经纬度坐标和投影坐标的转换

昨天，有朋友要我帮忙看看一个将经纬度坐标转换成墨卡托投影（墨卡托投影有很多变种，我也不知道他说的是哪一种）的程序，他说转换以后的坐标精度太差。当时，他的程序没怎么看懂，然后研究了一下Geotools，自己写了一个转换小程序，很简单的几行代码！

Geotools是Java语言编写的开源GIS工具包。该项目已有十多年历史，生命力旺盛，代码非常丰富，包含多个开源GIS项目，并且基于标准的GIS接口。Geotools主要提供各种GIS算法，各种数据格式的读写和显示。由于Geotools库依赖比较复杂，所以在Eclipse中我采用Maven进行建构。对于在Eclipse下怎么使用Maven，请自己百度之。貌似最新的Eclipse是直接集成Maven插件的不需要自己单独安装了。虽说Eclipse不需要安装Maven插件了，可以使用Eclipse自带的Eclipse Maven插件，但是还是建议自己安装一个最新的Maven二进制包。

在Eclipse新建Maven工程，添加库和依赖：

这里主要添加的是要下载Geotools的库：

<repositories>
    <repository>
        <id>maven2-repository.dev.java.netid>
        <name>Java.net repositoryname>
        <url>http://download.java.net/maven/2url>
    repository>
    <repository>
        <id>osgeoid>
        <name>Open Source Geospatial Foundation Repositoryname>    <url>http://download.osgeo.org/webdav/geotools/url>
    repository>
  repositories>

以及用到的Geotools库：

  <dependencies>
    <dependency>
      <groupId>junitgroupId>
      <artifactId>junitartifactId>
      <version>4.12version>
      <scope>testscope>
    dependency>
    <dependency>
        <groupId>org.geotoolsgroupId>
        <artifactId>gt-referencingartifactId>
        <version>${geotools.version}version>
    dependency>
    <dependency>
        <groupId>org.geotoolsgroupId>
        <artifactId>gt-epsg-wktartifactId>
        <version>${geotools.version}version>
    dependency>
    <dependency>
        <groupId>org.geotoolsgroupId>
        <artifactId>gt-apiartifactId>
        <version>${geotools.version}version>
    dependency>
    <dependency>
        <groupId>com.vividsolutionsgroupId>
        <artifactId>jtsartifactId>
        <version>1.13version>
    dependency>
  dependencies>

最后的pom.xml文件如下：

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0modelVersion>

  <groupId>cn.tzygroupId>
  <artifactId>geotoolsartifactId>
  <version>0.0.1-SNAPSHOTversion>
  <packaging>jarpackaging>

  <name>geotoolsname>
  <url>http://maven.apache.orgurl>

  <properties>
    <project.build.sourceEncoding>UTF-8project.build.sourceEncoding>
    <geotools.version>14.1geotools.version>
  properties>

  <repositories>
    <repository>
        <id>maven2-repository.dev.java.netid>
        <name>Java.net repositoryname>
        <url>http://download.java.net/maven/2url>
    repository>
    <repository>
        <id>osgeoid>
        <name>Open Source Geospatial Foundation Repositoryname>    <url>http://download.osgeo.org/webdav/geotools/url>
    repository>
  repositories>

  <dependencies>
    <dependency>
      <groupId>junitgroupId>
      <artifactId>junitartifactId>
      <version>4.12version>
      <scope>testscope>
    dependency>
    <dependency>
        <groupId>org.geotoolsgroupId>
        <artifactId>gt-referencingartifactId>
        <version>${geotools.version}version>
    dependency>
    <dependency>
        <groupId>org.geotoolsgroupId>
        <artifactId>gt-epsg-wktartifactId>
        <version>${geotools.version}version>
    dependency>
    <dependency>
        <groupId>org.geotoolsgroupId>
        <artifactId>gt-apiartifactId>
        <version>${geotools.version}version>
    dependency>
    <dependency>
        <groupId>com.vividsolutionsgroupId>
        <artifactId>jtsartifactId>
        <version>1.13version>
    dependency>
  dependencies>
project>

