def convexHull(self,
                   geometries,
                   sr=None):
        """
        The convexHull operation is performed on a geometry service resource. 
        It returns the convex hull of the input geometry. The input geometry can 
        be a point, multipoint, polyline, or polygon. The convex hull is typically 
        a polygon but can also be a polyline or point in degenerate cases.
        
        Inputs:
            geometries - array of geometries (structured as JSON geometry 
                         objects returned by the ArcGIS REST API).
            sr - spatial reference of the input geometries WKID.
        """
        url = self._url + "/convexHull"
        params = {
            "f" : "json"
        }

        if isinstance(geometries, list) and len(geometries) > 0:
            g = geometries[0]
            if sr is not None:
                params['sr'] = sr
            else:
                params['sr'] = g._wkid
            if isinstance(g, Polygon):
                params['geometries'] = {"geometryType": "esriGeometryPolygon",
                                        "geometries" : self.__geomToStringArray(geometries, "list")}
            elif isinstance(g, Point):
                params['geometries'] = {"geometryType": "esriGeometryPoint",
                                        "geometries" : self.__geomToStringArray(geometries, "list")}
            elif isinstance(g, Polyline):
                params['geometries'] = {"geometryType": "esriGeometryPolyline",
                                                        "geometries" : self.__geomToStringArray(geometries, "list")}
        else:
            return None
        return self._get(url=url,
                            param_dict=params,
                            securityHandler=self._securityHandler,
                            proxy_port=self._proxy_port,
                            proxy_url=self._proxy_url)
The convexHull operation is performed on a geometry service resource. 
        It returns the convex hull of the input geometry. The input geometry can 
        be a point, multipoint, polyline, or polygon. The convex hull is typically 
        a polygon but can also be a polyline or point in degenerate cases.
        
        Inputs:
            geometries - array of geometries (structured as JSON geometry 
                         objects returned by the ArcGIS REST API).
            sr - spatial reference of the input geometries WKID.