Clarification: Depth and Elevation terms

[dschristianson]

• Documentation reference (URL): https://github.com/ess-dive-community/essdive-hydrologic-monitoring/blob/main/HydroRF_Term_Guide.md#depth
• Submitter: Danielle Christianson (LBNL)
• Justification (why is this change necessary?): It takes some effort to figure out how the terms "Depth" and "Elevation" might work together and relate to the recommended vocabulary "Sensor_Depth" and "Sensor_Elevation". I'm assuming the following.
  • Per the definition of the term Elevation, it is the elevation of the sensor, not the elevation of a physical feature (e.g., ground surface).
  • Depth and/or Elevation should be used for permanently installed sensors that do not vary during the observation period, unless moved by the researcher or unforeseen events.
  • Sensor_Depth and Sensor_Elevation should be used when sensor vertical position is an observation, i.e., they are expected to vary during the observation period.

I suggest the following changes:

• Give an example of reporting Depth and Elevation together. For example: assuming I understand the RF correct: if Depth = 1, Depth_Reference = meters below ground surface, Elevation = 1000, Elevation_Reference = meters above mean sea level (NAVD88), then the elevation of the ground surface = 1001 meters above mean sea level. A simple figure would be helpful to quickly communicate these relationships. There may be other examples relevant to hydrological observations that are also common / better suited.
• Add guidance on when to use Depth v. Sensor_Depth and Elevation v. Sensor_Elevation.