In order to get polyline area and/or perimeter use render/calculate_polyline:

svc=render/calculate_polyline&params={"p":[
                                            {
                                                "x":<double>,
                                                "y":<double>
                                            },
                                            ...
                                      ],
                                      "flags":<uint>,
                                      "w":<uint>}

{
    "area":<double>,      /* area */
    "perimeter":<double>  /* perimeter */
}