Determine the longitude of two places. You can use a GPS or a longitude map. An example of a longitude map can be found online at World Atlas. A GPS system will typically display the latitude and longitude of your current location on the screen along with a map.
Write down the longitude for both places on a piece of paper.
Calculate the longitude between both places. Use the two longitude figures from the two locations and a longitude map. Any degree, minute and second between both points is considered to be a valid longitude. An example of this would be if one point was at 5 degrees and the other point was at 10 degrees, 6 degrees would be a longitude between the two points.
Average the two points for a midpoint. Convert the degrees, minutes and seconds into seconds by multiplying each minute by 60 and each degree by 1,800. Add all the seconds together and divide by two. Convert the two figures back to degrees, minutes and seconds by first dividing the seconds by 1,800 to calculate degrees and the remaining seconds by 60 to calculate minutes. A remainder of less than 60 seconds will exist after the calculation if the number is even.
