[gdal-dev] netcdf to geotiff and aaigrid

[Jeff Van Dorn]

On Jan 21, 2008, at 7:59 PM, Frank Warmerdam wrote:

> Jeff Van Dorn wrote:
>> Hello,
>> I'm hoping someone can set me straight in how to go about  
>> converting a bunch of netCDF files I have into both GTiff and  
>> AAIGrid formats, as my initial attempts have failed.  I'm using  
>> gdal version "GDAL 1.6.0dev, FWTools 2.0.4, released 2007/12/21".
> ...
>> I do seem to be able to create a mostly valid AAIGrid file by  
>> simply doing:  gdal_translate -of AAIGrid, BUT it appears upside- 
>> down when putting into something that can read that format, even  
>> though the header info seems MOSTLY correct (I'd think yll should  
>> really be 38.0000 (min of latitudes in netCDF file - 1/2 grid  
>> size, (or 38.0625 - 0.0625)):
>> ncols        110
>> nrows        80
>> xllcorner    -80.750000000000
>> yllcorner    28.000000000000
>> cellsize     0.125000000000
>> But, overall the image appears to be correct, other than inverted  
>> in the y direction, which I'm not really sure how to correct.
>
> Jeff,
>
> I tried translating this file with my linux build of GDAL and the  
> output
> AAIGRID header looks like:
>
> ncols        110
> nrows        80
> xllcorner    -80.750000000000
> yllcorner    38.000000000000
> cellsize     0.125000000000
> NODATA_value   -999
>
> I'm not at all sure why you get a yllcorner of 28 when I get 38.  I
> displayed the file directly, and the output aaigrid file and both  
> seemed
> "right side up" and otherwise normal.

Frank,

Thanks for the reply.

That is certainly strange that you get 38.0.  I just built version  
1.5.0 from scratch, but I still don't get 38.0 (here's my session):
jvandorn at temagami:~/public_html/gdal> gdal_translate --version
GDAL 1.5.0, released 2007/12/18
jvandorn at temagami:~/public_html/gdal> gdal_translate -of AAIGrid  
NE_1961-1990_Yearly_Max_Temp.nc NE_1961-1990_Yearly_Max_Temp.ASC
Warning 1: No UNIDATA NC_GLOBAL:Conventions attribute
Input file size is 110, 80
0...10...20...30...40...50...60...70...80...90...100 - done.
jvandorn at temagami:~/public_html/gdal> head -n 5  
NE_1961-1990_Yearly_Max_Temp.ASC
ncols        110
nrows        80
xllcorner    -80.750000000000
yllcorner    28.000000000000
cellsize     0.125000000000

Are you doing anything other than just gdal_translate -of AAIGrid,  
when converting that file?  I wouldn't really care all that much if I  
was just needing to convert this one file, as it's easy enough to  
manually change, but as I literally have thousands of files to  
convert to both AAIGrid and GTiff (with lots of different domains),  
it'd be nice to have it done automatically.

I don't know if this could be related to what Joaquim said, in that  
the appearing upside-down is a known issue with the netCDF driver.   
So, yours truly was not upside down?  A gif image of the data is  
here: http://temagami.tosm.ttu.edu/~jvandorn/gdal/ 
NE_1961-1990_Yearly_Max_Temp.gif

>> When I try to convert to the GTiff, from either the AAIGrid or  
>> netCDF format, all I get is a black rectangle as an image.
>
> I suspect this has more to do with the software you are displaying
> the GeoTIFF file with than the actual data.  Many software packages
> don't do particularly savvy autoscaling of floating point image data.
>
> Have you tried displaying it in OpenEV?

I'm not having any luck with OpenEV, just seg faults on me (using the  
FWTools version) when I attempt to open an image.
I've tried opening the generated GeoTiffs in ESRI's ArcMap.  I've  
been successful in opening other sample GeoTiffs with that program,  
but not the Tiff's I created (I also just get the black rectangle for  
GIF/JPEGs if that helps to diagnose at all).

>
> BTW, were you using the linux or win32 version of FWTools.

Using the linux version, and I've tried both OpenEV and  
gdal_translate on 2 different machines:
machine 1) Linux temagami 2.6.11.4-21.15-smp #1 SMP Tue Nov 28  
13:39:58 UTC 2006 x86_64 x86_64 x86_64 GNU/Linux
machine 2) Linux linuxlt 2.6.5-7.252-smp #1 SMP Tue Feb 14 11:11:04  
UTC 2006 i686 i686 i386 GNU/Linux

I built the latest GDAL code today on both machines.  I had to do -- 
without-libtool on the 64bit version, but otherwise, didn't specify  
anything when I ran configure on either machine.

Thanks for the help,
Jeff

>
> Best regards,
> -- 
> --------------------------------------- 
> +--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,  
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | President OSGeo, http:// 
> osgeo.org
>