Hi,
Since the projection code is very likely correct, I guess the error is in
interpretation of the reference coordinates and deltas given in the grib-files;
- In the unrotated image, the grid usually cannot be expressed in linear first
degree polynomials (longitude = lonRef + DX* i_index, latitude = latRef +
DY*j_index). But in the rotated lon/lat reference system (where the south pole
could be on equator), the grid is a regular, linear grid where parameters as
lower left corner and grid deltas makes sense:
Rotated LatLon Grid (468x378)
lon: -46.5 to 46.9 (dx 9.99990041077413E24)
lat: -36.5 to 38.9 (dy 9.99990041077413E24)
I interpret the dx,dy parameters above as undefined since they are too large to
give meaning (1 degree north is approx 111 km). Dividing delta lon by 467
(468-1 cells in transformed east-direction) gives 0.2 degrees grid resolution
east. Similiar calculation gives 0.2 degrees north:
| diff| N-1 | diff/(N-1)
lon -46,5 46,9 | 93,4 | 467 | 0,2 deg
lat -36,5 38,9 | 75,4 | 377 | 0,2 deg
PS. These lon, lat coordinates are not real, geographical east/north
coordinates, but angular positions relative to the rotated reference system.
The inverse rotated latlon (lon,lat) projection will give the real geographical
coordinates of any positions in the grid.
Hope this helps..
Tor C
----- Original Message -----
From: Robb Kambic <address@hidden>
Sent: Wed, 24-12-2008 01:03
To: Tor Christian Bekkvik <address@hidden>
Cc: decoders <address@hidden>
Subject: RE: [netcdf-java] Adding a new projection to GRIB netcdf decoder
Tor,
I changed the Lower Left corner of the unrotated image to be approximately
28 N and 47W and the image almost lines up with the map. Also it appears
that the image needs to be rotated some. the rotated coordinates I used
are:
La1 = -20.25
Lo1 = -43.5
yours are:
inv [-46.5, -36.5] -> [-37.030363026843844, 14.484143589394591]
since we have verified that the projection code is the same, what's the
next step.
Robb...
On Tue, 23 Dec 2008, Robb Kambic wrote:
Tor,
I was on vacation for a while, so no work was done on the projection.
I tested my code implemetation against the code you sent, I got all the same
values. Currently, my results are south and east of the desired map location.
Here's my thoughts, the Dx and Dy values in the file are not correct, I get
Dx = Dy = .2 degrees. These values are in millidegrees according to the
documentation. The values that you have are
grid_dx = 9.99990041077413E24 that I assume are meters. I think that if the
Dx and Dy values were correct then the image would line up correctly. What do
you think?
RObb...
C:\data\rotatedlatlon.grb
index_version = 6.0
grid_edition = 1
location = C:\data\rotatedlatlon.grb
length = 265454
created = 2008-12-23T21:44:09Z
center = 96
sub_center = 0
table_version = 1
tiles = 1
thin = false
ensemble = false
--------------------------------------------------------------------
1 0 -1 7 1 105 0.0 255 0.0 2003-12-15T18:00:00Z 0 2891248751 36 265454 0
false 96 0 1
--------------------------------------------------------------------
GDSkey = 2891248751
grid_type = 10
grid_name = Rotated latitude/longitude grid
grid_shape_code = 0
grid_shape = spherical
grid_radius_spherical_earth = 6367.47
Nx = 468
Ny = 378
La1 = -36.5
Lo1 = -46.5
ResCompFlag = 136
Winds = Relative
La2 = 38.9
Lo2 = 46.9
Dx = 0.2
Dy = 0.2
ScanningMode = 64
SpLat = -25.0
SpLon = 0.0
Angle = 0.0
On Mon, 24 Nov 2008, Tor Christian Bekkvik wrote:
Hi,
I did som debugging on our JGRIB application(rotatedlatlon.grb), hope this
helps:
1) Grib GDS Section content (JGRIB interpretation):
EARTH_RADIUS = 6367470.0
length = 46
