Vision Speed Estimation

Mavlink

VISION_SPEED_ESTIMATE (103) Speed estimate from a vision source.

Here is your data converted into a Markdown table:

Field Name	Type	Units	Description
usec	uint64_t	us	Timestamp (UNIX time or time since system boot)
x	float	m/s	Global X speed
y	float	m/s	Global Y speed
z	float	m/s	Global Z speed
covariance ++	float[9]		Row-major representation of 3x3 linear velocity covariance matrix (states: vx, vy, vz; 1st three entries - 1st row, etc.). If unknown, assign NaN value to first element in the array.
reset_counter++	uint8_t		Estimate reset counter. This should be incremented when the estimate resets in any of the dimensions (position, velocity, attitude, angular speed). Used when e.g. SLAM detects a loop-closure and the estimate jumps.

Explain Global

"global" often refers to a frame that is fixed relative to the Earth, as opposed to being fixed to the vehicle's body. However, it's not truly global in the sense of latitude and longitude (which would be a WGS84 ECEF or similar frame). Instead, it refers to a local tangent plane approximation of the Earth.

Why not use "Local X/Y/Z speed"
The reason they might use "Global" instead of "Local" is to distinguish it from a "body frame" velocity. A body frame velocity would be relative to the vehicle's own orientation

Global (WGS84) vs. Global (Local NED)

Mavros

vision_speed plugin