Store you current translate value or retrieve it from transform attribute and actualize it ...

Something as

tran_x = 0
tran_y = 0

function Up(dir){
switch(dir)
{
case "up" :
tran_y -= 100;
break
case "down" :
tran_y += 100;
break
case "right" :
tran_x += 100;
break
case "left" :
tran_x -= 100;
break
}
SVGRoot.setAttribute('transform','translate(' + tran_x + ',' + tran_y + ')');
}

Michel