nv = 1
octetPos = 43
grid_type = 10
grid_type_string = {java.lang.String@616}"Rotated LatLon Grid"
grid_nx = 468
grid_ny = 378
grid_lat1 = -36.5
grid_lon1 = -46.5
grid_lat2 = 38.9
grid_lon2 = 46.9
grid_mode = 136
grid_dx = 9.99990041077413E24
grid_dy = 9.99990041077413E24
grid_scan = 64
isUVEastNorth = false
grid_latsp = -25.0
grid_lonsp = 0.0
grid_rotang = 0.0
2) Then, from this Grib GDS section, the projection was instantiated with:
new RotLatLon(grid_lonsp, grid_latsp, grid_rotang)
3) And here is the resulting instantiated projection code:
public RotLatLon(double aLonsp,
double aLatsp,
double aRotsp)
{
lonsp = aLonsp;
latsp = aLatsp;
rotsp = aRotsp;
double dlat_rad = (latsp - (-90)) * DEG2RAD; //delta latitude
sinDlat = Math.sin(dlat_rad);
cosDlat = Math.cos(dlat_rad);
}
static final double DEG2RAD = Math.PI / 180;
static final double RAD2DEG = 180 / Math.PI;
public double[] fwd(double[] lonlat)
{
return transform(lonlat, lonsp, rotsp, sinDlat);
}
public double[] inv(double[] lonlat)
{
return transform(lonlat, -rotsp, -lonsp, -sinDlat);
}
private double[] transform(double [] lonlat, double rot1, double
rot2, double s)
{
double e = DEG2RAD * (lonlat[0] - rot1); //east
double n = DEG2RAD * lonlat[1]; //north
double cn = Math.cos(n);
double x = cn * Math.cos(e);
double y = cn * Math.sin(e);
double z = Math.sin(n);
double x2 = cosDlat * x + s * z;
double z2 = -s * x + cosDlat * z;
double R = Math.sqrt(x2 * x2 + y * y);
double e2 = Math.atan2(y, x2);
double n2 = Math.atan2(z2, R);
double rlon = RAD2DEG * e2 - rot2;
double rlat = RAD2DEG * n2;
return new double[]{rlon, rlat};
}
Thanks,
Tor C
-----Original Message-----
From: Robb Kambic [mailto:address@hidden]
Sent: 24. november 2008 02:51
To: Tor Christian Bekkvik
Subject: RE: [Fwd: [netcdf-java] FW: Adding a new projection
to GRIB netcdf decoder] (fwd)
Tor,
thanks for the example file, it helped. I'm still having
problems with the projection, it seems that it's -90 off. If
I enter 0, 0 into the projection transform I would expect
that the results would be the La1, Lo1 and that's not the
case. Is this the formula that you used in creating the grid?
Any help would be appreciated. I get all the parameters correct.
THnaks,
Robb...
On Thu, 20 Nov 2008, Tor Christian Bekkvik wrote:
Hi,
I have been a little busy; Sorry for the bad example file I sent
first, the attached file should have more meaningful values.
Some file info, logged from a java-application using JGRIB (JGRIB
supports rotated latlon for GRIB-1) :
Gribfile: rotatedlatlon.grb
Records: 1
Grid:
GDS section:
Rotated LatLon Grid (468x378)
lon: -46.5 to 46.9 (dx 9.99990041077413E24)
lat: -36.5 to 38.9 (dy 9.99990041077413E24)
south pole: lon 0.0 lat -25.0
rot angle: 0.0
mode(17):0x88
type(6) :0xa
scan(28):0x40
PDS cent tab proc: 96 1 1
-> 96 -1 1 ncep_opn.tab
Z type(105): fixed height above ground
Z #levels: 1
Dates(1):
min Mon Dec 15 19:00:00 CET 2003
max Mon Dec 15 19:00:00 CET 2003
Param: 7 HGT [gpm] Geopotential height [-275.23438 4048.7656]
PS. Hoping that rotated latlon projection could be supported for
GRIB-2
Thanks,
Tor C
-----Original Message-----
From: Robb Kambic [mailto:address@hidden]
Sent: 20. november 2008 00:44
To: Tor Christian Bekkvik
Subject: Re: [Fwd: [netcdf-java] FW: Adding a new
projection to GRIB
netcdf decoder] (fwd)
Tor,
An image of the result would be appreciated.
Thanks,
Robb...
On Wed, 19 Nov 2008, Robb Kambic wrote:
Tor,
I've been working on this problem but the sample file you
sent has all
the data values equal to 0. could you make available
another sample
file with a description so I can tell if I am displaying
correctly.
