Re: Lat/Lon from an Image

• To: hampton@xxxxxxxx
• Subject: Re: Lat/Lon from an Image
• From: John Caron <caron@xxxxxxxxxxxxxxxx>
• Date: Fri, 31 Mar 2006 10:59:12 -0700

Hi Shawn:

This is, as they say, non-trivial.

Abstractly you are working with two invertible transformations:

 latLon <- T1 -> Image Coords <- T2 -> screen coordinates

The Image coordinates are either Lat/Lon or projection coordinates. In the 
latter case, T1 is a complicated highly non-linear function, eg Lambert 
conformal. In our source jar, you will find implementations in 
ucar.unidata.geoloc.projection package.

T2 is always affine, and we use java.awt.geom.AffineTransform, which works well with the 
java.awt.Graphics2D stuff that does the actual screen drawing. By setting the correct 
AffineTransform on the graphics object, you can basically work in Image Coordinates, aka 
"User Coordinates" when you draw the Image. You only need the Lat/Lon when you 
want to display the coordinates based on the mouse position.

The ucar.nc2.ui.grid package is a Swing implementation that does all this. Its 
pretty complicated, and of course, not well documented. Have a look at 
GridRenderer if you're brave.


Shawn Hampton wrote:

John,

I am hoping that you can give me a leg up on dealing with lat/lon
coordinate conversions.  What I'm trying to do is this: show an image
on the screen, say a map of the US showing radar data.  I'd like to be
able to be able to view the lat/lon of the position under the cursor
as the mouse passes over it.

So, I need to convert (x,y) to (lat,lon) for a fixed image.

I have the code from the netcdf viewer, and I'm puzzling through the
various classes.  I  have an example working using LatLonProjection,
but the data makes no sense (lat is almost always 90 no matter where
the mouse moves).  I'm sure that I'm missing some steps, for one I'm
not setting the lat/lon coordinates of the map I'm using.  I'm also
fairly sure that there are many steps that I'm missing that I have no
idea about.

Thanks for your time,

Shawn


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