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20040505: Nexrad Level-III Plotting Question

Steve,

The problem with using the polar stereographic projection I believe 
is that projection defines the projection point as the opposite
side of the globe (eg south pole for a north pole tangent plane).
Objects on the plane are located to the earth from the ray passing from
the oposite point on the earth from the tangent point and extending to
intersect the tangent plane.

GEMPAK is using a special case of a satellite projection for viewing the
raster image on the plane.

Steve Chiswell
Unidata User Support


On Wed, 2004-05-05 at 15:42, Steve Ansari wrote:
> Hello Steve,
> I'm trying to geolocate some Level-III NCZ(38) Composite Reflectivity
> and I'm using GEMPAK as a point of reference to make sure the grid
> cells end up in the correct places.  I'm having a problem...  My grid
> cells differ in location from GEMPAK depending on the location within
> the grid.  There does not seem to be a pattern to the misalignment and
> it does not appear to be improperly shifted due to starting location. 
> I'm using the packet header information to assign the starting
> location and grid cell size (4km in this case).  I've tried both
> Albers Equal Area (tangent latitudes +- 1 deg) and Polar Stereographic
> projections (both Spherical Earth) centered over the Radar site with
> similar results.  I'm also using the ESRI States, Counties and
> Interstates shapefiles.  So...
> 
> I was wondering what method GEMPAK uses to geolocate the Level-III
> Nexrad?  
> 
> Thanks for any feedback and ideas!
> Steve
> 
> My test data is at:
> http://www1.ncdc.noaa.gov/pub/has/HAS999900001
> 
> -- 
> Steve Ansari
> GIS Programmer
> Data Processing Branch
> STG INC / National Climatic Data Center