>
>Hello, a couple of weeks ago I sought help on extracting
>point data from a grid, given a lat/lon that didn't necessarily
>correspond to the grid points. I was led to linint2_points() and
>have made "some" progress, but of course, I had to complicate
>the initial requirements a little bit :)
>
>Given a regular 3x3 grid varying from 45N to 47N and 112W to 114W,
>and a temperature field, I can easily extract/interpolate point
>data as follows:
>
>;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>begin
>
> temperature = (/ (/100, 100, 50/), \
> (/100, 100, 100/), \
> (/100, 100, 100/) /)
>
> lat = (/45, 46, 47/)
> lon = (/-114, -113, -112/)
>
> ; Here's the lat/lon I want station data for
> y0 = 45.5
> x0 = -112.5
>
> station_temperature = linint2_points(lon, lat, temperature, \
> False, x0, y0, 0)
>
> print(station_temperature)
>
>end
>;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>
>However, what if my grid is NOT so "regular?" Suppose, as is the
>case with some of my WRF output, a "row" of points might start at
>45N and end at 45.2N? Does linint2_points() allow for this? In
>the following, I've naively tried by placing all of my lat/lon grid
>points in arrays and passing those to linint2_points(), as follows
>
>;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>begin
>
> temperature = (/ (/100, 100, 100/), \
> (/100, 50, 100/), \
> (/100, 100, 100/) /)
>
> lat_points = (/ (/45, 45.1, 45.2 /), \
> (/46, 46.1, 46.2 /), \
> (/47, 47.1, 47.2 /) /)
>
> lon_points = (/ (/-114, -113, -112/), \
> (/-114.1, -113.1, -112.1/), \
> (/-114.2, -113.2, -112.2/) /)
>
> ; Here's the lat/lon of the location I want station data for
> y0 = 45.5
> x0 = -112.5
>
> station_temperature = linint2_points(lon_points, lat_points, \
> temperature, False, x0, y0, 0)
>
> print(station_temperature)
>
>end
>;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>
>
>but I get an error message saying:
>
>fatal:linint2_points: If xi is not one-dimensional, then it must have
>one less dimension than fi
>
>(Note that above, "xi" corresponds to "lon_points" and "fi" corresponds
>to "temperature").
>
>Indeed, the documentation confirms that xi must have one less dimension
>than fi, but
>I don't understand why this would be the case (which suggests I don't
>really
>understand the purpose of multi-dimension arguments for xi and yi). Is
>there any place
>I can go for a little more explanation? A Google search on
>"linint2_points" doesn't
>turn up much :)
============
Hello
linint2_points was developed to interpolate from
a "regular" grid to arbitrary locations within the regular grid.
By "regular", I mean a grid where any point on the grid
can be accessed via one-dimensional (1d) coordinate variables.
To be considered a "coordinate variabe" it must be a 1D
array of monotonically {in/de}creasing values. They need not
be equally spaced.
The WRF grid is curvilinear. It requires 2D lat/lon arrays to
describe the grid. It is not so easy to develop a fast, general
purpose code to interpolate from any arbitrary grid to a point.
Suffice it to say, some grids are *very* unusual.
I asked the MMM/WRF 'user support' person if they had
an interpolation function. Here is the response:
> We probably do not have the exact thing you
> ask for. We have some routines that converts
> x, y to lat, long, and vice versa. But the x,y
> here refers to the coordinate on projected maps.
> If that would be useful to you, I will pass them to you.
--- NCL's Natgrid interpolation library may work. I have never used Natgrid. Also, I have heard it can be slow. The following is untestedhttp://ngwww.ucar.edu/ngdoc/ng/ref/ncl/functions/nnptinit.html http://ngwww.ucar.edu/ngdoc/ng/ref/ncl/functions/nnpnt.html
Let's say lat2d(lat,lon) and lon2d(lat,lon) are the grid coordinates. for the variable x(ntim,lat,lon). Let latSta(N), lonSta(N), xSta(ntim,N) be the station observations
lat1d = ndtooned(lat2d) lon1d = ndtooned(lon2d) do nt=0,ntim-1 nnpntinit(lon1d, lat1d, ndtooned(x(nt,:,:)) ) xSta = nnpnts(lonSta,latSta) ; xSta(N) : end do Good Luck D
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