[ncl-talk] Masking points based on gsn_csm_contour_map limits

[Mary Haley]

Hi Dan,

Sorry for the long delay. Most of the NCL team was out last week for a
workshop.

I decided to create an example based on your question, since it's an
interesting one that we don't have an example of yet. Hopefully I
understood your question correctly!

See example mask_18.ncl at:

http://www.ncl.ucar.edu/Applications/mask.shtml#ex18

I started off on the same path you did, by creating a lambert conformal
plot. I then used this plot to retrieve the viewport coordinates of the
plot axes.  The viewport coordinates give me the four corners of the plot
in NDC space, which I then used "ndctodata" on to calculate the lat/lon
locations.

It's not enough to just provide the four corners of the plot, however,
because the lambert conformal boundary is curved. So, for each of the four
axes, I created an array of equally spaced viewport coordinates between the
start and end of each axis, so this would give me more values to
interpolate the lat/lon boundary.  See the "get_latlon_bounding_polygon"
function.

Once I had the lat/lon boundary, I used "gc_inout" to create a mask array.

This example uses a data array that is on a rectilinear grid, and which has
lat/lon coordinate arrays called "lat" and "lon". If your data is not
rectilinear or you don't have coordinate arrays, then you'll need to modify
the create_mask_array function to handle the type of lat/lon grid your data
is on.

Let me know if you have any questions.

--Mary


On Tue, Feb 6, 2018 at 12:49 PM, Daniel Adriaansen - NCAR <dadriaan at ucar.edu
> wrote:

> Hello,
>
> I am plotting a Lambert Conformal map using res at mpLimitMode set to
> "Corners". The map limits are configured to zoom in on a subset of the full
> domain of the gridded dataset in the x-y dimensions (lat-lon). I am then
> calling gsn_csm_contour_map to plot the data.
>
> What I would like to do, is use the "Corners" resources, to create a
> polygon so that I can perform statistics on the data only within the view
> shown in the map that is drawn by gsn_csm_contour_map. The problem is, I
> have not figured out how gsn_csm_contour_map determines what the bounds are
> for the domain drawn when using the "Corners" resources.
>
> My first attempt was to create a polygon from the "Corners" resources
> (res at mpLeftCornerLonF, res at mpLeftCornerLatF, res at mpRightCornerLonF, and
> res at mpRightCornerLatF) and then use gc_inout to identify which points
> from the gridded dataset are within the map limits. When I plot the
> resultant boolean mask however, the points "inside" the polygon do not
> directly match the view shown in the bounding box from gsn_csm_contour_map
> (see attached figure, Mask1.png).
>
> The end goal is to make sure the statistics being computed are created
> from exactly those grid cells shown within the bounding region created by
> gsn_csm_contour_map.
>
> Any suggestions or ideas are appreciated. Thanks!
>
> -Dan
>
>
>
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