Does Health app use GPS to measure distance?

My friend and I both have iPhones, today we went for a walk together.

It was the first time I’d picked my phone up today, she'd walked to her car then into my house, so less than 100 steps recorded.

At the end of our walk, my health app said we'd done exactly 3km, hers said just over 4km.

I know it uses your height to calculate your step length, and my friend is nearly 20cm taller than me, so I'm assuming it’s using step length & number of steps to calculate distance.

Obviously this isn’t accurate though, so shouldn’t it be using gps to measure the distance?

I used the ruler in Google maps to track the distance and came up with approx 3.9km, so my phone is underestimating my distance walked by nearly 25%, which makes it pretty useless.

Is there a easy way to measure distance using gps on an iPhone?