Re: CF-1.4 conventions for lat/lon datum.

My understanding is that NAD83 will have a slightly different PM than WGS84, but the difference is difficult to define. I'm still foggy on exactly how NAD83 changes... (it does, staying static over the "mean?" position of the north american continental plate). WGS84 is static relative to the center of mass of the earth and a number of star observations.

The important piece is that differences are quite small, and there is no great method of converting between the two (since NAD83 changes periodically you would need a bunch more metadata and a whole bunch of tectonic data to do the shift accurately). 

ahh,,, they don't have the same geoid exactly... they are whole micrometers different! heh... something about needing the accuracy for sattelite orbital calcs. (yay wikipedia!)

So, take home, I think we can ignore the distinction between NAD83 and WGS84 (for calculations on the GDP). I also think we shouldn't support data in NAD27 or any of the boutique state based reference systems.

Dave Blodgett
Center for Integrated Data Analytics (CIDA)
USGS WI Water Science Center
8505 Research Way Middleton WI 53562
608-821-3899 | 608-628-5855 (cell)
http://cida.usgs.gov

On Jun 14, 2010, at 12:27 AM, Tom Kunicki wrote:

Hey John,

I'm trying to tidy up the GeoTIFF IOSP.  For now we're going to only support WGS84 or NAD83 (as we can explicitly request these from the WCS server) and I want to make sure I declare these datums correctly..  I am looking at the example for WGS84 (4326) in the CF-1.4 spec [1].  When looking at the the explanation of the the "longitude_of_prime_meridian" it is stated as "0.0" in the example.    The definition of the attribute states "Specifies the longitude, with respect to Greenwich, of the prime meridian associated with the geodetic datum." [2]   It was my understanding that WGS84 used the IERS reference meridian, which is ~5.?? arcsec (~100m at 51N lat) east of the prime meridian [3].   If this is true, shouldn't the "longitude_of_prime_meridian" be non-zero, something around 0.001475?  Just want to make sure I understand these parameters correctly.

Out of curiosity, what does defining these CF CRS parameters do?  Is it really even necessary?

Dave, do NAD83 and WGS84 share the same prime meridian?  Between the they appear to share the same semi-major-axis with a minor deviation in inverse-flattening.

btw-  Thanks for the shwag.  I will be sporting my Unidata T-Shirt at the OGC meetings this week.  Let me know if you'll be there by chance.  I noticed a CF-netCDF SWG meeting on Thursday, I will pop in to see what's shaking out.

Below is a small example of the JAI GeoTIFF IOSP.readData.  Performance looks good but a potential 2-3x memory overhead on the read using the currently logic below (defensive copies in the JAI code)

[1] http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/ch05s06.html

[2] http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/apf.html

[3] http://en.wikipedia.org/wiki/Prime_Meridian#IERS_Reference_Meridian

Tom Kunicki
Center for Integrated Data Analytics
U.S. Geological Survey
8505 Research Way
Middleton, WI  53562

[...]

    @Override

    public Array readData(Variable variable, Section section)

            throws IOException, InvalidRangeException {

        int image = Integer.MIN_VALUE;

        int band = Integer.MIN_VALUE;

        for(Attribute attribute : variable.getAttributes()) {

            if (ATTRIBUTE_IMAGE.equals(attribute.getName())) {

                image = attribute.getNumericValue().intValue();

            } else if (ATTRIBUTE_BAND.equals(attribute.getName())) {

                band = attribute.getNumericValue().intValue();

            }

        }

        if (image < 0 || band < 0) {

            throw new IllegalArgumentException("ERROR:  Can't extract image "

                    + "or band number from variable.");

        }

        ImageReader imageReader = null;

        ImageInputStream imageInputStream = null;

        try {

            Range yRange = section.getRange(0);

            Range xRange = section.getRange(1);

            imageReader = createImageReader();

            if (section.getStride(1) > 1 &&

                variable.getDataType().isFloatingPoint() &&

                IMAGEIO_JAI_READER.equals(imageReader.getClass().getName())) {

                throw new IllegalArgumentException(

                        "ERROR:  Data corruption bug in Java JAI ImageIO TIFF "

                        + " ImageReader with floating point data and X strides > "

                        + " 1.  To utilize this functionality please obatain another"

                        + "JAI TIFF implementation such as ImageIO-Ext and place"

                        + "in the application classpath");

            }

            imageInputStream = ImageIO.createImageInputStream(new File(location));

            imageReader.setInput(imageInputStream, true, true);

            ImageReadParam imageReadParam = imageReader.getDefaultReadParam();

            imageReadParam.setSourceRegion(new Rectangle(

                    xRange.first(),

                    yRange.first(),

                    xRange.last() - xRange.first() + 1,

                    yRange.last() - yRange.first() + 1));

            imageReadParam.setSourceSubsampling(

                    xRange.stride(),

                    yRange.stride(),

                    0,

                    0);

            imageReadParam.setSourceBands(new int[] { band } );

            BufferedImage bufferedImage = imageReader.read(image, imageReadParam);

            Raster raster = bufferedImage.getData();

            Object arrayAsObject = createJavaArray(

                    variable.getDataType(), (int)section.computeSize());

            Array array = Array.factory(

                    variable.getDataType(),

                    section.getShape(),

                    arrayAsObject);

            raster.getDataElements(

                    0, 0,

                    bufferedImage.getWidth(), bufferedImage.getHeight(),

                    arrayAsObject);

            return array;

        } finally {

            if (imageReader != null) {

                imageReader.dispose();

            }

            if (imageInputStream != null) {

                try { imageInputStream.close(); } catch (IOException e) { }

            }

        }

    }

[...]