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PnmMercator User Manual(0)                          PnmMercator User Manual(0)



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NAME
       pnmmercator - transform a worldmap from rectangular projection to Mercator projection and vice-versa


SYNOPSIS
       pnmmercator [-inverse] [-nomix] [-[v]verbose] [filename]

       Minimum  unique  abbreviation  of option is acceptable.  You may use double hyphens instead of single hyphen to
       denote options.


DESCRIPTION
       This program is part of Netpbm(1).

       The pnmmercator utility, converts a rectangular projection worldmap to a Mercator projection  format,  as  used
       for  maps.google.com  and many other online maps.  The map used as input for pnmmercator must have rows for -90
       to 90 degrees latitude and columns for -180 to +180 degrees longitude. The file will typically be twice as wide
       as  high,  but  this  is not a requirement. The output file will be using the Mercator projection  and will get
       double the height of the input file.

       Maps using the Mercator projection are stretched more the closer a row is to the North or South Pole. The  last
       few  degrees  (> 85 or < -85 degrees) are not part of a Mercator map at all because they would be stretched too
       much and the rows close to the edge will show banding, because they originate from the same row in the original
       map.

       To  overcome this, the program will by default do interpolation of pixel colors, which will eliminate the band-
       ing effect, but will cause some blurring of the output. With the -nomix option, this  interpolation  of  colors
       isn't  applied.  You can obtain the highest quality output by starting with an input map of high resolution, so
       that you can follow the pnmmercator transformation with a pamscale reduction in size.

       This program can also convert a Mercator projection map back to a rectangular projection based.  As  said,  the
       Mercator  map  doesn't  have information about the latitudes close to the poles.  Therefore the top rows in the
       output image will be identical and copied from the row corresponding with latitude of 85 degrees. The  same  at
       the bottom of the map.

       Pnmmercator doesn't have any provision for scaling the image. You can scale by piping the output of the program
       through Netpbm programs such as pamscale.

       You can find maps to be used as input at flatplanet.sourceforge.net(1) or uic.edu/pape .

       The point of a Mercator projection map is that compass directions work on it.  If  you  draw  a  straight  line
       northeast  from  some point on the Mercator map, the line traces the course you would sail if you sailed a com-
       pass bearing of northeast from that spot.  Naturally, primitive navigators appreciated that.  The biggest draw-
       back of Mercator is that areas to the north and south appear much larger than they are in real life.  For exam-
       ple, Greenland appears to be larger than South America even though it only a ninth as large.  Note  that  areas
       away from the equator are stretched north-south as well as east-west.

       A  rectangular  projection  is  one where vertical distance is proportional to angular latitude distance of the
       represented area and horizontal distance is proportional to angular longitude.



PARAMETERS
       filename is the name of the input file.  If you don't specify this, pnmmercator reads the image  from  standard
       Input.


OPTIONS
       <dl compact="compact">

       -inverse

              With  this  option  a conversion from Mercator to degrees is applied.The output image will have half the
              height of the input map.


       -nomix

              Default behaviour is that color blending is applied in between two adjacent rows.  If  you  specify  the
              -nomix  parameter  there  is no blending. The consequence is a banding at the top and bottom of the map.
              With this option, the output map will also consist of exactly the same colors as the input.


       -verbose and -vverbose

              This parameter outputs some additional information. If you double the 'v', it  will  output  debug  data
              about the lat/long degree and Mercator conversions.




SEE ALSO
       pnm(1)and pamscale(1) ppmglobe(1)


HISTORY
       pnmmercator was new in Netpbm 10.49 (December 2009).


AUTHORS
       Willem van Schaik (of pnmtopng/pngtopnm fame) wrote this program in October 2009 and suggested it for inclusion
       in Netpbm.



netpbm documentation             October 2009       PnmMercator User Manual(0)