GeoJSON格式示例

{
    "type":"FeatureCollection",
    "features":[
        {
            "type":"Feature",
            "properties":{
                "area": 3865207830,
                "text": null
            },
            "id":"polygon.1",
            "geometry":{
                "type":"Polygon",
                "coordinates":[
                    [
                        [
                            116.19827270507814,
                            39.78321267821705
                        ],
                        [
                            116.04446411132814,
                            39.232253141714914
                        ],
                        [
                            116.89590454101562,
                            39.3831409542565
                        ],
                        [
                            116.86981201171876,
                            39.918162846609455
                        ],
                        [
                            116.19827270507814,
                            39.78321267821705
                        ]
                    ]
                ]
            }
        }
    ],
    "crs":{
        "type":"name",
        "properties":{
            "name":"EPSG:4326"
        }
    }
}

一、解析FeatureCollection對象文件

一個FeatureCollection對象文本，包含一個Feature要素。

1.1 geotools操作GeoJSON過程中的問題及相關源碼

public static void main(String[] a) throws Exception {
        // 座標順序是EAST_NORTH，即經度在前
        String json = "{\"type\":\"FeatureCollection\",\"features\":[{\"type\":\"Feature\",\"properties\":{\"area\":3865207830, \"text\": null},\"id\":\"polygon.1\",\"geometry\":{\"type\":\"Polygon\",\"coordinates\":[[[116.19827270507814,39.78321267821705],[116.04446411132814,39.232253141714914],[116.89590454101562,39.3831409542565],[116.86981201171876,39.918162846609455],[116.19827270507814,39.78321267821705]]]}}],\"crs\":{\"type\":\"name\",\"properties\":{\"name\":\"EPSG:4326\"}}}";
        // 指定GeometryJSON構造器，15位小數
        FeatureJSON fjson_15 = new FeatureJSON(new GeometryJSON(15));
        // 讀取爲FeatureCollection
        FeatureCollection featureCollection = fjson_15.readFeatureCollection(json);
        // 獲取SimpleFeatureType
        SimpleFeatureType simpleFeatureType = (SimpleFeatureType) featureCollection.getSchema();
        // 第1個問題。座標順序與實際座標順序不符合
        System.out.println(CRS.getAxisOrder(simpleFeatureType.getCoordinateReferenceSystem()));  //輸出：NORTH_EAST
 
        //第2個問題。查看空間列名稱
        System.out.println(simpleFeatureType.getGeometryDescriptor().getLocalName());  //輸出：geometry
 
        //第3個問題。座標精度丟失
        //第4個問題。默認無座標系和空值輸出
        OutputStream ostream = new ByteArrayOutputStream();
        GeoJSON.write(featureCollection, ostream);
        // 輸出：{"type":"FeatureCollection","features":[{"type":"Feature","geometry":{"type":"Polygon","coordinates":[[[116.1983,39.7832],[116.0445,39.2323],[116.8959,39.3831],[116.8698,39.9182],[116.1983,39.7832]]]},"properties":{"area":3865207830},"id":"polygon.1"}]}
        System.out.println(ostream);
 
        // 第5個問題。座標變換問題，由座標順序引發
        SimpleFeatureIterator iterator = (SimpleFeatureIterator) featureCollection.features();
        SimpleFeature simpleFeature = iterator.next();
        Geometry geom = (Geometry) simpleFeature.getDefaultGeometry();
        iterator.close();
        System.out.println(geom.getArea());  // 輸出：0.4043554020447081
        MathTransform transform_1 = CRS.findMathTransform(CRS.decode("EPSG:4326"), CRS.decode("EPSG:3857"),true);
        // 下面一行代碼會報異常：Exception in thread "main" org.geotools.referencing.operation.projection.ProjectionException: Latitude 116°11.8'N is too close to a pole.
        /*Geometry geom_3857 = JTS.transform(geom, transform_1);
        System.out.println(geom_3857.getArea());*/
    }

上述事例代碼給出了將GeoJSON解析成FeatureCollection時出現的一些問題。

第1個問題是得到的FeatureCollection座標順序是錯誤的，給出的GeoJSON座標順序是經度（EAST）在前，geotools讀取時給出了默認的座標順序（緯度在前）

看下面的org.geotools.geojson.feature.FeatureJSON的readFeatureCollection(Object input)方法源碼：

// org.geotools.geojson.feature.FeatureJSON
// input可以是File，Reader，InputStream等
public FeatureCollection readFeatureCollection(Object input) throws IOException {
 
        // 新建一個DefaultFeatureCollection對象，
        DefaultFeatureCollection features = new DefaultFeatureCollection(null, null);
 
