ucar.nc2.grib.grib2

Class Grib2Gds.SpaceViewPerspective

java.lang.Object

ucar.nc2.grib.grib2.Grib2Gds

ucar.nc2.grib.grib2.Grib2Gds.SpaceViewPerspective

Enclosing class:

Grib2Gds

public static class Grib2Gds.SpaceViewPerspective
extends Grib2Gds

Nested Class Summary

Nested classes/interfaces inherited from class ucar.nc2.grib.grib2.Grib2Gds

Grib2Gds.CurvilinearOrthogonal, Grib2Gds.GaussLatLon, Grib2Gds.LambertConformal, Grib2Gds.LatLon, Grib2Gds.Mercator, Grib2Gds.PolarStereographic, Grib2Gds.RotatedLatLon, Grib2Gds.SpaceViewPerspective

Field Summary

Fields

Modifier and Type	Field and Description
float	dX
float	dY
int	flags
float	LaP
float	LoP
float	Nr
float	orient
float	Xo
float	Xp
float	Yo
float	Yp

Fields inherited from class ucar.nc2.grib.grib2.Grib2Gds

data, earthRadius, earthShape, hashCode, lastOctet, majorAxis, minorAxis, nptsInLine, scanMode, template

Method Summary

Methods

Modifier and Type	Method and Description
boolean	equals(Object o)
int	hashCode()
GdsHorizCoordSys	makeHorizCoordSys() Make a Eumetsat MSG "Normalized Geostationary Projection" projection.
void	testHorizCoordSys(Formatter f)

Methods inherited from class ucar.nc2.grib.grib2.Grib2Gds

factory, finish, getEarth, getNameShort, getNptsInLine, getNx, getNxRaw, getNy, getNyRaw, getOctet, getOctet4, getOctetSigned, getRawBytes, getScaledValue, isLatLon, isThin, main, readNptsInLine

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

Field Detail

LaP

public float LaP

LoP

public float LoP

dX

public float dX

dY

public float dY

Xp

public float Xp

Yp

public float Yp

orient

public float orient

Nr

public float Nr

Xo

public float Xo

Yo

public float Yo

flags

public int flags

Method Detail

equals

public boolean equals(Object o)

Overrides:

equals in class Grib2Gds

hashCode

public int hashCode()

Overrides:

hashCode in class Grib2Gds

makeHorizCoordSys

public GdsHorizCoordSys makeHorizCoordSys()

Make a Eumetsat MSG "Normalized Geostationary Projection" projection. Fake coordinates for now, then see if this can be generalized.

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from simon.elliott@eumetsat.int

For products on a single pixel resolution grid, the scan angle is 83.84333 E-6 rad. So dx = 2 * arcsin(10e6/Nr) / 83.84333 E-6 = 3622.30, which encoded to the nearest integer is 3622. This is correctly encoded in our products.

For products on a 3x3 pixel resolution grid, the scan angle is 3 * 83.84333 E-6 rad = 251.52999 E-6 rad. So dx = 2 * arcsin(10e6/Nr) / 251.52999 E-6 = 1207.43, which encoded to the nearest integer is 1207. This is correctly encoded in our products.

Due to the elliptical shape of the earth, the calculation is a bit different in the y direction (Nr is in multiples of the equatorial radius, but the tangent point is much closer to the polar radius from the earth's centre. Approximating that the tangent point is actually at the polar radius from the earth's centre: The sine of the angle subtended by the Earths centre and the tangent point on the equator as seen from the spacecraft = Rp / (( Nr * Re) / 10^6) = (Rp * 10^6) / (Re * Nr)

The angle subtended by the Earth equator as seen by the spacecraft is, by symmetry twice the inverse sine above, = 2 * arcsine ((Rp * 10^6) / (Re * Nr))

For products on a single pixel resolution grid, the scan angle is 83.84333 E-6 rad. So dy = 2 * arcsine ((Rp * 10^6) / (Re * Nr)) / 83.84333 E-6 = 3610.06, which encoded to the nearest integer is 3610. This is currently encoded in our products as 3568.

For products on a 3x3 pixel resolution grid, the scan angle is 3 * 83.84333 E-6 rad = 251.52999 E-6 rad. So dy = 2 * arcsine ((Rp * 10^6) / (Re * Nr)) / 251.52999 E-6 = 1203.35, which encoded to the nearest integer is 1203. This is currently encoded in our products as 1189.

As you can see the dx and dy values we are using will lead to an error of around 1% in the y direction. I will ensure that the values are corrected to those explained here (3610 and 1203) as soon as possible.

Specified by:

makeHorizCoordSys in class Grib2Gds

testHorizCoordSys

public void testHorizCoordSys(Formatter f)

Specified by:

testHorizCoordSys in class Grib2Gds