상대좌표는 엘리먼트의 부모영역이 기준이고, position()함수를 이용해서 좌표를 알아내기만 할 수 있다.
<!DOCTYPE html>
<html>
<head>
<style>
#pink {
background-color: pink;
position:absolute;
top: 34px;
left: 56px;
width:100px;
height:100px;
}
#blue {
background-color: blue;
position:relative;
top: 5px;
left: 9px;
width:10px;
height:10px;
}
</style>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
</head>
<body>
<div id="pink">
<div id="blue"></div>
</div>
<script>
var p = $('#pink'),
b = $('#blue'),
px, py, bx, by;
px = p.offset().left;
py = p.offset().top;
alert('1p (' + px + ', ' + py + ')');
bx = b.position().left;
by = b.position().top;
alert('1b (' + bx + ', ' + by + ')');
p.offset({top:50, left:150});
px = p.offset().left;
py = p.offset().top;
alert('2p (' + px + ', ' + py + ')');
bx = b.position().left;
by = b.position().top;
alert('2b (' + bx + ', ' + by + ')');
</script>
</body>
</html>