        // FeatureCollectionIterator實現了FeatureIterator接口，是一個內部類，用於控制從geojson文本中讀取要素和座標系等信息。        
        FeatureCollectionIterator it = (FeatureCollectionIterator) streamFeatureCollection(input);
        while(it.hasNext()) {
            features.add(it.next());
        }
        
        if (features.getSchema() != null
                && features.getSchema().getCoordinateReferenceSystem() == null 
                && it.getHandler().getCRS() != null ) {
            try {
 
                // 只將座標系信息寫入，即只更改了座標系
                return new ForceCoordinateSystemFeatureResults(features, it.getHandler().getCRS());
            } catch (SchemaException e) {
                throw (IOException) new IOException().initCause(e);
            }
        }
        return features;
    }

ForceCoordinateSystemFeatureResults是FeatureCollection接口的一個子類，直接更改座標系信息，原數據中的座標信息不變。座標系的生成是在org.geotools.geojson.feature.CRSHandler類中，相關代碼如下：

//org.geotools.geojson.feature.CRSHandler
public boolean primitive(Object value) throws ParseException, IOException {
 
        if (state == 2) {
            try {
                try {
                    crs = CRS.decode(value.toString());   //座標順序默認NORTH_EAST，與實際數據不符
                }
                catch(NoSuchAuthorityCodeException e) {
                    //try pending on EPSG
                    try {
                        crs = CRS.decode("EPSG:" + value.toString());
                    }
                    catch(Exception e1) {
                        //throw the original
                        throw e;
                    }
                }
            }
            catch(Exception e) {
                throw (IOException) new IOException("Error parsing " + value + " as crs id").initCause(e);
            }
            state = -1;
        }
        
        return true;
    }

第1個問題可做如下修改，使座標系正常：

String srs = CRS.lookupIdentifier(simpleFeatureType.getCoordinateReferenceSystem(),true);  // 獲取EPSG
featureCollection = new ForceCoordinateSystemFeatureResults(featureCollection, CRS.decode(srs, true));

第2個問題，空間列名稱不是我們想要的"the_geom"，而是"geometry"，這使和另外一些數據源的FeatureCollection一起做操作時空間列不一致。

相關代碼在org.geotools.geojson.feature.FeatureHandler類中：

//org.geotools.geojson.feature.FeatureHandler
void addGeometryType(SimpleFeatureTypeBuilder typeBuilder, Geometry geometry) {
        // 空間列名"geometry"，而不是"the_geom" 
        typeBuilder.add("geometry", geometry != null ? geometry.getClass() : Geometry.class);
        typeBuilder.setDefaultGeometry("geometry");
    }

SimpleFeatureTypeBuilder類用於構建SimpleFeatureType，SimpleFeatureType描述了FeatureCollection對象屬性、數據類型、座標系等信息。

第2個問題可做如下修改，使空間列變爲"the_geom"：

    // 構建新的SimpleFeatureType
    public static SimpleFeatureType retype(SimpleFeatureType oldType){
        SimpleFeatureTypeBuilder typeBuilder = new SimpleFeatureTypeBuilder();
        typeBuilder.init(oldType);
        // the_geom
        if("geometry".equals(oldType.getGeometryDescriptor().getLocalName())){
            typeBuilder.remove("geometry");
            typeBuilder.add("the_geom",oldType.getType("geometry").getBinding());
        }
        //生成新的SimpleFeatureType
        return typeBuilder.buildFeatureType();
    }
 
    // 新建一個方法，用於變換feature的type
    public static SimpleFeature retypeFeature(SimpleFeature feature,SimpleFeatureType newType) {
        SimpleFeatureBuilder builder = new SimpleFeatureBuilder(newType);
        // 遍歷屬性
        for (AttributeDescriptor att : newType.getAttributeDescriptors()) {
            Object value = feature.getAttribute(att.getName());
            // 空間列
            if(Geometry.class.isAssignableFrom(att.getType().getBinding())){
                builder.set("the_geom", feature.getDefaultGeometry());
                continue;
            }
            builder.set(att.getName(), value);
        }
        return builder.buildFeature(feature.getID());
    }

在測試代碼中加入如下，得到最終的FeatureCollection：

    SimpleFeatureType newType = retype(simpleFeatureType);
    // ListFeatureCollection是FeatureCollection的一個子類
    ListFeatureCollection listFeatureCollection = new ListFeatureCollection(newType);
    SimpleFeatureIterator iterator_3 = (SimpleFeatureIterator) featureCollection.features();
    while (iterator_3.hasNext()){
        SimpleFeature newFeature = retypeFeature(iterator_3.next(),newType);
        listFeatureCollection.add(newFeature);
    }
    iterator_3.close();

