[Mapserver-users] Map Extent

[Eric Bridger]

Hi all.  I am a new mapserver, mapscript user, about 2 weeks now.  So
first my thanks to all involved in developing a truly excellent
application.  We are using Mapserver 3.6.4 and mapscript to develop an
Linux Apache/mod-perl oceanographic application.

I am having trouble mapping the users map image mouse click to map
coordinates with precision.  At first I thought it might be the
registration of the map datasets we were using (shape files) with one
shaded relief jpg, so I switched to using the states_ugl data and the
s1ex1.map from the 3.6 tutorial in a simple mapscript test and noticed
the same issues.

When adding a point symbol to the map at the location clicked, the
symbol appears near where I expected but not exactly where I expected. 
The closer I click to the center of the map the closer the symbol
registers.  It appears to me that the y-coordinate is always on target,
but that the x corrdinate is off. The closer to the edges of the map,
the further off the x coordinate is.

Perhaps this has to do with the fact that the map created 400x300 goes
all the way from y=0 to y=400, but not from x=0 to x=300?

The mapextent is gotten from the map object created from the map file.

I calculate the map coordinates with code adapted from the mapquakes.pl
example: 
if($q->param('img.x') && $q->param('imgext')) { # Make sure we got a
click
    @imgext = split(' ', $q->param('imgext'));
    $x = $q->param('img.x');
    $y = $q->param('img.y');

    $cx = ($imgext[2]-$imgext[0])/($map->{width}-1);  
    $cy = ($imgext[3]-$imgext[1])/($map->{height}-1);

    $x = $imgext[0] + $cx*$x;
    $y = $imgext[3] - $cy*$y;
}

Create a point object and add it to the map and display it.


Par of the map file:
MAP
EXTENT -97.238976 41.619778 -82.122902 49.38562
SIZE 400 300
SHAPEPATH "/home/eric/work/htdocs/mapserver/tutorial35/data"
# Added by EB
IMAGETYPE PNG
UNITS DD

LAYER # States
  NAME "lobster_zones"
  DATA states_ugl
  STATUS DEFAULT
  TYPE POLYGON
  CLASSITEM "STATE"
  CLASS
    NAME "Michigan"
    COLOR 255 255 128
    OUTLINECOLOR 0 0 0
  END # end of class object
END # end of layer object

....

Thanks.

Eric Bridger
eric at gomoos.org
Gulf of Maine Ocean Observing System www.gomoos.org