Re: help with coordinate systems and projections??

Hi Stuart-

Stuart Maclean wrote:
> Hi again Don, I mailed you before Xmas about some lack of understanding
> about an MM5 grid I had and wanted to plot on visad.  A few points I hoped
> you can help me with..
>
> My data grid wraps the dateline, i.e. some grid points's actual lon values
> are +ve, e.g. 175deg, whereas most of  the lons are -180 -> -120 say.
> This means that my plot looks wrong, since I imagine that the
> LambertConformal is producing +ve values for lon.  Can this be fixed?

I'm not familiar with the LamberConformal CoordinateSystem (we use
something else in the IDV), but you could send a note to the
visad-list and ask your question there.

> Also, the axes are in radians, and not degrees.  Do you know how I change
> this?  One hack is to override the toReference method of the LCC to scale
> up to degrees but I'm sure this is not right.

You could call the ScalarMap.setOverrideUnit(Unit) method on your
scalarmaps for lat/lon and use CommonUnit.degrees.

> One other thing bugs me now that I am beginning to understand visad a bit
> more.  With my grid in cartesian space, and applying the LCC, I guess we
> are mapping x,y -> lon,lat.  But if I am not specifying any 'display'
> projection, then Visad will just (correctly?) think that these lat,lon
> values have a linear mapping on to the display axes, and so I get a
> strange looking field??

The coordinate system translates x,y to lat,lon (or lon,lat depending on
the CS).  It will use the transform to make the correct values for
lat/lon at each point and then display them as such.

> I have downloaded the IDV system, inc source.  If I were to make a data
> selector for MM5 files, where might I start to poke around in the code?
> Is there any docs on building new selectors??

You could create a new MM5DataSource which would know how to read the
MM5 format and create the appropriate DataChoices.  You could use
the unidata.ucar.data.grid classes as a guide.  Once you have that,
you would just need to add an entry into
ucar.unidata.idv.resources.datasource.xml for your MM5DataSource.
You would not need a new selector - you could just open the file
through the file chooser at that point.

Alternatively, is there a way you could output netCDF from MM5?
I know there are various converters from MM5 to netCDF.  Some are
good, some are not. ;-)  Or, could you output in Vis5D format?
The IDV can read either of these directly.

As an aside, I'm going to cc this note to our support archives
and change the reply line to go to support.  I would like to
get this conversation into our archives for the benefit of others
and it is better to send questions to address@hidden
instead of me.  I'm going to be in and out of the office for
the next 6 months and if it goes to support, someone can get
to it quicker.

Don
*************************************************************
Don Murray                               UCAR Unidata Program
address@hidden                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307
http://www.unidata.ucar.edu/staff/donm
"There's someone in my head, but it's not me"    Roger Waters
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