shortlink class in C#

Today I wrote a VB.Net function for decoding an OSM shortlink, based on the Ruby example. This is now part of my map viewer application:
http://code.google.com/p/vataviamap/
The source file containing this function is available at:
http://code.google.com/p/vataviamap/source/browse/trunk/VataviaMap/Shared/clsServer.vb

Private Const OSMshortlinkEncoding As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_~"

    ''' <summary>
    ''' Parse an OpenStreetMap shortlink code into Latitude, Longitude and Zoom
    ''' </summary>
    ''' <param name="aCode">OpenStreetMap shortlink code</param>
    ''' <param name="aCenterLatitude">returns latitude at center of area</param>
    ''' <param name="aCenterLongitude">returns longitude at center of area</param>
    ''' <param name="aZoom">returns zoom level</param>
    ''' <returns>True if link was parsed and reasonable values for the ByRef arguments were found</returns>
    ''' <remarks>https://help.openstreetmap.org/questions/9566/shortlink-class-in-c</remarks>
    Private Function ParseOSMshortlink(ByVal aCode As String, _
                                       ByRef aCenterLatitude As Double, _
                                       ByRef aCenterLongitude As Double, _
                                       ByRef aZoom As Integer) As Boolean
        'http://osm.org/go/0MbEUuTq = http://www.openstreetmap.org/?lat=52.50547&lon=13.36932&zoom=16
        Dim x As Long = 0
        Dim y As Long = 0
        Dim z As Long = 0
        Dim z_offset As Long = 0

        Try
            ' replace @ in old shortlinks with ~
            aCode = aCode.Replace("@", "~")

            For Each ch As Char In aCode.ToCharArray
                Dim t As Integer = OSMshortlinkEncoding.IndexOf(ch)
                If t < 0 Then
                    z_offset -= 1
                Else
                    For index As Integer = 1 To 3
                        x <<= 1
                        If t And 32 Then x += 1
                        t <<= 1
                        y <<= 1
                        If t And 32 Then y += 1
                        t <<= 1
                    Next
                    z += 3
                End If
            Next
            ' pack the coordinates out to their original 32 bits.
            x <<= (32 - z)
            y <<= (32 - z)

            ' project the parameters back to their coordinate ranges.
            aCenterLongitude = (x * 360.0 / 2 ^ 32) - 180.0
            aCenterLatitude = (y * 180.0 / 2 ^ 32) - 90.0
            aZoom = z - 8 - (z_offset Mod 3)
            Return aCenterLatitude > -90 AndAlso aCenterLatitude < 90 AndAlso _
                aCenterLongitude >= -180 AndAlso aCenterLongitude <= 180
        Catch
            Return False
        End Try
    End Function