[ gpsdrive ] grib files

[Chris H]

On Thursday 31 July 2008 22:52:05 Hamish wrote:

> NetCDF is a bit complex, but things look rather good in GDAL-land:
>   http://www.gdal.org/frmt_grib.html

good news...

> > Now since gdal is used to prepare NOAA charts for example the same
> > program could in theory convert, converted grib files via the
> > process described above...theoretically of course.
>
> sure, presumably this is mostly useful at continental map scales?

played with zqGrib and it seems to work well. Base maps is poor as it uses the 
gshhs world boundaries map but it has zoom features so you can get relatively 
localized data display in a transparency overlay....say in a 1000 miles 
square region...which is about as low as you want to go I presume for the 
data to be relevant.

see http://zygrib.free.fr/ comes with source of course. 

> > The only part that is not known to me is if gpsdrive could
> > overlay the grib files over the geo referenced chart tiles??
>
> you would have to overlay the data upon processing the GRIB NetCDF.
> NetPBM offers a tool for that, we use a similar method in GRASS to overlay
> the map and decoration layers for display in the GUI window. It's pretty
> straight forward as long as the GRIB data is suitable for transparency.

It appears to be so at least as used by zygrib. The only problem with this 
application is that the background world map has zero detail. It may be 
easier to modify zygrib rather then gpsdrive but as I'm not a coder gotta 
stick with what works at present...:) The intend here is simply to display 
grib files in a geographical context. This it does well. 

> for updates keep your map.koords table the same and replace the gpsdrive
> map tiles (preserving filename). (or replace map dir and koords file
> together)  .. I don't know about gpsdrive tile caching, maybe there is a
> problem there.

tks..


> the only thing I worry about is how to ensure the tiles are up to date? use
> netpbm tool to overlay a watermark timestamp in the top left of each tile??

not sure either but zygrib has an interesting download feature whereby the 
grib file for a selected region is saved to the file system and then applied 
as an overlay for the selected region. 

for example: (two sets downloaded for different regions)
#zyGrib/grib ls -la
-rw-r--r-- 1 chris123 chris123  70311 2008-07-31 20:01 20080801_020108_643.grb
-rw-r--r-- 1 chris123 chris123 128463 2008-07-31 20:07 20080801_020724_272.grb

I think it uses the file's time stamp to keep things in order. 


> I'll put it on my virtual list of things to look at once I get back from
> away; if no one else gets to it first.

Many thanks for considering this as an added feature. Will see how far I can 
get with this.

-- 
/ch