In order to get polygon area and/or perimeter use render/calculate_polygon:

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

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