PnmMercator User Manual(0) PnmMercator User Manual(0)NAMEpnmmercator - transform a worldmap from rectangular projection to Mer‐
cator projection and vice-versa
SYNOPSISpnmmercator [-inverse] [-nomix] [-[v]verbose] [filename]
Minimum unique abbreviation of option is acceptable. You may use dou‐
ble 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 rec‐
tangular 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 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 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 nav‐
igators 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
<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 dou‐
ble the 'v', it will output debug data about the lat/long degree
and Mercator conversions.
SEE ALSOpnm(1)
and pamscale(1)ppmglobe(1)HISTORYpnmmercator 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)
