# How to: Implement a Custom Map Projection

In this example, the Hammer-Atoff map projection is implemented.

To create a custom projection, inherit the ProjectionBase class and override the following methods of the base class.

``````Imports DevExpress.Xpf.Map
Imports System
Imports System.Windows

Namespace CustomProjection
Friend Class HammerAitoffProjection
Inherits ProjectionBase

Public Overrides Property OffsetX() As Double
Get
Return 0.5
End Get
Set(value As Double)
End Set
End Property
Public Overrides Property OffsetY() As Double
Get
Return 0.5
End Get
Set(value As Double)
End Set
End Property
Public Overrides Property ScaleX() As Double
Get
Return 0.5
End Get
Set(value As Double)
End Set
End Property
Public Overrides Property ScaleY() As Double
Get
Return -0.25
End Get
Set(value As Double)
End Set
End Property

Private Const minLatitude As Double = -90.0
Private Const maxLatitude As Double = 90.0
Private Const minLongitude As Double = -180.0
Private Const maxLongitude As Double = 180.0

Private Shared Function RadianToDegree(ByVal value As Double) As Double
Return value * 180.0 / Math.PI
End Function

Private Shared Function DegreeToRadian(ByVal value As Double) As Double
Return value * Math.PI / 180.0
End Function

Private Function IsValidPoint(ByVal x As Double, ByVal y As Double) As Boolean
If Math.Pow(x, 2) + Math.Pow(y, 2) > 1 Then
Return False
End If

Return True
End Function

Public Overrides Function GeoPointToMapUnit(ByVal geoPoint As GeoPoint) As MapUnit
Dim z As Double = Math.Sqrt(1 + Math.Cos(latInRadian) * Math.Cos(lonInRadian / 2))
Dim x As Double = Math.Cos(latInRadian) * Math.Sin(lonInRadian / 2) / z
Dim y As Double = Math.Sin(latInRadian) / z

Return New MapUnit(x * ScaleX + OffsetX, y * ScaleY + OffsetY)
End Function

Public Overrides Function MapUnitToGeoPoint(ByVal mapUnit As MapUnit) As GeoPoint
Dim x As Double = (mapUnit.X - OffsetX) / ScaleX
Dim y As Double = Math.Min(1, Math.Max(-1, (mapUnit.Y - OffsetY) / ScaleY))

If IsValidPoint(x, y) Then
Dim z As Double = Math.Sqrt(1 - 0.5 * Math.Pow(x, 2) - 0.5 * Math.Pow(y, 2))
Dim c As Double = Math.Sqrt(2) * z * x / (2 * Math.Pow(z, 2) - 1)
Dim lon As Double = 2 * Math.Atan(c)
Dim lat As Double = Math.Asin(Math.Min(Math.Max(Math.Sqrt(2) * z * y, -1), 1))
Dim latInDegree As Double = lat * maxLongitude / Math.PI
Dim lonInDegree As Double = lon * maxLongitude / Math.PI

Else
Dim signX As Integer = If(x < 0, -1, 1)
Dim signY As Integer = If(y < 0, -1, 1)
Return New GeoPoint(maxLatitude * signY, maxLongitude * signX)
End If
End Function

Public Overrides Function GeoToKilometersSize(ByVal anchorPoint As GeoPoint, ByVal size As Size) As Size
Return New Size(size.Width * LonToKilometersRatio * Math.Cos(DegreeToRadian(anchorPoint.Latitude)), size.Height * LatToKilometersRatio)
End Function

Public Overrides Function KilometersToGeoSize(ByVal anchorPoint As GeoPoint, ByVal size As Size) As Size
Return New Size(size.Width / LonToKilometersRatio / Math.Cos(DegreeToRadian(anchorPoint.Latitude)), size.Height / LatToKilometersRatio)
End Function
End Class
End Namespace
``````