GPS detecting stationary?

can someone please point me at some code snippets to detect when the GPS is stationary. I am currently using tinyGPS library and it is working fine, but my data-logger has thousands of stationary details I’d like to not bother logging. Currently I am using this,

uint8_t speedZero() {
uint8_t spd = gps.speed.mph();
  if (spd < 5) {
    return 0;
  } else {
    return spd;
  }
}

But it is too granular at slow speeds, but the NEO-6M drifts a lot and 5mph standing still is not uncommon.

I have tried comparing Last Lat/Lon but not having any success as this Math is waaaay out of my skill-zone.