Diagram depicting how a field site of dimensions 100 m x 75 m can be turned into a coordinate system for selecting randomly chosen points. The trick here is to generate your random numbers so that all x-coordinates are between zero and one hundred, and all y-coordinates are between zero and seventy five. See the tips at the bottom of this page
for working with the random number generator in Excel if you would like to generate random numbers within a certain range. If using this method, it's wise to generate plenty of coordinate pairs while indoors and either add them to your field data sheet or print them out in a table before heading out to the field.