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
       ⟨http://en.wikipedia.org/wiki/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 banding 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
       ⟨http://www.evl.uic.edu/pape/data/Earth/⟩ .

       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 compass bearing of northeast from that spot.  Naturally,
       primitive navigators appreciated that.  The biggest drawback of Mercator is that areas  to
       the north and south appear much larger than they are in real life.  For example, 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

       In addition to the options common to all programs based on libnetpbm (most notably -quiet,
       see
        Common Options ⟨index.html#commonoptions⟩ ), pnmmercator recognizes the following command
       line options:

       -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.

DOCUMENT SOURCE

       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/pnmmercator.html