Simone,
As you might can tell from some of my recent posts, I am a meteorologist
focused primarily on meteorological data systems, so I am glad to see your
ongoing focus in that area. Particularly, I was excited to see details
about wind barbs on your blog:
http://www.geo-solutions.it/blog/developers-corner-supporting-wind-barbs-geoserver-geotools/
and I have been working with the RasterAsPointCollection transformation for
some time as shown here:
http://geoserver.geo-solutions.it/edu/en/wps/rendering_tx.html#point-extraction
I do want to raise one concern and/or question about the "Rotation" angle
for the Mark used to symbolize the vector.
>From a scientific perspective, we want the arrow or vector to point TOWARD
the direction wind is blowing, right, and conversely the barb to point in
the direction FROM which the wind is blowing.
However, doesn't the ROTATION parameter for the mark (arrow, barb, etc.)
only impact rotation of the mark within screen coordinates? In other
words, doesn't it just rotate a mark by a certain angle on the screen,
without respect to the underlying map projection?
If so, then the arrows or barbs displayed would only be valid for maps with
a Mercator projection, where true north aligns with "up" on the
screen/map/grid.
For many other projections, though, "up" on the screen/map/grid does not
align with true north. In fact, depending on the projection and location,
"up" may be toward the south!
So, again as a question and comment ... is ROTATION relative to the screen
or to the underlying map rotation? To get the vector to be properly
rotated with respect to true north and underlying land masses (and not
screen "up"), it would need to be relative to the underlying projection,
not the screen.
I am currently working on this aspect myself ... and am finding myself
needing to calculate the Grid Convergence Angle at each point ... where I
will be able to know how far off the "up" axis at that point is from true
north. I will then have to account for that angle and add it to / subtract
it from the screen ROTATION angle so that my vectors truly show actual
direction when I am using a non-mercator basemap. I had a separate
GeoTools post asking the best way to calculate Grid Convergence Angle ...
and I am still working on that.
Does this make sense or are you already compensating for any underlying map
projection which might rotate "up" away from true north? If so, do you
mind sharing how you are accomplishing that?
Thanks again!
- Mike Grogan
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