如何使用WinMapGIS或DotSpatial库在地图窗口中筛选出某些形状

Question

我有加州所有道路的大型街道形状档案。每当我在地图窗口重新绘制地图时，都要花很长时间(放大、缩小、移动地图.)。我想知道是否有任何方法告诉地图窗口只渲染一定数量的街道，根据特定的范围。这样，我就可以提高我的应用程序的整体性能。任何帮助都将不胜感激。

武川，

Answer 1

像DotSpatial这样的地图应用程序将尝试为加载的shapefile呈现屏幕上的所有向量。他们不试图划出任何不在当前范围内的界线。但是当你被放大的时候，他们会试图精确地画出所有的线，这是缓慢的。一种可能有帮助的策略是设置可见性刻度，以便只在放大后绘制图层。DotSpatial中的Layer类允许您为该层设置动态可见性，这样您只能在放大超过某个点时才绘制完整的shapefile。

myLayer.UseDynamicVisiblity = true;
myLayer.DynamicVisibilityMode = DynamicVisibilityMode.ZoomedIn;
myLayer.DynamicVisibiltyWidth = .2; // eg. .2 degrees of longitude in WGS84 

这种策略的唯一缺点是，在缩小到指定的范围之前，您根本看不到任何道路。因此，一种选择是创建一个低分辨率版本的道路形状，当你放大，但不包含任何“重复”点，为一个更大的网格大小。可以通过使用另一个行shapefile或创建图像或创建图像块来做到这一点。下面的第一个示例使用1000 x 1000的网格大小，从而将冗余点减少到最小表示形式。使用不同网格大小和动态可见性的组合，您应该能够更有效地呈现大行形状文件。Raster通常有总体视图，但是向量通常没有，因此这是为向量创建人工概述的一种方法。第二个示例将使用DP行简化，它保留所有形状，但使用较少的点来表示每个形状。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using DotSpatial.Data;
using DotSpatial.Projections;
using DotSpatial.Topology;

namespace WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Shapefile source = new Shapefile();
            source = Shapefile.OpenFile(@"C:\Data\Rivers\River.shp");

            bool[,] pointRepresented = new bool[1000, 1000];
            double width = source.Extent.Width;
            double height = source.Extent.Height;
            double dx = width / 1000;
            double dy = height / 1000;
            FeatureSet result = new FeatureSet(FeatureType.Line);
            result.Projection = KnownCoordinateSystems.Geographic.World.WGS1984; // lat lon in WGS84
            result.DataTable.Columns.Add("Index", typeof(int));
            int index = 0;
            foreach (ShapeRange shape in source.ShapeIndices) {

                bool started = false;
                List<LineString> linestrings = new List<LineString>();
                foreach (PartRange part in shape.Parts) {
                    List<Coordinate> coords = new List<Coordinate>();
                    foreach (Vertex vert in part) {
                        int i = (int)((vert.X - source.Extent.MinX) / dx);
                        int j = (int)((vert.Y - source.Extent.MinY) / dy);
                        if (i > 999) {
                            i = 999;
                        }
                        if (j > 999) {
                            j = 999;
                        }
                        if (pointRepresented[i, j] == true) continue;
                        coords.Add(new Coordinate(vert.X, vert.Y));
                        pointRepresented[i, j] = true;
                    }
                    if (coords.Count > 0) {
                        if (coords.Count == 1) {
                            coords.Add(coords[0]); // add a duplicate "endpoint" to the line if we only have one point.
                        }
                        linestrings.Add(new LineString(coords));
                    }
                }
                if (linestrings.Count > 0) {
                    IFeature feature;
                    if (linestrings.Count > 1)
                    {
                        feature = result.AddFeature(new MultiLineString(linestrings));
                    }
                    else {
                        feature = result.AddFeature(linestrings[0]);
                    }
                    feature.DataRow["Index"] = index;
                    index++;
                }
                result.SaveAs(@"C:\Data\Rivers\RiverPreview.shp", true);
            }

            MessageBox.Show(@"Finished creating file: C:\Data\Rivers\RiverPreview.shp");
        }
    }

}

第二种方法使用DP线简化，不会删除形状，而只是减少用于表示每个形状的点数。公差必须调整，以匹配您的数据集和坐标。

        /// <summary>
        /// This alternative uses DP line simplification, which keeps every shape, but "simplifies" the shape, simply reducing
        /// the number of points.  This will not accomplish the goal of reducing the number of shapes, but will
        /// give you a reduced representation that matches the original features in cases where slowness is caused
        /// by too much detail, which can be useful if you still want to keep all your features.
        /// </summary>
        private void button2_Click(object sender, EventArgs e)
        {
            Shapefile source = new Shapefile();
            source = Shapefile.OpenFile(@"C:\Data\Rivers\River.shp");
            FeatureSet result = new FeatureSet(FeatureType.Line);
            result.Projection = source.Projection;
            result.CopyTableSchema(source);
            foreach (IFeature feature in source.Features) {
                LineString linestring = feature.BasicGeometry as LineString;

                if (linestring != null)
                {
                    IList<Coordinate> points = linestring.Coordinates;
                    IList<Coordinate> simplified = DouglasPeuckerLineSimplifier.Simplify(points, .00002);
                    IFeature resultFeature = result.AddFeature(new LineString(simplified));
                    resultFeature.CopyAttributes(feature);
                }
                else {
                    MultiLineString multipleLines = feature.BasicGeometry as MultiLineString;

                    if (multipleLines != null)
                    {
                        List<LineString> resultLines = new List<LineString>();
                        foreach (IGeometry line in multipleLines.Geometries)
                        {
                            IList<Coordinate> points = line.Coordinates;
                            IList<Coordinate> simplified = DouglasPeuckerLineSimplifier.Simplify(points, .00002);
                            resultLines.Add(new LineString(simplified));
                        }
                        IFeature resultFeature = result.AddFeature(new MultiLineString(resultLines));
                        resultFeature.CopyAttributes(feature);
                    }
                }
            }

            result.SaveAs(@"C:\Data\Rivers\RiverSimplified.shp", true);
            MessageBox.Show(@"Finished creating file: C:\Data\Rivers\RiverSimplified.shp");
        }