25161 of 60 minutes, which is one degree, (60 x .25161) or 15.0966 minutes. Again now converting the decimal MINUTES of .0966 to seconds requires that you multiply the decimal minutes, the .0966 by 60 which is then shown as seconds to the tenth position. (60 x .0966 = 05.79 seconds.
So the final latitude of the decimal degrees position as stated above converts to N 30° 15 05.79. The (°) equals degrees, the ( )means minutes and the ( ) equals seconds.
You would then do the same with the Longitude value.
As you can see the Google position of N Latitude 30.251616° converts to 30° 15 05.79, which is just a few feet from the position you stated as in the GPS as 30°15 5.5. The discrepancy between the two is probably due to not placing the indicator at exactly the same location or different DATUM was used to determine the position.
Both the Google stated position and the mathematically converted position would describe the exact same location, within the confines of the amount of positions that are available for that particular format. Most GPS units only allow for a tenth of second (H,DDD,MM,SS.s and a thousandths of a minute in the H, DDD, MM.mmm position.
If having to do all of the math is a burden, I surely think so, then go to the SET UP in the MENU feature in your GPS unit and find the POSITION FORMAT or LOCATION which ever your brand of GPS calls it and change the H,DDD,MM,SS.s to the Decimal Degrees setting (H, DDD.dddddd) setting and enter the Google decimal degree positions. Then go back to the SETUP window and change the GPS back to whatever format you are comfortable with.
By the way, the GPS figure stats the Longitude value first as a LONG~LAT and the Google states the Latitude value first listing it as LAT~LONG.
In additional to the POSITION FORMAT settings, you should also consider the DATUM settings in the unit.
Datum is what the map makers thought was the shape of the earth. Older Datum was usually the North Americal Datum of 1929, (NAD 29) with the newer datum being determined via radar mapping and called World Geodetic Survey of 1984 (WGS 84). Waypoints, tracks and routes should be entered in the same DATUM used when they were plotted or captured by a GPS.
A difference between the two in this latitude could mean a difference of up to 250 feet which could be the error between your GPS and Google converted positions.
~ Captain Paul ~