GPS Science Report - Elevation Number Changes

Dr. William Whit Smith, Jr., GPS Scientist

Question: Why do the numbers for elevation on the GPS receiver change more than those for latitude and longitude?

Some students have noticed that their elevation values change more than their latitude and longitude values. Why is this?

The elevation measurement is being displayed to the nearest meter while the latitude and longitude are displayed to the nearest 0.01 arc minutes. If you work through the arithmetic (See the GPS Learning Activity "Offset GPS Measurements"), you find that there are about 18.5 meters in each 0.01 arc minutes of latitude. So, a change in your elevation reading of about 18.5 meters is the same as a change in the last digit of the displayed latitude. Therefore, your GPS receiver indicates smaller changes in elevation (in meters) than in latitude (in arc minutes). As changes occur, the elevation value appears to change more often and more dramatically than latitude presented to the nearest 0.01 arc minutes.

The values you record from your GPS receiver are quite good. We average them to generate a more accurate determination of location. (See the GPS Learning Activity "What Is the Right Answer" and the GPS Protocol in the GLOBE II Teacher's Guide.)

I would be interested in hearing from you. Send e-mail to: globe@ee.gatech.edu