第3（座標精度丟失）、第4（默認無座標系和空值輸出）、第5（由座標順序引發座標變換）這三個問題。

用GeoJSON的static void write(Object obj, Object output)靜態方法將FeatureCollection轉化成了json文本輸出，先看org.geotools.geojson.GeoJSON源碼：

// 該類用於FeatureCollection、Feature和座標系的JSON輸出
public class GeoJSON {
    static GeometryJSON gjson = new GeometryJSON();
    static FeatureJSON fjson = new FeatureJSON();  // 用的默認構造器
    public static Object read(Object input) throws IOException {
        throw new UnsupportedOperationException();
    }
    public static void write(Object obj, Object output) throws IOException {
        if (obj instanceof Geometry) {
            gjson.write((Geometry)obj, output);
        }
        else if (obj instanceof Feature || obj instanceof FeatureCollection ||  
                obj instanceof CoordinateReferenceSystem) {
            
            if (obj instanceof SimpleFeature) {
                fjson.writeFeature((SimpleFeature)obj, output); 
            }
            else if (obj instanceof FeatureCollection) {
                fjson.writeFeatureCollection((FeatureCollection)obj, output);
            }
            else if (obj instanceof CoordinateReferenceSystem) {
                fjson.writeCRS((CoordinateReferenceSystem)obj, output);
            }
            else {
                throw new IllegalArgumentException("Unable able to encode object of type " + obj.getClass());
            }
        }
    }
}

該類除寫Geometry外都是調用FeatureJSON的方法，在看下FeatureJSON的構造器和實例變量：

    // org.geotools.geojson.feature.FeatureJSON
    GeometryJSON gjson;  // 決定座標保留的位數
    SimpleFeatureType featureType;
    AttributeIO attio;
    boolean encodeFeatureBounds = false;   // true表示json文本中Feature輸出bbox
    boolean encodeFeatureCollectionBounds = false;    // true表示json文本中FeatureCollection輸出bbox
    boolean encodeFeatureCRS = false;   // true表示json文本中Feature輸出座標系
    boolean encodeFeatureCollectionCRS = false;   // true表示json文本中FeatureCollection輸出座標系
    boolean encodeNullValues = false;   // true表示識別值爲null的屬性
    
    public FeatureJSON() {
        this(new GeometryJSON());   // GeometryJSON默認保留4爲小數
    }
    public FeatureJSON(GeometryJSON gjson) {  //  自定義GeometryJSON，可控制小數位數
        this.gjson = gjson; 
        attio = new DefaultAttributeIO();
    }

GeometryJSON的相關代碼就不列出來了。

解決第3（座標精度丟失）、第4（默認無座標系和空值輸出）問題我們只需做一些設置。

如果想統一用GeoJSON.write()方法寫json文本，可以重寫該類，設置精度，代碼如下：

// 重寫後的GeoJSON
public class GeoJSON {
    static GeometryJSON gjson = new GeometryJSON(15);  // 15位小數
    static FeatureJSON fjson = new FeatureJSON(gjson);  // 指定GeometryJSON
    public static Object read(Object input) throws IOException {
        throw new UnsupportedOperationException();
    }
    public static void write(Object obj, Object output) throws IOException {
        if (obj instanceof Geometry) {
            gjson.write((Geometry)obj, output);
        }
        else if (obj instanceof Feature || obj instanceof FeatureCollection ||  
                obj instanceof CoordinateReferenceSystem) {
 
            // 值爲null的屬性也識別
            fjson.setEncodeNullValues(true);
            // 輸出座標系文本
            fjson.setEncodeFeatureCollectionCRS(true);            
            if (obj instanceof SimpleFeature) {
                fjson.writeFeature((SimpleFeature)obj, output); 
            }
            else if (obj instanceof FeatureCollection) {
                fjson.writeFeatureCollection((FeatureCollection)obj, output);
            }
            else if (obj instanceof CoordinateReferenceSystem) {
                fjson.writeCRS((CoordinateReferenceSystem)obj, output);
            }
            else {
                throw new IllegalArgumentException("Unable able to encode object of type " + obj.getClass());
            }
        }
    }
}

也可以不進行重寫，目前源碼已經支持在初始化的時候直接傳參即可，舉例如下：

 GeometryJSON gjson = new GeometryJSON(15);  // 初始化 精度爲15位小數
 FeatureJSON fjson = new FeatureJSON(gjson);

