Suunto Spartan Ultra uses barometric pressure to measure altitude. To get accurate readings, you need to define an altitude reference point. This can be your current elevation if you know the exact value. Alternatively, you can use FusedAlti (see FusedAlti) to set your reference point automatically.
Set your reference point from the settings under Outdoor.
By default, altitude is measured with FusedAlti during exercises that use GPS and during navigation. When GPS is switched off, altitude is measured with the barometric sensor.