Something based on sonar. If you had a speaker on the flag pole which emitted a short burst of audible or ultra-sonic sound, and then had a microphone where you were standing, ready to pick up the noise. Then all you need to do is measure how much time there is between the speaker making the sound and the microphone picking it up, multiply that by the speed of sound and you have your distance.
I assume you want this to be wireless? If so, it gets a bit more complicated because you have to factor in the time it takes for your receiver to send a signal to the flag pole telling it to make a sound, but there are certainly ways of doing it. And it's likely to be cheaper than GPS.