[fitswcs] Question about the Mercator projection.

[David Berry]

Hi Bill,
           As I understand it DS9 uses the AST library for plotting
its coordinate grids, and AST does produce curved lines, so I'm
confused. Maybe Bill Joye can comment. If you go to

http://starlink.jach.hawaii.edu/cgi-bin/ast/fits-plotter

and paste your FITS headers into the "Headers" box,  then enter
"grid=1" into the "plot settings" box (to get grid lines rather than
just tick marks) and then press submit, you will see what AST
produces.

David



On 9 December 2010 22:32, William Thompson <William.T.Thompson at nasa.gov> wrote:
> I'm running into an issue with the Mercator projection that makes me ask whether
>  I understand the way that cylindrical projections work.  I have an image with
> the following FITS header (relevant keywords only):
>
> SIMPLE  =                    T / file does conform to FITS standard
> BITPIX  =                  -32 / number of bits per data pixel
> NAXIS   =                    2 / number of data axes
> NAXIS1  =                  500 / length of data axis 1
> NAXIS2  =                  272 / length of data axis 2
> CTYPE1  = 'HGLN-MER'
> CTYPE2  = 'HGLT-MER'
> CRPIX1  = 250.500000
> CRPIX2  = 136.500000
> CRVAL1  = 59.889179
> CRVAL2  = 21.644938
> CDELT1  = 0.030000
> CDELT2  = 0.030000
> CUNIT1  = 'degree'
> CUNIT2  = 'degree'
> END
>
> In the WCS software that I wrote, the resulting lines of longitude and latitude
> are slightly curved, because the reference pixel is not on the equator.
> However, if I open this same file in ds9, the lines of longitude and latitude
> are straight across.  Am I missing something?
>
> Bill Thompson
>
> --
> William Thompson
> NASA Goddard Space Flight Center
> Code 671
> Greenbelt, MD  20771
> USA
>
> 301-286-2040
> William.T.Thompson at nasa.gov
>
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