SharpMap学习10-比例尺

代码下载地址： http://download.csdn.net/source/3327777

看了SharpMap的比例尺相关代码，看的是一知半解，主要原因在于代码中的Zoom和以往对比例尺的概念联系不上，但是代码的意思，我想应该是一个坐标系（也就是地理坐标，或者叫WordView）到窗口坐标的转换吧。这个似乎在图形编程的时候经常用到，自己理解不是很深刻，那就将就着写点感悟吧。

地理坐标系下是笛卡尔坐标系

而屏幕坐标系：

假设地理坐标系下，也就是现实中一个点如下左图示，要转为屏幕点（右侧图）显示，就需要通过比例转换得到：

具体转换公式很简单，分别处理X,Y即可。简单实践代码如下，首先定义一个显示的点集合：

public class Geometry


{


    private List<PointF> points;


    public List<PointF> Points


    {


        get


        {


            return points;


        }


    }





    public Geometry()


    {


        this.points = new List<PointF>();           


    }





    public void ReadData()


    {


        PointF pt;





        pt = new PointF(13183368.7f, 3836485.263f);


        points.Add(pt);





        pt = new PointF(13183709.796f, 3785207.202f);


        points.Add(pt);





        pt = new PointF(13206335.814f, 3784297.613f);


        points.Add(pt);





        pt = new PointF(13210770.059f, 3802716.783f);


        points.Add(pt);





        pt = new PointF(13230667.311f, 3803398.975f);


        points.Add(pt);





        pt = new PointF(13231804.297f, 3820453.762f);


        points.Add(pt);





        pt = new PointF(13206790.608f, 3839327.727f);


        points.Add(pt);


    }





    public Geometry Colon()


    {


        Geometry geometry = new Geometry();


        foreach (PointF ptItem in this.points)


        {


            geometry.points.Add(ptItem);


        }


        return geometry;


    }


}

然后类似定义一个Map对象

class Map


{


   private int imgWidth;


   private int imgHeight;





   public int ImgWidth


   {


       get


       {


           return imgWidth;


       }


       set


       {


           imgWidth = value;


       }


   }





   public int ImgHeight


   {


       get


       {


           return imgHeight;


       }


       set


       {


           imgHeight = value;


       }


   }





   private float zoomScale;


   public float ZoomScale


   {


       get


       {


           return zoomScale;


       }


       set


       {


           if (value < 1)


               return;


           zoomScale = value;


       }


   }





   public Map(int imgWidth,int imgHeight)


   {


       this.imgWidth = imgWidth;


       this.imgHeight = imgHeight;





       this.zoomScale = 100.0f;





       this.data = new Geometry();


       data.ReadData();


   }





   private Geometry data = null;





   public PointF GetCenter()


   {


       PointF resultPt = new PointF();


       if (this.data != null)


       {


           BoundingBox box = Utility.GetEnvolope(this.data.Points);


           resultPt.X = (box.MinX + box.MaxX) / 2.0f;


           resultPt.Y = (box.MinY + box.MaxY) / 2.0f;


       }


       return resultPt;


   }





   public void GetLineMap(Graphics g)


   {


       Geometry lineGeo = data.Colon();


       for (int i = 0; i < lineGeo.Points.Count; i++)


       {


           lineGeo.Points[i] = Utility.WordToImage(lineGeo.Points[i], this);


       }





       GraphicsPath gpPath = new GraphicsPath();


       gpPath.AddLines(lineGeo.Points.ToArray());


       


       g.DrawPath(linePen, gpPath);


   }





   public void GetRegionMap(Graphics g)


   {


       Geometry regionGeo = data.Colon();


       for (int i = 0; i < regionGeo.Points.Count; i++)


       {


           regionGeo.Points[i] = Utility.WordToImage(regionGeo.Points[i], this);


       }





       GraphicsPath gpPath = new GraphicsPath();


       gpPath.AddPolygon(regionGeo.Points.ToArray());





       g.FillPath(regionBrush, gpPath);


   }





   private Pen linePen = new Pen(Color.Red, 3);


   private Brush regionBrush = new SolidBrush(Color.FromArgb(120, Color.Blue));





   public float GetExtentZoomScale()


   {


       if (this.data != null)


       {


           return Utility.GetEnvolope(this.data.Points).BoundingWidth;


       }


       else


       {


           return 0;


       }


   }


}

在绘制前，首先要遍历每一个点，将其从地理坐标转为窗口坐标，以下是具体的转换函数：

public static PointF WordToImage(PointF wordPt, Map map)


{


   PointF imgPt = new PointF();


   float wordHeight = map.ZoomScale * (float)map.ImgHeight / (float)map.ImgWidth;





   float wordLeft = map.GetCenter().X - map.ZoomScale * 0.5f;


   float wordTop = map.GetCenter().Y + wordHeight * 0.5f;





   imgPt.X = (wordPt.X - wordLeft) / map.ZoomScale * map.ImgWidth;


   imgPt.Y = (wordTop - wordPt.Y) / wordHeight * map.ImgHeight;





   return imgPt;


}

解释下，Map对象的ZoomScale是作为“比例尺”的“比例”概念存在的，按照一些博文中的解释，是作为地理坐标显示范围的宽度处理的，也就是说地理显示范围(真实范围)的宽度=ZoomScale，通过它和窗口中高、宽比再求得地理显示范围的高，从而进行转换操作的。SharpMap中还涉及到其他的参数处理，不过我觉得大概意思应该是这样的吧。

最终效果：

改变比例尺：