下面开始写程序：

package cn.tzy.geotools;

import org.geotools.geometry.jts.JTS;
import org.geotools.referencing.CRS;
import org.geotools.referencing.crs.DefaultGeographicCRS;
import org.opengis.geometry.MismatchedDimensionException;
import org.opengis.referencing.FactoryException;
import org.opengis.referencing.crs.CoordinateReferenceSystem;
import org.opengis.referencing.operation.MathTransform;
import org.opengis.referencing.operation.TransformException;

import com.vividsolutions.jts.geom.Coordinate;
import com.vividsolutions.jts.geom.GeometryFactory;
import com.vividsolutions.jts.geom.Point;

/**
 * Hello world!
 *
 */
public class CoordConverter 
{
    public static double[] convert(double lon, double lat) 
            throws FactoryException, MismatchedDimensionException, TransformException {
        // 传入原始的经纬度坐标
        Coordinate sourceCoord = new Coordinate(lon, lat);
        GeometryFactory geoFactory = new GeometryFactory();
        Point sourcePoint = geoFactory.createPoint(sourceCoord);

        // 这里是以OGC WKT形式定义的是World Mercator投影，网页地图一般使用该投影
        final String strWKTMercator = "PROJCS[\"World_Mercator\","
                + "GEOGCS[\"GCS_WGS_1984\","
                + "DATUM[\"WGS_1984\","
                + "SPHEROID[\"WGS_1984\",6378137,298.257223563]],"
                + "PRIMEM[\"Greenwich\",0],"
                + "UNIT[\"Degree\",0.017453292519943295]],"
                + "PROJECTION[\"Mercator_1SP\"],"
                + "PARAMETER[\"False_Easting\",0],"
                + "PARAMETER[\"False_Northing\",0],"
                + "PARAMETER[\"Central_Meridian\",0],"
                + "PARAMETER[\"latitude_of_origin\",0],"
                + "UNIT[\"Meter\",1]]";
        CoordinateReferenceSystem mercatroCRS = CRS.parseWKT(strWKTMercator);
        // 做投影转换，将WCG84坐标转换成世界墨卡托投影转
        MathTransform transform = CRS.findMathTransform(DefaultGeographicCRS.WGS84, mercatroCRS);
        Point targetPoint = (Point) JTS.transform(sourcePoint, transform);

        // 返回转换以后的X和Y坐标
        double[] targetCoord = {targetPoint.getX(), targetPoint.getY()};
        return targetCoord;
    }

    // 将目标投影坐标系作为参数输入，其实和第一个程序类似，我懒得提取公共部分再抽取函数了
    public static double[] convert(double lon, double lat, String strWKT) 
            throws FactoryException, MismatchedDimensionException, TransformException {
        Coordinate sourceCoord = new Coordinate(lon, lat);
        GeometryFactory geoFactory = new GeometryFactory();
        Point sourcePoint = geoFactory.createPoint(sourceCoord);

        CoordinateReferenceSystem mercatroCRS = CRS.parseWKT(strWKT);
        MathTransform transform = CRS.findMathTransform(DefaultGeographicCRS.WGS84, mercatroCRS);
        Point targetPoint = (Point) JTS.transform(sourcePoint, transform);

        double[] targetCoord = {targetPoint.getX(), targetPoint.getY()};
        return targetCoord;
    }

    // main函数进行验证
    public static void main( String[] args ) throws Exception
    {
        double longitude = 113.926982;
        double latitude = 22.53089;
        double[] coordinate = convert(longitude, latitude);

        System.out.println("X: " + coordinate[0] + ", Y: " + coordinate[1]);
    }
}

程序很简单：CRS.findMathTransform()方法定义转换的坐标系，JTS.transform()进行坐标的转换。

其中第一个方法是我专门写的将WGS84经纬度坐标转成World Mercator投影坐标的函数。第二个函数是对第一个函数的抽象，将要转换到的投影坐标提取出来作为参数，如果想要转换到某种投影坐标系，只需要传递一个该投影坐标系的OGC WKT（Well Known Text）给函数，就可以做转换了。常见投影的WKT表示可以在http://spatialreference.org/进行查询！
看懂了该程序的朋友，就可以写自己的实现了，上面只是一个简单的Hello World示例程序。