man > pnmmercator(1)

PnmMercator User Manual(1)             General Commands Manual            PnmMercator User Manual(1)

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 de‐
       grees longitude. The file will typically be twice as wide as high, but this is not a require‐
       ment. The output file will be using the ]8;;http://en.wikipedia.org/wiki/Mercator_projection\Mercator projection]8;;\  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 band‐
       ing, 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 op‐
       tion,  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 ]8;;http://www.evl.uic.edu/pape/data/Earth/\uic.edu/pape]8;;\ .

       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  longi‐
       tude.



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


OPTIONS
       In  addition  to  the options common to all programs based on libnetpbm (most notably -quiet,
       see ]8;;index.html#commonoptions\ Common Options]8;;\ ), 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  sug‐
       gested 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

netpbm documentation                        October 2009                  PnmMercator User Manual(1)