[Gdal-dev] how is yllcorner determined and why
janeen at splash.princeton.edu
Mon Nov 27 09:18:33 EST 2006
I've a correction to my original post. I discovered that in trying to
figure out how the value for yllcorner is calculated by gdal_translate,
I was using the dy value NOT the dx value, as the line of code states.
See corrections within mail.
But that leaves me with 2 new questions:
Why is the dx value used to calculate the yllcorner in the Ascii Grid?
And what in the original .nc file is the yllcorner = -269 related to?
Any light shed on this would be appreciated.
> Hello All,
> I've just tried the gdal_translate utility on a NetCDF file,
> converting it to the AAIGrid format. I'm new to the world of GIS and
> NetCDF also. So in using the utility, I want to be sure that I
> understand how all the data from the .nc file makes it over to the
> ascii file.
> I can't quite figure out how the yllcorner value being reported is
> calculated, and why it is transformed the way it is. I've looked at
> the code and gotten as far as this line in aaigriddataset.cpp:
> adfGeoTransform- nYSize * adfGeoTransform
> I can't quite get what the variable adfGeoTransform represents from
> the original .nc file. The part of the equation:
> nYSize * adfGeoTransform
> I understand where those values come from in my sample file (i.e. 180
> * 0.994444444444 = 179 ).
in my example this should be 180 * 0.997222222222 = 179.5
> Given that yllcorner is being reported as -269, that means that the
> value for adfGeoTransform would be -90. But what does that value
> represent in my original netCDF file given the info below?
the actual value for adfGeoTransform becomes -89.5 which is the
southern most latitude value in the .nc file.
> If anyone can give me a description of what is going on here, I would
> appreciate it. Below is header and gdalinfo.
> Here is the gdal header info from my converted .nc file:
> cols 360
> nrows 180
> xllcorner 0.500000000000
> yllcorner -269.000000000000
> dx 0.997222222222
> dy 0.994444444444
> NODATA_value -10000000000
> Here is (relevant) gdalinfo from my .nc file
> Driver: netCDF/Network Common Data Format
> Size is 360, 180
> Coordinate System is `'
> Origin = (0.500000,-89.500000)
> Pixel Size = (0.99722222,0.99444444)
> Corner Coordinates:
> Upper Left ( 0.5000000, -89.5000000)
> Lower Left ( 0.5000000, 89.5000000)
> Upper Right ( 359.500, -89.500)
> Lower Right ( 359.500, 89.500)
> Center ( 180.0000000, 0.0000000)
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