Cartesian coordinate systems:
- The x coordinate is longitude or the distance east or west from the prime meridian
- The y coordinate is latitude or the angular distance north or south from the equator
- There are 360 degrees in a circle. There are 60 minutes in a degree, and there are 60 seconds in a minute. So there are 3600 seconds in a degree.
The GIS software reads and understands coordinates in decimal degrees. If you will be collecting latitude/longitude data in decimal minutes or degrees-minutes-seconds (dms), you will need to convert the coordinates to decimal degrees in order to map data in a GIS. See Creating Tables to Import into GIS for instructions on using MS Excel to convert your data.
To convert decimal minutes to decimal degrees, divide the decimal minutes by 60 and add the number of degrees. For example, if your GPS gives you longitude in decimal minutes: 70o 33.452’W, divide 33.452 minutes by 60 to get 0.55753 degrees (this should seem just about right to you since 33 minutes is a little more than half a degree). Then add 70 degrees to get 70.55753oW. Once you know how to do this by hand, you can easily have Excel or another spreadsheet program perform the calculations with your data.
When we import spatial data into a GIS, the program cannot understand coordinates that are given in both numbers and letters. Instead of using letters to determine the direction from the prime meridian or the equator, we use positive or negative numbers. ArcMap reads west longitude as negative and east longitude and positive. ArcMap reads north latitude as positive, and south latitude as negative. So in the example given above, our longitude would be –70.55753. back to top
GPS to GIS: Entering & Converting Coordinates