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[netCDFJava #JHY-783257]: Term Definitions

Hi,

To follow up, these radar data are stored in a polar/spherical coordinate 
system:

- Gate represents discrete locations along range, moving away from the radar 
location
- Radial represents all gates as a particular pointing directions. The radial 
increases along the direction that the radar is scanning; for these data, the 
radar is scanning horizontally (called the azimuthal direction)
- Scan represents a collection of one or more radials, all of which have the 
same pointing direction in either the vertical (called elevation) or horizontal 
(called azimuth). For these data, each scan represents a single nominal 
elevation angle.

Hope this helps,

Ryan

> Thank you so much! This is wonderful!
> 
> address@hidden> wrote:
> 
> > Greetings Allie!
> >
> > These terms are dimensions:
> >
> > > netcdf /home/sansari/stage4/java/KPAH20080330_131826_V03 {
> > > dimensions:
> > > scanR_HI = 4;
> > > gateR_HI = 1832;
> > > radialR_HI = 720;
> > > scanR = 12;
> > > gateR = 458;
> > > radialR = 360;
> > > scanV_HI = 2;
> > > gateV_HI = 1192;
> > > radialV_HI = 720;
> > > scanV = 12;
> > > gateV = 912;
> > > radialV = 360;
> >
> > which tell you about the "shape" of the data. Dimensions alone don't tell
> > you much about the actual data in the file. You really need to look at the
> > variables to see what those dimensions might mean. In this case, you have a
> > variable called
> >
> > > byte Reflectivity_HI(scanR_HI=4, radialR_HI=720, gateR_HI=1832);
> > > :units = "dBZ";
> > > :long_name = "Reflectivity_HI";
> > > :missing_value = 1b, 0b; // byte
> > > :signal_below_threshold = 0b; // byte
> > > :scale_factor = 0.5f; // float
> > > :add_offset = -33.0f; // float
> >
> > Here we can see from the variable name and the attribute "long_name" that
> > we are dealing with Reflectivity. The "_HI" suffix tends to mean this is a
> > "super resolution" product (more data samples available). Based on the name
> > and knowing a little bit about radar data, I *think* what we have here,
> > dimension-wise, is:
> >
> > scanR_HI: tilt level (or scan) of the radar
> > radialR_HI: which way the radar is pointing (sometimes called sweep)
> > gateR_HI: distance from the radar
> >
> > When accessing the array (details depend on what library you are using),
> > you may do something like:
> >
> > Reflectivity_HI[0, 0:5, 100:200]
> >
> > that would mean you are getting data from a single tilt of the radar,
> > using the first 5 angular samples, with 100 samples along the beam of the
> > radar. The indexes 0, 0:5, 100:200 tell you nothing about the actual tilt
> > angle, compass direction, or physical distance from the radar. It would be
> > good to know if there are variables with those same names in the file, as
> > they would tell you the actual values of what, say, the 1st element of the
> > scanR_HI means (probably 0.5 degrees, but not guaranteed).
> >
> >
> > The data are stored as type byte - this is typically called "packed" data
> > (packed because less space required to store a byte than a float, for
> > example). What you really want is the data as a floating point value
> > representing the data in dBz. The "units" attribute tell you that, when you
> > apply the scale and offset, you will have values with units in dBz. For
> > information on how to apply the scale_factor and add_offset values, see the
> > "Packed Data Values" section here:
> >
> > https://www.unidata.ucar.edu/software/netcdf/docs/BestPractices.html
> >
> > Note that many popular libraries will do this automatically when you read
> > the data.
> >
> >
> > >
> > >
> > > I'd also like to inquire about the "add offset" value in the following
> > > part of the script.
> > >
> > > variables:
> > > byte Reflectivity_HI(scanR_HI=4, radialR_HI=720, gateR_HI=1832);
> > > :units = "dBZ";
> > > :long_name = "Reflectivity_HI";
> > > :missing_value = 1b, 0b; // byte
> > > :signal_below_threshold = 0b; // byte
> > > :scale_factor = 0.5f; // float
> > > :add_offset = -33.0f; // float
> > >
> > > I'm assuming the offset is related to the dbZ scale but wanted to be
> > sure.
> > >
> > > Basically, i'm trying to create radar imagery that will only display dbZ
> > > values between 50 and 75.  I'd also like to display velocity data >55kts.
> > > How might I do that?
> > >
> > > Please forgive my ignorance in the computer programming aspect.  I
> > > appreciate any information you may give me.
> > >
> > > Thank you!
> > > --
> > >
> > > [image: WeatherCheck] <https://weathercheck.co/>
> > > *Allie Garrett*
> > > Meteorologist - Severe Weather Specialist
> > > M: 270-836-9060
> > > E: address@hidden | W: WeatherCheck.co <https://weathercheck.co/>
> > >
> > >
> >
> >
> > Ticket Details
> > ===================
> > Ticket ID: JHY-783257
> > Department: Support netCDF Java
> > Priority: Normal
> > Status: Open
> > ===================
> > NOTE: All email exchanges with Unidata User Support are recorded in the
> > Unidata inquiry tracking system and then made publicly available through
> > the web.  If you do not want to have your interactions made available in
> > this way, you must let us know in each email you send to us.
> >
> >
> >
> 
> --
> 
> [image: WeatherCheck] <https://weathercheck.co/>
> *Allie Garrett*
> Meteorologist - Severe Weather Specialist
> M: 270-836-9060
> E: address@hidden | W: WeatherCheck.co <https://weathercheck.co/>
> 
> 

Ticket Details
===================
Ticket ID: JHY-783257
Department: Support netCDF Java
Priority: Normal
Status: Closed
===================
NOTE: All email exchanges with Unidata User Support are recorded in the Unidata 
inquiry tracking system and then made publicly available through the web.  If 
you do not want to have your interactions made available in this way, you must 
let us know in each email you send to us.