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Native Grid Projections

WARNING

Is your data on a native grid? A native grid is a model that was designed from the beginning with a particular map projection. In order to plot the data exactly as the designers planned, we do not want to transform the data to a projection, but simply plot it. This is the essence of the examples on this page.

Just because your data has 2D lat/lon arrays, does not make it a native grid. Native grids are plotted differently than other data with 2D coordinates.

In all cases, you MUST use the mpLimitMode="Corners" method to specify the grid. Other methods will result in an incorrect mismatch between the data and the map. Additionally, you must set tfDoNDCOverlay = True, so that the data is not transformed to the projection, but is simply placed there since it is already on a projection.

pmTickMarkDisplayMode = "Always" turns on the automatic tickmarks (available since NCL version 4.2.0.a023).

Lambert conformal native grid projections 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.

STEREOGRAPHIC: FROM ASCII

native_4.ncl: Wu's plot