Fieldwork with GPS receivers
Use a map or atlas to determine the approximate coordinates, as latitude and longitude, of your school. Demonstrate how to operate a handheld GPS receiver and read the display. Discuss why indoor readings may be not possible, or inaccurate (radio waves may be disrupted by buildings).
On a playing field make sure the GPS unit can "see" the sky, away from buildings and trees. Discuss where in the sky satellites might be (in the UK, mostly to the South -or southeast/southwest - of the observer. At less than 55 degrees North - Newcastle - orbits may reach points overhead and to the North of overhead. Above this latitude, GPS satellites will always be in areas of sky South of overhead, or below the horizon).
Ask different pairs/groups of students to measure the coordinates of the same fixed point, and compare results. Discuss sources of error. Get students to "geocache" small markers on, for example, a playing field for other students to find, using the coordinates supplied by the hiders.
Ask groups/pairs to log the coordinates of two fixed points (waypoints) on either side of the field. Have them time precisely how long it takes them to walk (or jog!) between the points. Calculate the distance between the points. If required, use a latitude/longitude distance calculator such as the one at:
http://www.wcrl.ars.usda.gov/cec/java/lat-long.htm
Use the time and distance measurements to calculate the speed of travel between the points. This is the principle used by vehicle tracking to determine the vehicle's speed.
The JavaScript calculator mentioned above uses a formula assuming the Earth is spherical. This introduces a small error, because the Earth is actually ellipsoid. Its shape is like a sphere squashed from top to bottom, so it bulges in the middle. GPS mapping is based on the actual shape of the Earth. This is known as geodetic mapping.