Help me understand different GPS readings

January 10, 2007 9:40 AM

Help me understand the differences in the way Latitude and Longitude are reported by my GPS and Google Maps

I have a GlobalSat BC-307. I took a reading and it reported my lat/long as: Lat N 40'54.6140'/Long W 074'58.0233'

I typed this into Goggle Maps as: 40 54.6140 -074 58.0233

While the map reported the correct location at the time, the pop-up window above the map "re-intrepreted" the coordinates in the following way:

In bold text were these coordinates: 40.910233, -74.967055

In regular text below that were these coordinates:
+40 54' 36.84", -74 58' 1.40"

Can you help me understand why the coordinates are displayed in different ways?

posted by frogmoses to Technology (6 answers total)

Here's a good page that summarizes the differences:

Lat/Lon Formats and Symbols
posted by llamateur at 9:45 AM on January 10, 2007

Your GPS is reporting lat/long in the minutes and seconds format, while Google is using decimal format. To convert, remember there are 60 seconds in a minute. 54.6140×(100/60) = 91.023, hence 40.91023 is the decimal equivalent of 40'54.6140
posted by matthewr at 9:50 AM on January 10, 2007

Best answer: Its a question of when the software starts displaying decimals vs. when it keeps showing base 60 numbers.

Compare these:

40 54' 36.84"
(Degrees Minutes' Seconds.decimalSeconds")

Change the seconds to a decimal by dividing by 60: 36.84 / 60 = 0.614

40 54.6140'
(Degrees Minutes.decimalMinutes')

Change the minutes to a decimal by dividing by 60: 54.6140 / 60 = 0.910233

40.910233
(Degrees.decimalDegrees)
posted by Maastrictian at 9:51 AM on January 10, 2007

Everything said above is correct. And for what it's worth, you can usually modify the GPS unit's preferences to display coordinates in whichever format you prefer. And if you ever use Google Earth (as opposed to Google Maps), you can also go into Preferences >> 3D View and set your lat/lon preferences there as well. There are many online calculators that will convert degrees/minutes to decimal and back again in the interim.
posted by Hankins at 10:44 AM on January 10, 2007

It would be analogous to expressing e.g. three hours and thirty minutes as either "3:30" or "3.5 hours".
posted by Rhomboid at 10:48 AM on January 10, 2007

Although not directly relevant to your question (which others have answered well), another thing you may need to keep in mind is the 'datum' that's used. GPS generally uses the WGS84 datum, but NAD27 is commonly used in other applications. Even though the coordinates have the same format, they can differ in the actual location they represent by as much as 200 meters. This page has some more info.
posted by Emanuel at 12:06 PM on January 10, 2007