[ncl-talk] No coordinates to coordinate

[Gus Correa]

Hi

This is the thread mentioned by Dave Allured:
https://mailman.ucar.edu/pipermail/ncl-talk/2021-February/017831.html

You probably have variables lat(x_t,y_t), and lon(x_t,y_t).
Hopefully also the inverse functions x_t(lat,lon) and y_t(lat,lon),
which would allow you to plot the parallels and meridians overlaid to the
map.
These variables may be in this data file or
in a separate file with grid information.

Ideally one of these files also has information on the map projection used
to generate x_t and y_t.
It looks like a Transverse Mercator projection, centered in a meridian
along the middle of
the Atlantic ocean (30W or 35W), but it may be a different  specialized
projection.
https://en.wikipedia.org/wiki/Transverse_Mercator_projection

I would guess this projection (the one you used in your plot)
is probably the one in which the whole map looks nicer,
especially if you overlay the parallels and meridians as suggested above,
because this will show geographic location more clearly.

Since this data includes the North Pole, which is a singularity in the
standard cylindrical Mercator projection,
you won't be able to plot the Arctic using standard Mercator projection,
everything will be more and more distorted near the North pole.
https://en.wikipedia.org/wiki/Mercator_projection

You could use the stereographic projection centered on the North Pole (is
that what you mean by "polar map"?),
with meridians appearing as radial straight lines, and parallels as
concentric circles,
but that will also make the equatorial region and the Southern Hemisphere
appear distorted,
although it may be OK if you want to plot only the Arctic region.
https://en.wikipedia.org/wiki/Stereographic_projection_in_cartography

I hope this helps,
Gus Correa

On Sat, Mar 13, 2021 at 10:46 AM Dave Allured - NOAA Affiliate via ncl-talk
<ncl-talk at mailman.ucar.edu> wrote:

> Check your original source data file for an extra set of coordinates, lat
> and lon, or similar names.  x_t and y_t are not lat and lon, they are
> projection coordinates, probably in meters.  If lat and lon are not in the
> original file, you might also be able to request them from the data source.
>
> See recent thread about OSGB grid for method of using the supplemental
> coordinates.
>
>
> On Sat, Mar 13, 2021 at 1:38 AM Chathurika via ncl-talk <
> ncl-talk at mailman.ucar.edu> wrote:
>
>> Hello,
>>
>>
>> Thank you very much for reading my email.
>>
>>
>> I have downscaling data in Atlantic and Arctic Oceans. But It has no
>> attached coordinates. I want to plot these data on a polar map with
>> coordinates.
>>
>>
>> This is the variable summary;
>>
>>
>>  Copyright (C) 1995-2018 - All Rights Reserved
>>
>>  University Corporation for Atmospheric Research
>>
>>  NCAR Command Language Version 6.5.0
>>
>>  The use of this software is governed by a License Agreement.
>>
>>  See http://www.ncl.ucar.edu/ for more details.
>>
>> Variable: zos1
>>
>> Type: float
>>
>> Total Size: 2548039680 bytes
>>
>>             637009920 values
>>
>> Number of Dimensions: 3
>>
>> Dimensions and sizes: [Time | 240] x [y_t | 1728] x [x_t | 1536]
>>
>> Coordinates:
>>
>>             Time: [7.260877e+09..7.8894e+09]
>>
>>             y_t: [0.5..1727.5]
>>
>>             x_t: [0.5..1535.5]
>>
>> Number Of Attributes: 5
>>
>>   grid : TS
>>
>>   missing_value : -1e+34
>>
>>   units : m
>>
>>   long_name : Sea Surface Height
>>
>>   _FillValue : -1e+34
>>
>> Variable: xt
>>
>> Type: float
>>
>> Total Size: 6144 bytes
>>
>>             1536 values
>>
>> Number of Dimensions: 1
>>
>> Dimensions and sizes: [x_t | 1536]
>>
>> Coordinates:
>>
>>             x_t: [0.5..1535.5]
>>
>> Number Of Attributes: 2
>>
>>   long_name : X on T grid
>>
>>   units :
>>
>>
>> If I plot the data as a contour plot using "gsn_csm_contour" the figure
>> looks like the attached png. Please help me to solve this.
>>
>>
>> Thank you very much and best regards,
>>
>> Chathu
>>
>> [image]
>>
>>
>> Wickramage Chathurika Hemamali
>> Msc in Physical Oceanography
>> State Key Laboratory of Tropical Oceanography
>> South China Sea Institute of Oceanology
>> University of Chinese Academy of Science
>> China
>>
>> *Specialized in Oceanography and Marine Geology (Bachelor)*
>> *University of Ruhuna*
>> *Matara*
>> *Sri Lanka*
>>
>> Email : wickramagechathurika at rocketmail.com
>> chatu at scsio.ac.cn
>>
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