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19990524: grid spacing for various map projections



Steve,

The quick answer is no, I don't have a quick way of plotting
out the grid distance. However, let me qualify this with the following:

You can get the gddx and gddy values for the ddx and ddy 
functions you mention by inserting a write statement in to the
dgmscl.f gemlib routine, like:
~line 187:
C*      Make sure grid increments are positive.
C
        gddx = ABS ( gddx )
        gddy = ABS ( gddy )
        write(*,*) 'dx = ',gddx,' dy = ',gddy

Then relink gdlist and try gfunc = msfx it will call that routine.


However that these are the distances of the grid points as projected onto
a 2d map plane from the cone that the map projection is from, so it
is not be the nominal "grid distance" you may be expecting.

For example the gddx and gddy values for the #211 projection are:
 dx =    74114.42 m    dy =    74115.74 m

The above values are fixed for the projection.

However, if you actually want the grid distance between two adjacent
points on the grid, which vary over the grid- eg, the nominal distance
of the 211 grid is 81.2km at 45 degrees N, then the only way is to
print out the longitudes using gfunc=mul(lonr,rtd) and
then get out your calculator and do the subtraction.

For more on a similar question:
http://www.unidata.ucar.edu/glimpse/gempak/2385

Steve

On Fri, 21 May 1999, Steven L. Mullen wrote:

> Steve,
> 
> A quickie:
> 
> Do you know how I can access (from gdlist e.g.)
> the spacing between grid points (in km, meters,
> in units of distance) for the various map
> projections. These would be the denominators
> in the ddx(s) and ddy(s) GFUNC online functions.
> 
> Thanks,
> Steve
> 
> --
> -------------------------------------------------------
>      {}               Steven L. Mullen
>      {} {}            Dept. of Atmospheric Sciences
>   {} {} {}            University of Arizona
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