# [ncl-talk] mean value of all displayed grid points / pixel in a map

Kyle Griffin ksgriffin2 at wisc.edu
Thu Sep 4 08:22:18 MDT 2014

```Hi Frank,

There is no such magic step - plotting is commonly done by simple
contouring of the data, there is no array of 'pixels' to be found anywhere
as NCL will plot the data. To answer your earlier question, a point of data
cannot be "partially" plotted - it is either in the plot or it isn't. As
such, you might be able to determine lines that bound the plotted data and
use the vertices of these lines alongside the gc_inout function (
https://www.ncl.ucar.edu/Document/Functions/Built-in/gc_inout.shtml) to
find which points are within these boundaries. At that point, you could
calculate the average.

NCL is meant to work with the actual data, not a graphical interpretation
of the data. You must be able to calculate the locations of the boundaries
of the area you want to average over, as NCL can only do math over the data
you provide it. If you want to avoid calculating these boundaries, I
suggest either determining by trial and error (plot many lat/lon lines on
your map, estimate the vertices along the edge, use this to create a
bounding polygon) OR you can try creating your own boundaries, drawing them
on the map with a polyline function, and then simply stating what the
average is within this known area.

Hope this provides some insight and simplifies your thoughts some...

Kyle

----------------------------------------
Kyle S. Griffin
Department of Atmospheric and Oceanic Sciences
Room 1421
1225 W Dayton St, Madison, WI 53706
Email: ksgriffin2 at wisc.edu

On Thu, Sep 4, 2014 at 5:25 AM, Kreienkamp Frank <Frank.Kreienkamp at dwd.de>
wrote:

>  Yes the idea of removing all grid points that are not displayed might be
> an solution.
>
>
>
> But from my current view I have several problems. The two central are:
>
> a)      I define the plot parameter using:
>
>    res at mpProjection                  = "Stereographic"
>
>    res at mpCenterLatF                  = 50                 ; Centered over
> Germany
>
>    res at mpCenterLonF                  = 10                 ; Centered over
> Germany
>
>    res at mpLimitMode                   = "Corners"          ; choose range
> of map
>
>    res at mpLeftCornerLatF              =  55.8
>
>    res at mpLeftCornerLonF              =   2.6
>
>    res at mpRightCornerLatF             =  45.1
>
>    res at mpRightCornerLonF             =  19.5
>
> So two points of the rectangle are missing. If the res at mpCenterLonF is
> not central, calculating the two needed points is not easy.
>
> b)      What to do with points which are only partly drawn. I would like
> to have this part inside the average value. For fine grids with many
> points, missing few might not be an issue. With coarse grids this might
> produce a problem.
>
>
>
> My hope was, that there is point when I can catch a ‘transfer array’ in
> the moment when the grid is transferred to the (png-) workstation, after
> all the projection part is done. Like there would be an matrix of
> points(pixel) which are displayed.
>
>
>
>
> Frank
>
>
>
> *Von:* ncl-talk-bounces at ucar.edu [mailto:ncl-talk-bounces at ucar.edu] *Im
> Auftrag von *Maria Gehne - NOAA Affiliate
> *Gesendet:* Mittwoch, 3. September 2014 17:31
>
> *An:* Kreienkamp Frank
> *Cc:* ncl-talk
> *Betreff:* Re: [ncl-talk] mean value of all displayed grid points / pixel
> in a map
>
>
>
> Hi Frank,
>
>
>
> how are you deciding which area to plot? You must have something in your
> code that limits the area shown in the figure. Why don't you just use
> coordinate indexing to extract the region of interest from your data and
> set everything else to _FillValue. That way you can compute the area
> average and only the points that are not missing will be used for the
> average. Is that what you were looking for?
>
>
>
> Maria
>
>
>
> On Wed, Sep 3, 2014 at 12:07 AM, Kreienkamp Frank <Frank.Kreienkamp at dwd.de>
> wrote:
>
> Hello Dennis,
>
>
>
> if i read the webpage correctly the suggested functions are not what i am
> looking for.
>
>
>
> Yes, I am looking for an areal average.
>
>
>
> No, it is not for an rectangular grid.
>
>
>
> I have an rectangular grid as the ‘base’. Using this grid and a polar
> stereographic projection I am drawing a map. This map is showing  only a
> part of the hole ‘base’ grid. I am looking for an average of this
> shown/drawn part.
>
>
>
> Writing the same from a different view. I am looking for a function what
> calculates the average values of the presented map. See the example in my
> previous e-mail.
>
>
>
> As far a I understand the description correctly this in not what the
> function wgt_areaave_wrap provides.
