Latitude and longitude 2

In the first discussion of latitude and longitude, we investigated how the latitude-longitude grid was established. In this post we will look at how that relates to the display on your handheld GPS unit.

If I stand outside with my GPS and direct the unit to display my position, it may do so in a couple of different ways. First, it might show my position on a map background, pinpointing my location with regard to other features such as streets, buildings, or landforms. This shows me my location as if I were walking around on the map.

I might want to know my latitude and longitude coordinates. Perhaps I want to record them so I can return to this spot in the future. (You can understand why a “bone digger” would want that!) In most handheld GPS units you can save your location as a waypoint. (See my review of the best GPS units.)

A really useful thing to be aware of about has to do with the format of latitude and longitude number displays. These numbers can be expressed in several ways, and you must know which format the numbers are in or risk making errors. After GPS units first came out, I used one to locate some fossil sites where I collected. I happily recorded the coordinates for the fossils I picked up, and dutifully wrote them in my field book. Only later did I realize that I was not perfectly clear which form the coordinates were in, making the records almost useless!

Most handheld GPS units allow you to display the coordinates in several formats. You could show the form degrees-minutes-seconds, sometimes denoted as DMS. This might look something like 38°53’22.49″N, 99°17’58.73″W. Latitude is given as degrees north or south from the equator, and longitude is given as degrees east or west from the prime meridian.

But we could also give these same numbers in another format which would look like: 38° 53.375’N, 99° 17.979’W. This format is in the form of degrees, minutes, and decimal minutes, or DMM.

Finally, we could show these coordinates as full decimal degrees (DDD) and it would look like: 38.889583°, -99.299650°. Note that in this form, the positive or negative form of the number is important as that gives the direction. Positive latitude numbers are north, and negative longitude numbers are west.

Notice that these forms are all equal: 1° 30′ 30″ (DMS); 1° 30.5′ (DMM); 1.5083° (DDD). And you can see if you just wrote down numbers on a page, and were not very clear about which form the numbers were in, you could be very far off the mark in terms of location.

There is another common coordinate form called UTM which we will examine in another post. Bonus points to anyone who can tell me what is at the coordinates used in this post.

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