What could be the reason for no counting runs without GPS?

[Saqib_ins]

Fitbit instructs logging GPS-tracked runs to get precise/more accurate cardio scores (vo2 max).

But I don't see why these should be GPS-tracked runs.

The tracker still gets the heartbeat, pace, and distance data without GPS.

It's not that GPS tracking enables other important data such as terrain characteristics (whether running area is flat/uphill, etc). We know this because Fitbit recommends running on a flat terrain, meaning, evidently, that it cannot process this extra variable (which is fine and not something that affects me).

So, what really could be the reasons for not counting runs without GPS.

Please note, to avoid potential miscommunication, this post is not about improving vo2 max (cardio score/performance/health). It is about giving data input to make cardio score more accurate. I also realise that runs get logged into the software even if not GPS-tracked. I'm talking about these runs counting toward calculating vo2 max (cardio score) more precisely. According to Fitbit, to make the score more accurate you will have to do "several" runs that are at least 10 minutes in length, and GPS-tracked.

The reason why this even matters to me is that I just dragged myself through one of these at-least-10-min runs that I'm doing to make the vo2 max calculation more accurate. And after 12 min of beat down my unfit body took, I find out that gps wasn't activated for whatever unknown reasons. So, it will not count toward cardio score update, the whole reason I'm doing the runs.

Yesterday, it worked just fine. I went to the 'exercise' menu, tapped 'run', started running, and it was gps-tracked. Today, it wasn't. I will make sure next time that I don't start before I see a GPS notification. But that's a different matter. Right now, I'm wondering why Fitbit needs gps tracking at all to to use the running data toward vo2 max calculations.

[LZeeW]

@Saqib_ins   Fitbit's algorithm needs a start/stop times and the distance for its calculation.  It's the way it's programmed.  There are other ways to estimate VO2max.  For example, there is a walk test, but you must walk one mile and use a stopwatch for your start/stop times.

You should run on a flat terrain for this reason.  You want to have as low a heart rate as possible while running.  Fitbit's own explanation -- "individuals with higher VO2max have a lower heart rate while running at the same pace compared to individuals with lower VO2max."  Your heart rate will increase when you need to run up a hill.