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20000125: GINI MERCator projection products (cont.)

>From: Unidata User Support <address@hidden>
>Organization: Unidata Program Center
>Keywords: McIDAS ADDE GINI

Barb,

I got to the bottom of my problem with the Puerto Rico and Hawaiian
GINI sectors in NOAAPORT.  The problem basically boiled down to
the explanation for the "Latin" parameter in the GINI header.

The Interface Control Document (ICD) for AWIPS-National... describes
this as:

Octet      Scaling                       Contents
------+----------------+-----------------------------------------------
39-41  Integer x 10000  Latin - The latitude(s) at which the Mercator
                        projection cylinder intersects the earth
 42    Integer          Image Resolution - See Table 4.9 - 4.12

The way this is worded made me think that the Mercator projection was a
secant one (i.e., the projection cylinder was smaller than the earth
and interesected it at +Latin N and -Latin S).  Latin is really the
latitude at which the image resolution is that described in octet 42.

If I set the latitude at which the projection cylinder is tangent to
the earth at 0 (zero, the equator), then I can correctly navigate the
images in McIDAS.

So, it seems to me that the documentation for Latin is incorrect and
should be changed (I don't know if you have anything to do with this,
but...).

The reason that this may not have come up as an issue before is: AWIPS
doesn't even read these header values; and GEMPAK only uses the LAT,LON
of the "first gridpoint" and "last gridpoint" to do its navigation
(i.e., it assumes that it is a tangent Mercator projection).

Tom Yoksas