I am trying to round latitude / longitude positions to the nearest 50m accurately using Oracle Spatial. Currently I am converting each position to British National Grid to obtain coordinates in meters. This then allows me to round the X and Y coordinate to the nearest 50m. I then convert the result back to lat / lon format. This works fine for a series of lat / lon positions in Britain because the rounded positions overlay well with the original data. However, for other areas the rounded values become skewed and the overall series of positions is rotated. Obviously this is because I am using the wrong coordinate system.

I would like to be able to round the positions to the nearest 50m without doing a coordinate system transform (stay in WGS84). Since ‘SDO_DISTANCE’ returns the distance in meters (accurate?) I am wondering if it does a conversion within the function to get the lat lon into x/y coordinates. Looking at the function within MDSYS does not help much as it is in hex.

A simplified example of what I am doing is shown in the code below. Any help is much appreciated.

	-- stores lat / lon of each position	
	pos1Lat FLOAT;
	pos1Lon FLOAT;
	pos2Lat FLOAT;
	pos2Lon FLOAT;
	-- distance between 2 points
	theDistance FLOAT;
	-- spatial
    x FLOAT;
    y FLOAT;

	pos1Lat := 51.413481;
	pos1Lon := -0.182495;
	pos2Lat := 51.415825;
	pos2Lon := -0.189712;
	theDistance := 0;
	-- 8307 = WGS84
	-- 81989 = British National Grid
	-- this gives the distance between the 2 positions in meters
		(mdsys.sdo_geometry(2001,8307,mdsys.sdo_point_type(pos1Lon, pos1Lat, NULL), NULL, NULL),
                mdsys.sdo_geometry(2001,8307,mdsys.sdo_point_type(pos2Lon, pos2Lat, NULL), NULL, NULL),

	-- my current method of rounding a position to a 50m grid
	-- set up spatial instance
    latlon := MDSYS.SDO_GEOMETRY(2001, 8307, mdsys.sdo_point_type(pos1Lon, pos1Lat, NULL), NULL, NULL);
    -- convert to British National Grid
    xy := MDSYS.SDO_CS.TRANSFORM(latlon, 81989);
    x := xy.sdo_point.x;
    y := xy.sdo_point.y;
    --Round to nearest 50m
    x := 50 * ROUND(x / 50);
    y := 50 * ROUND(y / 50);
    xy := MDSYS.SDO_GEOMETRY(2001, 81989, mdsys.sdo_point_type(x, y, NULL), NULL, NULL);
    -- convert back to lat/lon format
    latlon := MDSYS.SDO_CS.TRANSFORM(xy, 8307);	

	dbms_output.put_line('The actual distance is ' || theDistance);
	dbms_output.put_line('Position 1 rounded ' || latlon.sdo_point.x || ' ' || latlon.sdo_point.y);