Harold,
I use my own projection code for most of the projected grids that Panoply
handles, but I have also written versions for all of them that use the Unidata
netCDF library's projection code. My method for getting the projection from the
Unidata code is a bit different from yours, mostly because the various
projections I have implemented extend from an abstract projection handler. That
abstract class doesn't need to know what the specific projection is; it just
works on a ProjectionImpl class, and LambertConformal extends from
ProjectionImpl.
private ProjectionImpl njProj;
private void initUnidataProj ( )
{
final VariableDS njvarDS = ...;
final List<CoordinateSystem> csList = njvarDS.getCoordinateSystems ( );
if (csList == null || csList.isEmpty ( ))
{ throw new RuntimeException ("No coordinate systems found."); }
final CoordinateSystem cs = csList.get (0);
// Get coordinate system projection. CS will return LatLonProjection
if no projection detected but
// there is an auxiliary 2D grid in place.
njProj = cs.getProjection ( );
// If projection is not Lambert Conformal Conic, that makes us unhappy.
if (! (njProj instanceof LambertConformal))
{
throw new RuntimeException ("CS returned " + njProj.getClass ( ) +
" instead of LambertConformal");
}
}
And then as you indicate, call njproj.projToLatLon to translate grid
coordinates to lon-lat pairs.
But your final question is the opposite direction, given a lon-lat pair, to
find the data value at that location.
When gridding such data, Panoply creates an x-y grid using the projected grid
coordinates. It then creates an output lon-lat grid, and calls lonLatToProj on
each of the points in that grid, takes each returned x-y pair, and finds the
nearest data point in the x-y grid to that pair. Since it is doing this an
array of locations rather than just looking up single points on demand, there's
some optimization in place.
rbs
--
Robert B. Schmunk
Webmaster / Senior Systems Programmer
NASA Goddard Institute for Space Studies
2880 Broadway, New York, NY 10025
-----Original Message-----
From: <netcdf-java-bounces@xxxxxxxxxxxxxxxx> on behalf of Harold
<harold@xxxxxxxxxxxxxx>
Date: Tuesday, June 18, 2019 at 12:38
To: "netcdf-java@xxxxxxxxxxxxxxxx" <netcdf-java@xxxxxxxxxxxxxxxx>
Subject: [netcdf-java] Reading grib2 with LambertConformal_Projection -> lat
/ lng
Hello,
First, thanks for this lib; it is very helpful and I don't know where we'd
be without it.
I encountered my first netcdf (.grib2) files recently, and hope to access
the data therein to verify the output of another system.
Here's an example file, if that helps:
-
https://s3.us-east-2.amazonaws.com/tech.public.data/grib2-test/3600.grib2
<https://s3.us-east-2.amazonaws.com/tech.public.data/grib2-test/3600.grib2>
I found PanoplyJ, which works great, and summarizes the file like this:
- https://i.imgur.com/VJXnpWH.png
I am using the java libs `edu.ucar/cdm` and `edu.ucar/grib`, both at
version 4.6.13 - which also work great and open the file without trouble.
My intention is to verify the contents of this file against a different
dataset which expresses surface location with lat/lng, while this grib2 file
has x/y (under a Lambert conformal mapping).
The path I found through the lib yesterday is:
1) NetcdfDataset/openDataset
2) .getVariables and store a reference to the `LambertConformal_Projection`
variable
3) Construct a `LambertConformalConic` and call `.makeCoordinateTransform`
with the dataset and the variable
4) From that I can `.getProjection`
5) Then I make a `float[][]` out of the separate x/y variables
6) Then I can call `.projToLatLon` with the projection and the `float[][]`
7) After this I go linear over the contents of the `Temperature_surface`
variable associating each one with the correct lat/lng.
This actually works, but I am wondering if there is a better way.
I have a feeling that there is some higher level API that I missed that
would do all this more automatically, and possibly more generally. My code
feels brittle and over-specific to this particular file.
Or, put another way, how best to (given a lat/lng) query the temp at that
location in this file?
With warm appreciation for your time,
-Harold
