PnmMercator User Manual(1) General Commands Manual PnmMercator User Manual(1)
NAME
pnmmercator - transform a worldmap from rectangular projection to Merca-
tor 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 longi-
tude. The file will typically be twice as wide as high, but this is not
a requirement. 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 de-
grees) 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, be-
cause 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 rec-
tangular projection based. As said, the Mercator map doesn't have in-
formation about the latitudes close to the poles. Therefore the top
rows in the output image will be identical and copied from the row cor-
responding 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 navi-
gators 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 dis-
tance is proportional to angular longitude.
PARAMETERS
filename is the name of the input file. If you don't specify this, pnm-
mercator reads the image 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 ap-
plied.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 Oc-
tober 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
netpbm documentation October 2009 PnmMercator User Manual(1)
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