func load_map ( ffn= ) {
/* DOCUMENT load_map(ffn=)

   Load a NOAA/USGS geographical coastline lat/lon map into the present 
   window.  This is useful if you want to project some GPS positions 
   onto a map easily.  Yorick lets you quickly and easily pan and zoom 
   on the map and your data.

   Get the maps from: http://crusty.er.usgs.gov/coast/getcoast.html
   Use the "mapgen" format option. After you download the map, remove 
   the first line (which is a comment beginning with a # sign).  This 
   function expects the map files to end with a .amap extension.  This 
   function uses sel_file to solicite a filename from the user.

   C. W. Wright wright@web-span.com  99-03-20

   See also:   sel_file

*/
extern map_path

if ( is_void( map_path )  ) {

// Set the following to where the maps are stored on your system.
  map_path = "/home/wright/maps"	
}

if ( is_void( ffn ) ) { 
  ffn = sel_file(ss="*.amap", path="/home/wright/maps/") (1);
}

f = open(ffn, "r" );
lsegs = 0
str = array(string,1);
lat = array(float, 1000)
lon = array(float, 1000)

 if (catch(0x02) ) { 
    close,f
    return;
  }

// load upto 100,000 line segments
for (i=0; i<100000; i++) {
 n = read(f,format="%f %f", lon,lat)
 if ( n == 0 ) {
   close,f
   write,i," line segments loaded"
   return; 
 }
 n = n/2
 lsegs++
 if ( lat(1) == 0 ) break;
 if ( (i % 1000) == 0  ) { 
   print,i
   redraw
 }
 plg,lat(1:n),lon(1:n),marks=0
 gridxy,1,1
}
 n=write(format="%d line segments read from %s\n", lsegs, fn)
 close,f
}