[Gdal-dev] Convert PNG map / Mercator -> equirectangular projection

[Andreas Weller]

Hi!
I used the attached Perl script for grabbing maps from
http://maps.google.com
The tiles are automatically stitched together to one big file -> now
I've got a 12000x12000 px PNG file  :-)

I wish to use it with Xastir - an amateur radio mapping program.

Google Maps seems to use mercator projection - for working
correctly the file to needs to match equirectangular projection, also
called the equidistant cylindrical projection.

Any hint how to do it? I think gdalwarp might do it but I don't know how...

Thank you!


Regards,
  Andreas Weller, DF1PAW


Here the Perl file:
#!/usr/bin/perl

# download a bunch of google map tiles and stitch them together
# Shifty Death Effect Done by Noah Vawter in May, 2005.
# Computing Culture Groop, MIT Media Lab.
# Modified for compatibility with maps.7.js -> Mmaps.10.js by Ian (June
2005)
# Modified for currently URL format used by google
# and PNG support (instead GIF) by Andreas Weller, DF1PAW (Aug. 2007)

# with width 12
$sx = 2137;$sy = 1375;   # DF1PAW home

$w=10; # Width
$h=10; # Hight

open(PEDG,">pedg.html");

for($yd=0;$yd<$h;$yd++)
{
    for($xd=0;$xd<$w;$xd++)
    {
	$x=int($sx+$xd-$w/2);
	$y=int($sy+$yd-$h/2);
	$localnem = "tile$x,$y.png";

	# do we already have it locally?
	$val = open(CHECK,$localnem);
	if($val == 0){
	    # http://mt.google.com/mt?v=w2.4&x=4190&y=6012&zoom=3
	    #   523  wget "http://mt.google.com/mt?v=.38&x=5376&y=-730&zoom=2"
	    # have a look at the Url and modify these lines:
	    $req="http://mt.google.com/";
	    $nem="mt?v=w2.60&x=$x&y=$y&zoom=5";
	    $url = $req . $nem;
	    print "$url\n";
	    $cmd1="wget \"$url\" ";
	    print "$cmd1\n";
	    system($cmd1);
	    # $nem =~ s/\?/@/;
	    $cmd2="mv \"$nem\" $localnem";
	    print "$cmd2\n";
	    system($cmd2);
	}

	print PEDG "<img src=$localnem>\n";
    }
}

# concatenate horizontal maps
# convert  tile005000.png  -page +129+0 tile006000.png -page +258+0
tile007000.png -mosaic o.png

for($yd=0;$yd<$h;$yd++)
{
    $cmd3 = "convert ";

    for($xd=0;$xd<$w;$xd++)
    {
	$x=int($sx+$xd-$w/2);
	$y=int($sy+$yd-$h/2);
	$localnem = "tile$x,$y.png";
	
	$xsh = $xd*128;
	$ysh = $yd*128;
	$cmd3 .= "$localnem ";
    }

    $cmd3 .= "+append tmp$yd.png";
    print "$cmd3\n";
    system($cmd3);
}

# concatenate horizontal strips
$cmd4 = "convert ";

for($yd=0;$yd<$h;$yd++)
{
    $localnem = "tmp$yd.png";
    $cmd4 .= "$localnem ";
}

$cmd4 .= "-append output.png";
print "$cmd4\n";
system($cmd4);