[OSM-newbies] Mercator problems

[Bettersoftware]

I'm implementing a "slippy" map using VB.Net in a Windows application and everything works quite well at zooms 10 to 17.  On the "Slippy" map I indicate the lat/lon of the cursor position and the lat/lon of the map centre. However, at zooms < 10 the accuracy of the Latitude degrades considerably. The Longitude accuracy also degrades but to a lesser extent.

After two weeks of debugging I'm stumped as my maths knowledge is not up to the task in hand.

The error I think is on in these two pieces of code :- ( You Javascript people should be able to follow this easily)
Private Function getMercatorLatitude(ByVal lat As Double, ByVal Zoom As Integer) As Single



If (lat > 89.5) Then

lat = 89.5

End If

If (lat < -89.5) Then

lat = -89.5

End If

Dim phi As Double = Math.PI * lat / 180

Dim res As Double = 0.5 * Math.Log((1 + Math.Sin(phi)) / (1 - Math.Sin(phi)))

Dim maxTileY As integer = Math.Pow(2, Zoom)

Dim Tile As Single = ((1 - res / Math.PI) / 2) * maxTileY

Return Tile

End Function

Private Function getOSMTileDetails(ByVal x As Integer, ByVal y As Integer, ByVal zoom As Integer) As Object

' 

' Returns a Rectangle2D with x = lon, y = lat, width=lonSpan, height=latSpan 

' for an x,y,zoom as used by OSM/Google. 

' 

Dim lon As Double = -180 ' x 

Dim lonWidth As Double = 360 ' width 360 

Dim lat As Double = -1

Dim latHeight As Double = 2

Dim tilesAtThisZoom As Integer = Math.Pow(2, zoom)

lonWidth = 360.0 / tilesAtThisZoom

lon = -180 + (x * lonWidth)

latHeight = 2.0 / tilesAtThisZoom

lat = ((tilesAtThisZoom / 2 - y - 1) * latHeight)

'

' Convert lat and latHeight to degrees in a transverse mercator projection 

' note that in fact the coordinates go from about -85 to +85 not -90 to 90! 

'

latHeight += lat

latHeight = 2 * Math.Atan(Math.Exp(Math.PI * latHeight)) - (Math.PI / 2)

latHeight *= (180 / Math.PI)

'

lat = (2 * Math.Atan(Math.Exp(Math.PI * lat))) - (Math.PI / 2)

lat *= (180 / Math.PI)

'

latHeight -= lat

If lonWidth < 0 Then

lon = lon + lonWidth

lonWidth = -lonWidth

End If

If latHeight < 0 Then

lat = lat + latHeight

latHeight = -latHeight

End If

Dim tileDetails As TileDetails

tileDetails.X = lon

tileDetails.Y = lat

tileDetails.Height = latHeight

tileDetails.Width = lonWidth

Return tileDetails

End Function

I apologise if this is not the correct place for a query such as this. 

Any assistance with this will be much appreciated

Best regards

Barry
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