Geocaching & GPS location based games

#Höhendaten aus SRTM Files
#1.) Download Höhendaten von ftp://e0srp01u.ecs.nasa.gov/srtm/version2/SRTM3/Eurasia
#2.) Unzip Höhendaten
#3.) Dieses Perl Script aus gleichem Verzeichnis starten
#usage: perl srtm_height.pl lat lon
#Online Check per http://www.earthtools.org/webservices.htm, z.B. http://www.earthtools.org/height/49/12

$numArgs = $#ARGV + 1;
if ($numArgs < 2)
{
  printf "usage: perl srtm_height.pl lat lon";
  exit;
}

#input
my $lat=$ARGV[0];
my $lon=$ARGV[1];

#lower left corner
my $ll_lat=int($lat);
my $ll_lon=int($lon);

#filename
my $filename = "";
if ($ll_lat >= 0) { $filename = "N"; } else {$filename = "S"; };
$filename.=sprintf("%02d",abs($ll_lat));
if ($ll_lon >= 0) { $filename .= "E"; } else {$filename .= "W"; };
$filename.=sprintf("%03d.hgt",abs($ll_lon));
#printf ("%s\n", $filename);

#read file in one big array
open(FILE, $filename);
binmode(FILE);
my $buffer;
my $num_bytes = 1201*1201*2;
read (FILE, $buffer, $num_bytes);
close(FILE);

#calculate offset, see ftp://e0srp01u.ecs.nasa.gov/srtm/version2/Documentation/SRTM_Topo.pdf
my $row_offset = ( ($ll_lat+1.0-$lat)*1200.0);
#printf ("%f\n", $row_offset);
$row_offset = int($row_offset+0.5); #round
my $col_offset = ( ($lon-$ll_lon)*1200.0);
#printf ("%f\n", $col_offset);
$col_offset = int($col_offset+0.5); #round
my $byte_offset = $row_offset*1201+$col_offset;
#printf ("%f\n", $byte_offset);

#get height from buffer
my $height = vec($buffer, $byte_offset, 16);
print $height;