OGR

[Gregor Mosheh]

Hi, Andreas.
Yeah, I found the OGR documentation very dense too -- then again, the OGR
stuff is for C instead of PHP, so it takes a little imagination and
experimentation to map it over. Here are a few functions I whipped up a
few months ago, which should get you started.

The first just opens a data source and returns the spatial extent....

function find_extent($shapefilename) {
   // open the data source
   // cheesy: we iterate over drivers 'til we find the one we know is right
   OGRRegisterAll();
   $numDrivers = OGRGetDriverCount();
   for ($i=0; $i<$numDrivers; $i++) {
      $driver = OGRGetDriver($i);
      if (OGR_Dr_GetName($driver) == "ESRI Shapefile" ) { break; }
   }
   $datasource = OGR_Dr_Open($driver, $shapefilename, NULL);

   // fetch the layer and its extent
   $layer = OGR_DS_GetLayer($datasource,0);
   OGR_L_GetExtent($layer,$extent,1);
   return $extent;
}


This second one is more complex. It takes an array of associative arrays,
each one representing a point. It builds a pair of shapefiles, the sample
points and also lines connecting the points. We wrote this for a vehicle
tracking system.

function create_data_sources($basename,$points) {
   global $TEMPDIR;
   $shapefilename_points = "$TEMPDIR/points_$basename.shp";
   $shapefilename_tracks = "$TEMPDIR/tracks_$basename.shp";

   // fetch the driver we want: shapefile
   // sadly, fetching a driver by name is rather roundabout
   OGRRegisterAll();
   $numDrivers = OGRGetDriverCount();
   for ($i=0; $i<$numDrivers; $i++) {
      $driver = OGRGetDriver($i);
      if (OGR_Dr_GetName($driver) == "ESRI Shapefile" ) { break; }
   }

   // delete any existing data source by this name, in preparation for
overwriting
   @unlink($shapefilename_points);
   @unlink(basename($shapefilename_points,'.shp').'.shx');
   @unlink(basename($shapefilename_points,'.shp').'.dbf');
   @unlink($shapefilename_tracks);
   @unlink(basename($shapefilename_tracks,'.shp').'.shx');
   @unlink(basename($shapefilename_tracks,'.shp').'.dbf');

   // fetch the SRS reference.
   // we always use latlong NAD83, EPSG:4326 so "nothing" is fine for us.
   $srs = NULL;

   // create the target data sources, then create a layer in each source
   $points_file = OGR_Dr_CreateDataSource($driver, $shapefilename_points,
NULL);
   $tracks_file = OGR_Dr_CreateDataSource($driver, $shapefilename_tracks,
NULL);

   // create the layer definition for the points file
   $points_layer = OGR_DS_CreateLayer($points_file, 'points', $srs,
wkbPoint, NULL);
   $field = OGR_Fld_Create( "POINTID", OFTInteger );
   OGR_L_CreateField($points_layer, $field, 0);
   $field = OGR_Fld_Create( "DIRECTION", OFTString );
   OGR_L_CreateField($points_layer, $field, 0);
   $points_layerdefn = OGR_L_GetLayerDefn($points_layer);
   // create the layer definition for the tracks file
   $tracks_layer = OGR_DS_CreateLayer($tracks_file, 'tracks', $srs,
wkbLineString, NULL);
   $tracks_layerdefn = OGR_L_GetLayerDefn($tracks_layer);

   // create a single line in the tracks file
   $track_feature = OGR_F_Create($tracks_layerdefn);
   $track_geom = OGR_G_CreateGeometry(wkbLineString);

   // iterate through the supplied points, stuffing them into the points
and also forming a track
   global $ZOOMMULTIPLIER;
   for ($i=0; $i<sizeof($points); $i++) {
      // figure out this point's relation to the next point; northeast,
south, etc.
      $points[$i]['x'] = (float) $points[$i]['x'];
      $points[$i]['y'] = (float) $points[$i]['y'];
      $direction = '';
      if ($i < sizeof($points)-1) {
         $xdiff = $points[$i+1]['x'] - $points[$i]['x'];
         $ydiff = $points[$i+1]['y'] - $points[$i]['y'];
         if ($ydiff >= $ZOOMMULTIPLIER/10) { $direction .= 'N'; }
         elseif ($ydiff <= -$ZOOMMULTIPLIER/10) { $direction .= 'S'; }
         if ($xdiff >= $ZOOMMULTIPLIER/10) { $direction .= 'E'; }
         elseif ($xdiff <= -$ZOOMMULTIPLIER/10) { $direction .= 'W'; }
      }

      // add the point to the points file
      $point_feature = OGR_F_Create($points_layerdefn);
      OGR_F_SetFieldInteger( $point_feature, 0, $points[$i]['id']);
      OGR_F_SetFieldString( $point_feature, 1, $direction);
      $point_geom = OGR_G_CreateGeometry(wkbPoint);
      OGR_G_SetPoint($point_geom, 0, $points[$i]['x'], $points[$i]['y'], 0);
      OGR_F_SetGeometry($point_feature, $point_geom);
      OGR_L_CreateFeature( $points_layer, $point_feature );

      // add the point to the track
      OGR_G_AddPoint($track_geom, $points[$i]['x'], $points[$i]['y'], 0);
   }

   // go ahead and terminate the line
   OGR_F_SetGeometry($track_feature, $track_geom);
   OGR_L_CreateFeature($tracks_layer, $track_feature );

   // done, close the data source
   OGR_DS_Destroy($points_file);
   OGR_DS_Destroy($tracks_file);
   return true;
}