[Github]shapefile转json——对Github上shp2json项目的修改

一. 前言

大二开始正式学编程，这条路走得磕磕绊绊，基本上想写点东西时，都会先在Github上看有没有现成的......

在一开始还不会配环境的时候，最痛苦的就是Github上拉下来能直接就用的代码寥寥无几。

后面又渐渐尝试，接触了maven，pip以及npm，开始在Github上搬运代码了。

又因为专业和GIS相关，GIS又是个冷门的方向，往往搜索结果惨淡。

有时候想做个功能，这时候就只能找到别人类似的代码，在他人基础上修改了，这里的坑也是数不胜数......

下面介绍一个冷门的Github项目，该项目的地址

https://github.com/substack/shp2json

二. 修改

当初是想实现一个将shapefile文件转为json的功能，

其实在Github上类似功能的代码还算多。

但是，在那么多的代码里，限于能力窘迫，当时我能运行的也就那么一两个，再从中间挑一个评分高的，那就选中shp2json了。

在运行完shp2json，发现json数据能正确显示在控制台里。

那么呵呵呵，怎么把控制台的所谓的“流”变成json字符串呢？

当时各种搜索，各种关键词都一无所获，

最后走投无路时只能打开源码。

其实我连源码怎么看都不知道，

但还好，所以就挑了个最长最大的index.js开始瞎几把改。

最后，就莫名其妙的成功了。

呵呵呵~算是新手第一关通过了。

下面是代码修改：

原来的index.js

function fromShpFile (file, outStream) {
    outStream = outStream || duplex.obj();
    var shp = gdal.open(file);
    var layerCount = shp.layers.count();

    var before = '{"type": "FeatureCollection","features": [\n';
    var after = '\n]}\n';
    var started = false;
    var currentLayer, currentFeature, currentTransformation;
    var nextLayer = 0;

    var to = gdal.SpatialReference.fromEPSG(4326);

    function getNextLayer() {
      currentLayer = shp.layers.get(nextLayer++);
      var srs = currentLayer.srs || gdal.SpatialReference.fromEPSG(4326);
      currentTransformation = new gdal.CoordinateTransformation(srs, to);
    }

    getNextLayer();

    var layerStream = from(function(size, next) {
      var out = '';
      writeNextFeature();

      function writeNextFeature() {
          var feature = currentLayer.features.next();
          if (!feature) {
              // end stream
              if (nextLayer === layerCount) {
                  // push remaining output and end
                  layerStream.push(out);
                  layerStream.push(after);
                  return layerStream.push(null);
              }
              getNextLayer();
              feature = currentLayer.features.next();
          }

          try {
              var geom = feature.getGeometry();
          } catch (e) {
              return writeNextFeature();
          }

          geom.transform(currentTransformation);
          var geojson = geom.toJSON();
          var fields = feature.fields.toJSON();
          var featStr = '{"type": "Feature", "properties": ' + fields + ',"geometry": ' + geojson + '}';

          if (started) {
              featStr = ',\n' + featStr;
          } else {
              featStr = before + featStr;
          }

          started = true;
          out += featStr;

          if (out.length >= size) {
              next(null, out);
          } else {
              writeNextFeature();
          }
      }

    })

    outStream.setReadable(layerStream);
    outStream.end(after);

    return outStream;
}

 

修改之后的代码：

function fromShpFile (file, outStream) {
    var out = '';
    outStream = outStream || duplex.obj();
    var shp = gdal.open(file);
    var layerCount = shp.layers.count();
    
    var countLimit = 0;//要素读取数量限制
    
    var before = '{"type": "FeatureCollection","features": [\n';
    var after = '\n]}\n';
    var started = false;
    var currentLayer, currentFeature, currentTransformation;
    var nextLayer = 0;

    var to = gdal.SpatialReference.fromEPSG(4326);

    function getNextLayer() {
      currentLayer = shp.layers.get(nextLayer++);
      var srs = currentLayer.srs || gdal.SpatialReference.fromEPSG(4326);
      currentTransformation = new gdal.CoordinateTransformation(srs, to);
    }

    getNextLayer();

    var layerStream = from(function(size, next) {
      writeNextFeature();

      function writeNextFeature() {
          var feature = currentLayer.features.next();
          if (!feature) {
              // end stream
              if (nextLayer === layerCount) {
                  // push remaining output and end
                  layerStream.push(out);
                  layerStream.push(after);
                  return layerStream.push(null);
              }
              getNextLayer();
              feature = currentLayer.features.next();
          }

          try {
              var geom = feature.getGeometry();
          } catch (e) {
              return writeNextFeature();
          }

          geom.transform(currentTransformation);
          var geojson = geom.toJSON();
          var fields = feature.fields.toJSON();
          var featStr = '{"type": "Feature", "properties": ' + fields + ',"geometry": ' + geojson + '}';

          if (started) {
              featStr = ',\n' + featStr;
          } else {
              featStr = before + featStr;
          }

          started = true;
          out += featStr;

         countLimit++;  //要素读取数量
                  
         if(countLimit < 8000)
            writeNextFeature();  //这个方法报错,栈溢出

      }

    })

    outStream.setReadable(layerStream);
    outStream.end(after);

    return (out += after) ;  
}

三. 问题

这个包转化的速率并不理想，甚至很慢......

而且当要素数量过大时，会报栈溢出，这应该和机器的内存有关......

......

 [作者: 阿泫] https://www.cnblogs.com/KissXuan/