The phrase "This angle is measured starting from the horizontal plane directly in front of sensor.? is problematic. A plane is typically understood to be infinite in all directions, so the horizontal plane is not ?in front of the sensor?, but coincident with the sensor. (And, as noted elsewhere, perpendicular to the gravity-based zenith. I agree with the flow of discussion regarding zenith, and think ?perpendicular to the zenith direction? would be commonly understood and would work so long as zenith is in fact defined elsewhere. Else,' perpendicular to Earth gravity' is OK.)
Relevant note: I believe in a number of sensitive applications zenith is necessary calculated as perpendicular to center of earth mass, which is different than local gravity because of inconsistently dense materials near the earth?s surface. So whether you want to define *the* zenith for all CF uses, or *a* zenith for the current CF uses, may be worth discussing. Doing the former would be hard to undo later.
John
---------------------------------------
John Graybeal
jbgraybeal at mindspring.com
650-450-1853
skype: graybealski
linkedin:
http://www.linkedin.com/in/johngraybeal/
> On Mar 20, 2019, at 02:10, Martin Juckes - UKRI STFC <martin.juckes at stfc.ac.uk> wrote:
>
> Hi Ken,
>
>
> We don't need to have "zenith" in the table, but we do want to have a consistent meaning. There is a table (probably incomplete) of terms used in standard names here: http://cfconventions.org/Data/cf-standard-names/docs/guidelines.html#id2798851, it is probably worth adding "zenith" to this table.
>
>
> It looks to me as though the definition in sensor_zenith_angle is a simple mistake. The term was added in 2013, and the initial proposal defines it as relative to the local vertical (http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2013/005883.html). The subsequent email discussion makes it clear that it is intended to be consistent with platform_zenith_angle. Alison may be able to say more about this (I haven't reviewed the whole discussion thread).
>
>
> To be on the safe side, can we define horizontal as "perpendicular to Earth's gravity", to eliminate the time varying lunar component which may not matter to your measurements, but is bound to worry somebody sooner or later.
>
>
> regards,
>
> Martin
>
>
>
>
>
>
>
> ________________________________
> From: Kehoe, Kenneth E. <kkehoe at ou.edu>
> Sent: 19 March 2019 18:48
> To: Juckes, Martin (STFC,RAL,RALSP); CF Metadata List
> Subject: Re: Adding sensor_elevation_angle to standard_name
>
> Hi Martin,
>
> Yeah I went round and round with this trying to reuse the current definitions, but they were a little confusing to me. Using the statement "Local zenith is a line perpendicular to the Earth's surface at a given location." in sensor_zenith_angle (and platform_zenith_angle) does not work for me because I want zenith to be defined by direction of gravity not the Earth's surface. The Earth's surface is often tilted. I think that needs to be updated according to my understanding of the term zenith. I assume there could be a use for angle between local surface of Earth, but I think that would be a different term than zenith. I think zenith_angle is currently OK. Is it worth defining zenith in the table for referencing by other standard_name's? I don't really know if that is how the standard_name table is designed to work.
>
> I want the reference horizontal plane to be perpendicular to gravity, not the other definition where horizon is where the Earth surface and sky meet<https://www.merriam-webster.com/dictionary/horizon>. Is it worth creating a horizontal_plane standard name? I guess we could add something to allow a user to add "A comment attribute should be added to a data variable with this standard name to specify the reference direction." like sensor_azimuth_angle if they wanted to change it? But to be honest I don't understand this instruction in the sensor_azimuth_angle description statement and would probably never use it.
>
> You may also need to update sensor_view_angle and sensor_zenith_angle from "line of sight from the sensor" to "line of sight of the sensor", but I don't want this proposal to get bogged down with changing the other standard name definitions. It may just be my interpretation and the existing definitions of sensor_zenith_angle and sensor_view_angle are correct.
>
>
> Here is an updated definition:
>
> proposed definition = sensor_elevation_angle is the angle measured in the vertical plane between the line of sight of the sensor and a horizontal plane perpendicular to local zenith. This angle is measured starting at zero from the horizontal plane directly in front of sensor. The angle is positive above the horizontal plane, and negative below the horizontal plane. Local zenith is directly overhead in the direction of gravity. A standard name also exists for sensor_view_angle and sensor_zenith_angle.
>
> Ken
>
>
>
> On 2019-3-19 03:56, Martin Juckes - UKRI STFC wrote:
>
> Hello Ken,
>
>
> this looks like a straight forward extension of existing names, but I'm puzzled by the definition of "zenith" in sensor_zenith_angle, where it is defined as "Local zenith is a line perpendicular to the Earth's surface at a given location." For the standard name "zenith_angle" the definition is different: "Zenith angle is the angle to the local vertical". The latter, with vertical defined by the direction of gravity, appears to be the standard usage: would you agree with this?
>
>
> Is the reference to the Earth's surface in the definition of sensor_zenith_angle a mistake, which should be modified to bring it into line with zenith_angle, or is there a real need to have angles measured relative to the local plane of the Earth's surface?
>
>
> For the sensor_elevation_angle, do you want a specific interpretation of "horizontal" (e.g. perpendicular to the direction of gravity), or is it acceptable to use this term with other interpretations of the reference horizontal plane?
>
>
> regards,
>
> Martin
>
>
> ________________________________
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu><mailto:cf-metadata-bounces at cgd.ucar.edu> on behalf of Kehoe, Kenneth E. <kkehoe at ou.edu><mailto:kkehoe at ou.edu>
> Sent: 19 March 2019 00:27
> To: CF Metadata List
> Subject: [CF-metadata] Adding sensor_elevation_angle to standard_name
>
> I would like to add sensor_elevation_angle to complement the existing sensor_azimuth_angle in the standard_name table.
>
> proposed standard_name = sensor_elevation_angle
>
> proposed canonical units = degree
>
> proposed definition = sensor_elevation_angle is the angle measured in the vertical plane between the line of sight of the sensor and a horizontal plane perpendicular to local zenith. This angle is measured starting from the horizontal plane directly in front of sensor. The angle is positive above the horizontal plane, and negative below the horizontal plane. Local zenith is directly overhead. A standard name also exists for sensor_view_angle and sensor_zenith_angle.
>
> Thanks,
>
> Ken
>
>
> --
> Kenneth E. Kehoe
> Research Associate - University of Oklahoma
> Cooperative Institute for Mesoscale Meteorological Studies
> ARM Climate Research Facility - Data Quality Office
> e-mail: kkehoe at ou.edu<mailto:kkehoe at ou.edu><mailto:kkehoe at ou.edu><mailto:kkehoe at ou.edu> | Office: 303-497-4754
>
>
>
> --
> Kenneth E. Kehoe
> Research Associate - University of Oklahoma
> Cooperative Institute for Mesoscale Meteorological Studies
> ARM Climate Research Facility - Data Quality Office
> e-mail: kkehoe at ou.edu<mailto:kkehoe at ou.edu> | Office: 303-497-4754
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Received on Wed Oct 02 2019 - 21:32:48 BST
