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Native Grid Projections
WARNING
Is your data on a native grid? A native grid is a
grid that is on a particular map projection.
In order to plot the data correctly over a map, you must know the
exact map projection and all its parameters. You will then plot
the data over this map projection without needing to do any kind of
mathematical transformation.
To plot native grids, you must set the resource tfDoNDCOverlay = True, so that the data
doesn't go through a mathematical transformation to be overlaid on the
map. Additionally, you commonly want to set the resource mpLimitMode="Corners" to specify the area of
the map to zoom in on. This resource must be accompanied by resources
for setting the four corners of the map area to view: mpLeftCornerLatF, mpLeftCornerLonF, mpRightCornerLatF, and mpRightCornerLonF.
pmTickMarkDisplayMode = "Always"
turns on the automatic tickmarks.
If your data comes with 2D lat/lon arrays, you then have the option of
plotting it on a different map projection. In this case, you do not
want to set tfDoNDCOverlay (it is
False by default). Instead, you will need to set the special
lat2d/lon2d attributes. See example 3 on the Ice Model page, example 1 on the EASE page, or example 3 on the Raster contouring page,
Lambert conformal native grids have been
given their own page.
STEREOGRAPHIC
native_1.ncl: An example of a
Stereographic native grid.
mpProjection = "Stereographic", sets
the projection.
The four pieces of information that are required for this projection
(with example values) are:
mpRelativeCenterLon = True
mpCenterLonF = lon(147,172)
mpRelativeCenterLat = True
mpCenterLatF = 90.
For this grid, you must set the center lat (-90 or 90) and center lon,
sometimes referred to as the "orientation". A common way that these
grids are described is to give the orientation, the delta X and delta
Y at some arbitrary latitude and the coordinates of the pole in
integer grid coordinates. Coming up with corner lat/lons is difficult
when this is the only information provided.
MERCATOR
native_2.ncl: An example of a
Mercator native grid.
mpProjection = "Mercator", sets the
projection.
This projection requires that the "corners"
method of specifying the map boundaries be used. (see example 1
above).
STEREOGRAPHIC: FROM GRIB
native_3.ncl: Like example 4, this
GRIB file has attributes that make plotting the native stereographic
projection easy.
native_5.ncl: This
example shows how to plot geoscientific data in its native projection,
and then how to calculate the lat/lon values of the projection
so you can overlay the data on a different map projection.
