# Re: calculating mercator coordinates

From: David Brown <dbrown_at_nyahnyahspammersnyahnyah>
Date: Thu Apr 25 2013 - 14:39:34 MDT

Hi Micah,
There is no specific function to do this in NCL. However, it is possible to do what you want with a bit of effort. The recipe goes something like this:
Create a map using the native projection of the data. That is, a mercator map with corners as specified by the metadata (left: lat 15, lon 284.5 right: lat 22.005, 297.491).
Discover the location and width and height of the viewport using getvalues;
getvalues mapid
"vpXF" : x
"vpYF" y
"vpWidthF" : w
"vpHeightF" : h
end get values

Now you can use the function datatondc to find the NDC locations of any data location within the boundaries of the map.

The index values of the coordinate can now be determined using a simple proportion:

ix =((data_x - x) / w) * (nlon -1)

Of course this produces a floating point number which usually lies in between two indexes.

Hope this helps.
-dave

On Apr 24, 2013, at 2:42 PM, Micah Sklut <micahs2005@gmail.com> wrote:

> Hi there,
>
> I have a dataset that is from a mercator projection.
> So, my usual routine to find the data coordinates of a particular lat/lon don't exactly work, since the grid isn't equidistant.
>
> Are there any NCL function that will calculate or convert lat/lon from mercator data.
>
> Example. lat 38.5, lon -100.
> What coordinates in my dataset contains this point.
> Lat/Lon attributes are below.
>
> float lat_0 ( lat_0 )
> long_name : latitude
> GridType : Mercator Projection Grid
> units : degrees_north
> Dj : 2.5
> Di : 2.5
> Lo2 : 297.491
> La2 : 22.005
> Lo1 : 284.5
> La1 : 15
>
> float lon_0 ( lon_0 )
> long_name : longitude
> grid_type : Mercator
> units : degrees_east
> Dj : 2.5
> Di : 2.5
> Lo2 : 297.491
> La2 : 22.005
> Lo1 : 284.5
> La1 : 15
>
>
> Thank you,
>
> --
> Micah
>
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Received on Thu Apr 25 14:39:43 2013

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