>
>
>
>
> Frank
>
>
>
> *Von:* Dennis Shea [mailto:shea at ucar.edu]
> *Gesendet:* Montag, 1. September 2014 16:13
>
>
> *An:* Kreienkamp Frank
> *Cc:* ncl-talk
> *Betreff:* Re: [ncl-talk] mean value of all displayed grid points / pixel
> in a map
>
>
>
> As previously noted:
>
> For a rectilinear grid: wgt_areaave_Wrap calculates an area average
>
> https://www.ncl.ucar.edu/Document/Functions/Contributed/wgt_areaave_Wrap.shtml
>
> For a curvilinear grid:
> http://www.ncl.ucar.edu/Document/Functions/Built-in/wgt_areaave2.shtml
>
> or a simple arithmetic average: avg
>
> http://www.ncl.ucar.edu/Document/Functions/Built-in/avg.shtml
>
>
>
>
>
>
>
> On Mon, Sep 1, 2014 at 7:57 AM, Kreienkamp Frank <Frank.Kreienkamp at dwd.de>
> wrote:
>
> Hello Dennis,
>
>
>
> sorry that my question wasn’t clear enough.
>
>
>
> Let my try again.
>
> The attached map is a displayed result of RCM-dataset. The original grid
> covers Europe. The displayed area covers Germany and a bit more.
>
>
>
> What I am looking for is a routine that calculates the mean value of the
> displayed area.
>
>
>
>
> Frank
>
>
>
> *Von:* ncl-talk-bounces at ucar.edu [mailto:ncl-talk-bounces at ucar.edu] *Im
> Auftrag von *Dennis Shea
> *Gesendet:* Montag, 1. September 2014 15:11
> *An:* Kreienkamp Frank
> *Cc:* ncl-talk
> *Betreff:* Re: [ncl-talk] mean value of all displayed grid points / pixel
> in a map
>
>
>
> The question is vague
>
> "I want to calculate the mean value of all displayed grid points in a map.
>
> This means:
> - i have gridded data for the entire world (gcm-results)
> - i plot a map (stereographic projection) for Europe
>
> NCL draws a rectangle map based on the given resource parameters. I am
> looking for a function which counts all displayed values (pixel?) and
> calculates an average value."
>
> NCL handles "gridded data for the entire world (gcm-results)" all the time.
>
>
>
> [1]  dim_avg_n_Wrap will calculate the means at each grid point over all
> times.
>
> [2] wgt_areaave_Wrap calculates an area average
>
> [3] NCL can display much more than a "rectangular map"
>
> [4] wks = gsn_open_wks("png" ,"frank")
>
>      will send graphics to a png file
>
> What do you mean by:
>
> "Is there a chance that i can get the matrix-values which are send to the
> (png-)workstation?"
>
>
> =======
>
> https://www.ncl.ucar.edu/Document/Functions/Contributed/dim_avg_n_Wrap.shtml
> https://www.ncl.ucar.edu/Applications/climo.shtml
>
> https://www.ncl.ucar.edu/Document/Functions/Contributed/wgt_areaave_Wrap.shtml
> https://www.ncl.ucar.edu/Applications/maponly.shtml
>
> https://www.ncl.ucar.edu/Document/Graphics/Interfaces/gsn_csm_contour_map.shtml
>
> Good luck
>
>
>
> On Mon, Sep 1, 2014 at 2:31 AM, Kreienkamp Frank <Frank.Kreienkamp at dwd.de>
> wrote:
>
> Hello ,
>
> since having no one answering the original question.
>
> Is there a chance that i can get the matrix-values which are send to the
> (png-)workstation?
>
> Frank Kreienkamp
>
> -----Ursprüngliche Nachricht-----
> Von: ncl-talk-bounces at ucar.edu [mailto:ncl-talk-bounces at ucar.edu] Im
> Auftrag von Frank Kreienkamp
> Gesendet: Mittwoch, 27. August 2014 06:37
> An: ncl-talk
> Betreff: [ncl-talk] mean value of all displayed grid points / pixel in a
> map
>
>
> Hello,
>
> i have searched the function list but i have not found a solution for the
> following problem.
>
> I want to calculate the mean value of all displayed grid points in a map.
>
> This means:
> - i have gridded data for the entire world (gcm-results)
> - i plot a map (stereographic projection) for Europe
>
> NCL draws a rectangle map based on the given resource parameters. I am
> looking for a function which counts all displayed values (pixel?) and
> calculates an average value.
>
> Frank
> _______________________________________________
> ncl-talk mailing list
> List instructions, subscriber options, unsubscribe:
> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
> _______________________________________________
> ncl-talk mailing list
> List instructions, subscriber options, unsubscribe:
> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>
>
>
>
> _______________________________________________
> ncl-talk mailing list
> List instructions, subscriber options, unsubscribe:
> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>
>
>
>
> _______________________________________________
> ncl-talk mailing list
> List instructions, subscriber options, unsubscribe:
> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>
>
>
> _______________________________________________
> ncl-talk mailing list
> List instructions, subscriber options, unsubscribe:
> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.ucar.edu/pipermail/ncl-talk/attachments/20140904/24eb5748/attachment.html
```