如果就想用FeatureJSON操作輸出，可以在測試代碼中添加如下代碼解決：

// fjson_15已經保留15位
fjson_15.setEncodeFeatureCollectionCRS(true);
fjson_15.setEncodeNullValues(true);
fjson_15.writeFeatureCollection(featureCollection,System.out);  // 控制檯輸出和原始geojson一致

針對第5（由座標順序引發座標變換）個問題

“org.geotools.referencing.operation.projection.ProjectionException: Latitude 116°11.8’N is too close to a pole”異常其實是由座標順序不正確導致，經緯度順序調換後識別的座標超出了範圍，不是當前座標系能表示的值了。

這是一個隱藏問題，在處理另一些原數據或變換不同的座標系時，不一定會產生這個異常，那用不合理的座標順序得到的結果是不正確的。

調整代碼如下即可以解決：

String srs = CRS.lookupIdentifier(simpleFeatureType.getCoordinateReferenceSystem(),true);
// CRS.decode()方法可以設置經緯度順序
MathTransform transform_2 = CRS.findMathTransform(CRS.decode(srs,true), CRS.decode("EPSG:3857",true),true);
Geometry geom_3857 = JTS.transform(geom, transform_2);
System.out.println(geom_3857.getArea());  // 輸出：6.501222710260582E9

測試代碼裏輸出了幾何對象geom的面積，但這個面積很粗糙，只做測試用。給出的GeoJSON文本中的"area"值也只做參考，不是最精確的面積。

大家都知道，EPSG:3857以EPSG:4326地理座標系和投影方式爲僞墨卡託的平面座標系，給出的面積偏差較大。

讀取本地txt文件進行解析

FeatJson.class

import com.geomesa.spark.SparkJTS.Operation;
import com.vividsolutions.jts.geom.Coordinate;
import com.vividsolutions.jts.geom.Geometry;
import com.vividsolutions.jts.geom.GeometryFactory;
import com.vividsolutions.jts.operation.distance.DistanceOp;
import org.geotools.data.simple.SimpleFeatureIterator;
import org.geotools.feature.FeatureCollection;
import org.geotools.geojson.GeoJSON;
import org.geotools.geojson.feature.FeatureJSON;
import org.geotools.geojson.geom.GeometryJSON;
import org.geotools.geometry.jts.JTSFactoryFinder;
import org.opengis.feature.simple.SimpleFeature;
import org.opengis.feature.simple.SimpleFeatureType;
import java.io.*;

public class FeatJson {
    static GeometryFactory geometryFactory = JTSFactoryFinder.getGeometryFactory(null);
    public static void main(String[] args) throws IOException {
        //讀取本地文件
        FileReader reader = new FileReader("D:/GitProjects/GeoMesa/GeoMesaSpark/src/main/resources/gsmc.txt");
        BufferedReader bufferReader = new BufferedReader(reader);
        String dict = bufferReader.readLine();
        //按行讀取文件
        //構造FeatureJSON對象，GeometryJSON保留15位小數
        FeatureJSON featureJSON = new FeatureJSON(new GeometryJSON(15));
        FeatureCollection featureCollection = featureJSON.readFeatureCollection(dict);
        SimpleFeatureType simpleFeatureType = (SimpleFeatureType) featureCollection.getSchema();
        System.out.println(simpleFeatureType.getGeometryDescriptor().getLocalName());
        OutputStream ostream = new ByteArrayOutputStream();
        GeoJSON.write(featureCollection, ostream);
        System.out.println(ostream);

        SimpleFeatureIterator iterator = (SimpleFeatureIterator) featureCollection.features();
        SimpleFeature simpleFeature = iterator.next();
        Geometry geom = (Geometry) simpleFeature.getDefaultGeometry();
        iterator.close();
        System.out.println(geom.getLength());
        System.out.println(geom.getCoordinate());
        System.out.println(geom.getBoundary());
        System.out.println(geom.getGeometryType());

        //新建一個經緯度座標對象
        Coordinate coordinate1 = new Coordinate(1.357846020181606E7, 4505819.87283728);
        Coordinate[] coordinates2 = geom.getCoordinates();
        Operation op = new Operation();
        //求點到線的距離
        System.out.println("距離："+op.distanceGeo(geometryFactory.createPoint(coordinate1),geometryFactory.createLineString(coordinates2)));
        //求點到線的最近一個點
        System.out.println(DistanceOp.nearestPoints(geometryFactory.createPoint(coordinate1),geometryFactory.createLineString(coordinates2)));

        bufferReader.close();
        reader.close();
    }
}

推薦api: FeatureJSON

解析FeatureCollection（Geotools對geojson操作出現的問題）

一、解析FeatureCollection對象文件

1.1 geotools操作GeoJSON過程中的問題及相關源碼

讀取本地txt文件進行解析

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