gredddd was talking about flying at 500m which is 1,640ft (the software limit of the Inspire's barometer differential).
Please note that msl has nothing to do with the height you are legally allowed to fly at. The barometer on the Inspire zeros at power on and only measures relative pressure difference (to 10cm) upon changing altitude.
In the UK my ops manual operational ceiling I can legally fly at is 4,809ft (1,465m). This is taken as the highest point in the country (Ben Nevis) at 4,409ft plus the permissible height above ground level for UAV operation which is 400ft. Of course this is only achievable flying from the summit.
The important point is height of operation is ALWAYS measured from ground level so gregdd scenario would be taking off at the base of a mountain and then following the terrain at say a height of 30m above the ground, always staying well within 400ft of ground level but easily reaching 1000's feet in altitude but never breaking the 400ft legal limit.
Of course going outside unaided VLOS of 500m could be another matter.