使用MapObjects绘制缓冲区
MapObjects使用Buffer函数生成缓冲区，并返回一个pologon。需要注意的是，Buffer函数的第二个参数，即一个地图范围，通常设置为Map.Extent，但该extent必须能够包含所生成的缓冲区，否则缓冲区无法生成，返回null。
为避免缓冲区生成失败，可以认为设定Buffer的extent参数。以点状shape的缓冲区生成为例，可以获取缓冲区的包络矩形的范围，然后使得extent参数的范围能够完全容纳这个矩形即可。
代码如下
If listCity.Text <> "" Then Dim distance As Double '缓冲距离 Dim leftside As Double Dim rightside As Double Dim topside As Double Dim bottomside As Double Dim rect As MapObjects2.Rectangle Dim p As MapObjects2.Point Dim city As New MapObjects2.Recordset distance = 1050000 Set city = cityLayer.SearchExpression("mc = '" + listCity.Text + "'") 'mc = '" + listCity.Text + "' If Not city Is Nothing Then city.MoveFirst If Not city.EOF Then Set p = city.Fields("shape").Value MsgBox (p.X) leftside = p.X - distance rightside = p.X + distance topside = p.Y + distance bottomside = p.Y - distance Set rect = New Rectangle If leftside < map.Extent.Left Then rect.Left = leftside - 1 '不能重叠，所以-1 Else rect.Left = map.Extent.Left End If If rightside > map.Extent.Right Then rect.Right = rightside + 1 Else: rect.Right = map.Extent.Right End If If topside > map.Extent.Top Then rect.Top = topside + 1 Else: rect.Top = map.Extent.Top End If If bottomside < map.Extent.Bottom Then rect.Bottom = bottomside - 1 Else: rect.Bottom = map.Extent.Bottom End If ' map.Extent = rect 'map.Refresh Set bufferCircle = p.Buffer(distance, rect) MsgBox (bufferCircle.Area) End If MsgBox (city.Count) MsgBox (listCity.Text) End If map.TrackingLayer.Refresh True Else MsgBox "请首先选中一个地级市！" End If End Sub