Numpty's Progress

Tag: maidenhead

  • More radio amateur grid squares

    Toronto, as understood by the Maidenhead Locator system

    After yesterday’s post, I went a bit nuts with working out the whole amateur radio grid locator thing (not that I’m currently likely to use it, though). I’d hoped to provide a shapefile of the entire world, but that would be too big for the format’s 2GB file size limit.

    What I can give you, though, is:

    • A Perl program that will generate a shapefile of an entire Maidenhead grid field, down to the subsquare level: make_grid.pl. You’ll need Geo::Shapelib to make this work. 324 (= 182) of these files would cover the whole world, and at 8MB or so a pop, things get unwieldy quickly.
    • A Google Earth KML file covering the whole world in 20° by 10° grid fields: Maidenhead_Locator_World_Grid. (If you’re feeling nerdy, here it is in Shapefile format: Maidenhead_Locator_World_Grid-shp).

    If anyone would like their grid square in Google Earth format, let me know, or read on …

    Making KML Files

    Several people have asked, so here’s how you convert to KML. You’ll need the OGR toolkit installed, which comes in several open-source geo software bundles: FWTools/osgeo4w/QGis. Let’s assume we want to make the grid square ‘EN’.

      1. Run make_grid.pl: 
        make_grid.pl en
      2. Convert to KML using ogr2ogr: 
        ogr2ogr -f KML EN-maidenhead_grid.kml EN-maidenhead_grid.shp
      3. Alternatively, if you just want to extract a square (say EN82), you can use ogr2ogr’s ‘where’ clause to select just the geometry you want: 
        ogr2ogr -f KML -where "Square='82'" EN82-maidenhead_grid.kml EN-maidenhead_grid.shp

     

  • Maidenhead Grid Locator, in Perl

    Amateur radio operators sometimes use the Maidenhead Grid Locator Square system to identify location. Here’s my attempt at a translator, written in slightly squirrelly Perl:

    #!/usr/bin/perl -w
# maidenhead - 6 character locator
# created by scruss/VA3PID on 02011/04/01
# RCS/CVS: $Id: maidenhead,v 1.3 2011/04/03 11:04:38 scruss Exp $

use strict;
sub latlong2maidenhead;
sub maidenhead2latlong;

if ( $#ARGV >= 1 ) {    # two or more args
                        # convert lat/long to grid
  print latlong2maidenhead( $ARGV[0], $ARGV[1] ), "\n";
}
elsif ( $#ARGV == 0 ) {    # one arg
                           # convert grid to lat/long
  print join( ", ", maidenhead2latlong( $ARGV[0] ) ), "\n";
}
else {                     # no args

  # print usage
  print 'Usage: ', $0, ' dd.dddddd ddd.dddddd', "\n",
    ' or', "\n",
    $0, ' grid_location', "\n";
}
exit;

sub maidenhead2latlong {

  # convert a Maidenhead Grid location (eg FN03ir)
  #  to decimal degrees
  # this code could be cleaner/shorter/clearer
  my @locator =
    split( //, uc(shift) );    # convert arg to upper case array
  my $lat      = 0;
  my $long     = 0;
  my $latdiv   = 0;
  my $longdiv  = 0;
  my @divisors = ( 72000, 36000, 7200, 3600, 300, 150 )
    ;                          # long,lat field size in seconds
  my $max = ( $#locator > $#divisors ) ? $#divisors : $#locator;

  for ( my $i = 0 ; $i <= $max ; $i++ ) {
    if ( int( $i / 2 ) % 2 ) {    # numeric
      if ( $i % 2 ) {             # lat
        $latdiv = $divisors[$i];    # save for later
        $lat += $locator[$i] * $latdiv;
      }
      else {                        # long
        $longdiv = $divisors[$i];
        $long += $locator[$i] * $longdiv;
      }
    }
    else {                          # alpha
      my $val = ord( $locator[$i] ) - ord('A');
      if ( $i % 2 ) {               # lat
        $latdiv = $divisors[$i];    # save for later
        $lat += $val * $latdiv;
      }
      else {                        # long
        $longdiv = $divisors[$i];
        $long += $val * $longdiv;
      }
    }
  }
  $lat  += ( $latdiv / 2 );         # location of centre of square
  $long += ( $longdiv / 2 );
  return ( ( $lat / 3600 ) - 90, ( $long / 3600 ) - 180 );
}

sub latlong2maidenhead {

  # convert a WGS84 coordinate in decimal degrees
  #  to a Maidenhead grid location
  my ( $lat, $long ) = @_;
  my @divisors =
    ( 72000, 36000, 7200, 3600, 300, 150 );    # field size in seconds
  my @locator = ();

  # add false easting and northing, convert to seconds
  $lat  = ( $lat + 90 ) * 3600;
  $long = ( $long + 180 ) * 3600;
  for ( my $i = 0 ; $i < 3 ; $i++ ) {
    foreach ( $long, $lat ) {
      my $div  = shift(@divisors);
      my $part = int( $_ / $div );
      if ( $i == 1 ) {    # do the numeric thing for 2nd pair
        push @locator, $part;
      }
      else {              # character thing for 1st and 3rd pair
        push @locator,
          chr( ( ( $i < 1 ) ? ord('A') : ord('a') ) + $part );
      }
      $_ -= ( $part * $div );    # leaves remainder in $long or $lat
    }
  }
  return join( '', @locator );
}

    Grid square to lat/long and grid square shapefiles to follow.