elevation and depth units

[crotwell]

Text should say that elevation_above_sea_level and depth_below_surface are in meters.

Those names seem a bit wordy, but...

[crotwell]

Also strike and dip should say degrees and text should describe what they are referenced to. See stationxml docs for azimuth and dip:

Azimuth of the component in degrees clockwise from geographic (true) north.

Dip of the component in degrees, positive is down from horizontal. For horizontal dip=0, for vertical upwards dip=-90 and for vertical downwards dip=+90.

Also very helpful if range is given so that all users either use -180 to 180 or 0 to 360.

Elevation can be confusing, so helpful to say that elevation is the elevation of the cable, not the ground surface.

[vhlai-seis]

Agree.

Along this topic, there are some confusion on the definition, which I will attempt to clarify below:

In current DAS metadata:
elevation_above_sea_level: Elevation or height above sea level.
depth_below_surface: Depth below surface (downwards positive).

In StationXML:
Elevation: Elevation of local ground surface level at station, in meters.
Depth: The depth of the sensor relative to the local ground surface level, in meters.

I could see why StationXML specify local ground surface as there are many borehole sensors, and this arrangement works particularly well for point sensors. I will now apply our definitions to different common scenarios:

Scenario #1: horizontal fibre in a conduit, which is buried at some depth.
Elevation_above_sea_level = Elevation of local ground surface (typically obtained from DEM/GPS; positive value unless at Salton/Dead Sea…)
Depth_below_surface = conduit burial depth (positive value)

Scenario #2: offshore cable, laid directly on seafloor
Elevation_above_sea_level = seafloor bathymetry (*negative value)
Depth_below_surface = zero.

Scenario #3: vertical borehole.
Elevation_above_sea_level = elevation of local ground surface (* will be the same for every channel)
Depth_below_surface = increasing positive values of depth…

I think it will be prudent if we update the definitions to avoid any ambiguities.
elevation_above_sea_level: The elevation or height above sea level of the local ground surface, where the fibre is located. In meters.
depth_below_surface: The depth or vertical distance from the local ground surface to the fibre, measured downward as positive. In meters.