how to call action controller method from razor using ajax

[HttpPost]
        public JsonResult WriteCoordiatesNew(int catid)
        {
            try
            {
                List<SubCategory> sublstcontent = new List<SubCategory>();
                {
                    //Get all page content from TblHome table
                    sublstcontent = DatabaseAccess.SubCategory(catid);

                };

                List<SelectListItem> subcatlist = new List<SelectListItem>();
                //List<string> items = new List<string>();
                foreach (var item in sublstcontent)
                {
                    subcatlist.Add(new SelectListItem
                    {
                        Text = item.sub_cate,
                        Value = item.sub_cat_id.ToString()
                    });
                }

                return Json(subcatlist, JsonRequestBehavior.AllowGet);
            }
            catch (Exception ee)
            {
                Console.WriteLine(ee.Message);

                //WriteLog("Exception in Saving Canvas " + txtLocation + " " + ee.Message);
            }

            return null;
        }

@Html.Label("Select Category")
                    @Html.DropDownList("category_name", (IEnumerable<SelectListItem>)ViewBag.categorylist, "--Select--", new { id = "drpCate", onChange = "dropdownsub()" })
                    <br />
                    @Html.Label("Select Sub-Category")
                    @Html.DropDownList("subcat_name", (IEnumerable<SelectListItem>)ViewBag.SubCatLst, "--Select--", new { id = "drpSubCate" })

function dropdownsub() {

                var urlMap = document.getElementById('SubCat').value;
                //alert("URL: "+$("#hidURL").val());                
                var sel = document.getElementById('drpCate').value;
                alert(sel);
                try {

                    //var newDataURl=dataURL.replace('data:image/png;base64,', '');

                    $.ajax({
                        //url: "/AnswerSheetValuation/Main/WriteCoordiatesNew",
                        url: urlMap,
                        type: "POST",
                        contentType: 'application/json; charset=utf-8',
                        data: JSON.stringify({ 'catid': sel }),
                        success: function (mydata) {
                            alert('Success');
                        }
                    });

                }
                catch (e) {
                    alert(e);
                }

                //return dataURL;
            }