Robb...
-------- Original Message --------
Subject: [netcdf-java] FW: Adding a new projection to GRIB netcdf
decoder
Date: Wed, 29 Oct 2008 09:35:55 +0100
From: Tor Christian Bekkvik <address@hidden>
To: netcdf-java <address@hidden>
-----Original Message-----
From: Tor Christian Bekkvik
Sent: 27. oktober 2008 14:48
To: 'address@hidden'
Cc: Kjell Røang
Subject: Adding a new projection to GRIB netcdf decoder
Hi,
We would like to add support for Rotated LatLon projection when
reading GRIB files. (using netcdf decoders)
To open the gribfile, we have used class NetcdfDataset:
NetcdfDataset.openDataset(gribFile,...)
But this fails for with rotated latlong projection, as it
is not supported:
ucar.grib.NoValidGribException: GDS: Unknown Grid Type :
10) is not
supported.
at
ucar.grib.grib1.Grib1GridDefinitionSection.<init>(Grib1GridDef
initionSection.java:401)
at ucar.grib.grib1.Grib1Input.scan(Grib1Input.java:155)
at
ucar.grib.grib1.Grib1Indexer.writeFileIndex(Grib1Indexer.java:90)
at
ucar.nc2.iosp.grib.GribServiceProvider.writeIndex(GribServiceP
rovider.java:163)
at
ucar.nc2.iosp.grib.GribServiceProvider.getIndex(GribServicePro
vider.java:139)
at
ucar.nc2.iosp.grib.GribServiceProvider.open(GribServiceProvide
r.java:75)
at ucar.nc2.NetcdfFile.<init>(NetcdfFile.java:1092)
at ucar.nc2.NetcdfFile.open(NetcdfFile.java:485)
at ucar.nc2.NetcdfFile.open(NetcdfFile.java:278)
at
ucar.nc2.dataset.NetcdfDataset.openFile(NetcdfDataset.java:341)
at
ucar.nc2.dataset.NetcdfDataset.openDataset(NetcdfDataset.java:175)
at
ucar.nc2.dataset.NetcdfDataset.openDataset(NetcdfDataset.java:161)
To add this new projection, it seems that at least the
following code
needs
update:
1) ProjectionImpl (I can implement RotLatLonProjection extends
ProjectionImpl)
2) Grib1GridDefinitionSection (to fix
NoValidGribException, listed above)
3) GribHorizCoordSys.makeProjection (read necessary
parameters from Grib
GDS section and return RotLatLonProjection):
private void makeProjection(NetcdfFile ncfile) {
switch (lookup.getProjectionType(gdsIndex)) {
case TableLookup.PolarStereographic:
makePS();
break;
case TableLookup.LambertConformal:
makeLC();
break;
case TableLookup.Mercator:
makeMercator();
break;
case TableLookup.Orthographic:
makeSpaceViewOrOthographic();
break;
default:
throw new UnsupportedOperationException("unknown
projection = "
+ gdsIndex.grid_type);
}
Currently, I don't have sufficient GRIB/NetCDF knowledge
(or source
code) to do this.
But I have already added Rotated LatLon projection for the JGRIB
project (http://jgrib.sourceforge.net/), and we would like
to do the
same with NetCDF/GRIB.
- Any comments how to best do this ?
Thanks,
Tor Chr Bekkvik
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==============================================================
=================
Robb Kambic Unidata Program Center
Software Engineer III Univ. Corp for
Atmospheric Research
address@hidden WWW:
http://www.unidata.ucar.edu/
==============================================================
=================
==============================================================
=================
Robb Kambic Unidata Program Center
Software Engineer III Univ. Corp for
Atmospheric Research
address@hidden WWW:
http://www.unidata.ucar.edu/
==============================================================
=================
===============================================================================
Robb Kambic Unidata Program Center
Software Engineer III Univ. Corp for Atmospheric
Research
address@hidden WWW: http://www.unidata.ucar.edu/
===============================================================================
===============================================================================
Robb Kambic Unidata Program Center
Software Engineer III Univ. Corp for Atmospheric Research
address@hidden WWW: http://www.unidata.ucar.edu/
===============================================================================
