Could not access SharePoint Area information from WebService

I am creating dropdown navigation menu for SharePoint. It should be dynamic as any user makes any changes should be reflected automatically.

I developed an C$.NET windows application to access SharePoint Portal information using Microsoft.SharePoint.Portal.SiteData. It is working fine and i am able to get all the information I need.

Then, I developed a WebService which have the same sequence of code as windows application. But I am getting error as follows.

---ERROR MESSAGE---

An unhandled exception of type 'System.Web.Services.Protocols.SoapException' occurred in system.web.services.dll

Additional information: System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> Microsoft.SharePoint.Portal.Security.AccessDeniedException: Access Denied
   at Microsoft.SharePoint.Portal.Security.aj.c()
   at Microsoft.SharePoint.Portal.SiteData.AreaManager.GetSystemAreaGuid(PortalContext ctx, SystemArea type)
   at SharePointPortalWS.SharePointPortalWS.LoadPortalStructure(String SiteName) in c:\inetpub\wwwroot\webservicesforspp\sharepointportalws\sharepointportalws.asmx.cs:line 42
   --- End of inner exception stack trace ---

---CODE OF WebService---

TopologyManager topologyManager = new TopologyManager();

PortalSiteCollection sites = topologyManager.PortalSites;

PortalContext portalContext = PortalApplication.GetContext(sites[new Uri( SiteName)].ID );

Guid homeGuid = AreaManager.GetSystemAreaGuid(portalContext, SystemArea.Home); // Here error comes.

Area homeArea = AreaManager.GetArea(portalContext, homeGuid);

AreaCollection areasTopics = homeArea.Areas;

site=new SharePointPortalSite(sites[new Uri( SiteName)].ID.ToString() ,SiteName );

--END OF CODE--

It seems there is some issue with rights required to access SharePoint Portal site through WebService and I am unable to solve this problem.

Thank you,

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