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

Posted 2021-01-14 kissxuan

一. 前言

作为编程菜鸡，我有事没事都会上Github上抄代码。

基本上想写点东西时，都会先在Github上看有没有现成的......

又因为写的代码往往和GIS相关，GIS又是个炒鸡冷门的方向，所以往往搜索结果惨淡，拉下来能用的代码更是寥寥无几呀。

这时候就只能在别人代码基础上修改了，愁......

该Github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，

在Github上类似的代码还算多。

但是，作为菜鸡，那么多的代码里，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。

作为菜鸡，真的心累。

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？

这对连json是什么都不知道的我真是头大呀。

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~

下面是代码修改：

原来的index.js

 1 function fromShpFile (file, outStream) {
 2     outStream = outStream || duplex.obj();
 3     var shp = gdal.open(file);
 4     var layerCount = shp.layers.count();
 5 
 6     var before = ‘{"type": "FeatureCollection","features": [
‘;
 7     var after = ‘
]}
‘;
 8     var started = false;
 9     var currentLayer, currentFeature, currentTransformation;
10     var nextLayer = 0;
11 
12     var to = gdal.SpatialReference.fromEPSG(4326);
13 
14     function getNextLayer() {
15       currentLayer = shp.layers.get(nextLayer++);
16       var srs = currentLayer.srs || gdal.SpatialReference.fromEPSG(4326);
17       currentTransformation = new gdal.CoordinateTransformation(srs, to);
18     }
19 
20     getNextLayer();
21 
22     var layerStream = from(function(size, next) {
23       var out = ‘‘;
24       writeNextFeature();
25 
26       function writeNextFeature() {
27           var feature = currentLayer.features.next();
28           if (!feature) {
29               // end stream
30               if (nextLayer === layerCount) {
31                   // push remaining output and end
32                   layerStream.push(out);
33                   layerStream.push(after);
34                   return layerStream.push(null);
35               }
36               getNextLayer();
37               feature = currentLayer.features.next();
38           }
39 
40           try {
41               var geom = feature.getGeometry();
42           } catch (e) {
43               return writeNextFeature();
44           }
45 
46           geom.transform(currentTransformation);
47           var geojson = geom.toJSON();
48           var fields = feature.fields.toJSON();
49           var featStr = ‘{"type": "Feature", "properties": ‘ + fields + ‘,"geometry": ‘ + geojson + ‘}‘;
50 
51           if (started) {
52               featStr = ‘,
‘ + featStr;
53           } else {
54               featStr = before + featStr;
55           }
56 
57           started = true;
58           out += featStr;
59 
60           if (out.length >= size) {
61               next(null, out);
62           } else {
63               writeNextFeature();
64           }
65       }
66 
67     })
68 
69     outStream.setReadable(layerStream);
70     outStream.end(after);
71 
72     return outStream;
73 }

View Code

 

修改之后的代码：

 1 function fromShpFile (file, outStream) {
 2     var out = ‘‘;
 3     outStream = outStream || duplex.obj();
 4     var shp = gdal.open(file);
 5     var layerCount = shp.layers.count();
 6     
 7     var countLimit = 0;//要素读取数量限制
 8     
 9     var before = ‘{"type": "FeatureCollection","features": [
‘;
10     var after = ‘
]}
‘;
11     var started = false;
12     var currentLayer, currentFeature, currentTransformation;
13     var nextLayer = 0;
14 
15     var to = gdal.SpatialReference.fromEPSG(4326);
16 
17     function getNextLayer() {
18       currentLayer = shp.layers.get(nextLayer++);
19       var srs = currentLayer.srs || gdal.SpatialReference.fromEPSG(4326);
20       currentTransformation = new gdal.CoordinateTransformation(srs, to);
21     }
22 
23     getNextLayer();
24 
25     var layerStream = from(function(size, next) {
26       writeNextFeature();
27 
28       function writeNextFeature() {
29           var feature = currentLayer.features.next();
30           if (!feature) {
31               // end stream
32               if (nextLayer === layerCount) {
33                   // push remaining output and end
34                   layerStream.push(out);
35                   layerStream.push(after);
36                   return layerStream.push(null);
37               }
38               getNextLayer();
39               feature = currentLayer.features.next();
40           }
41 
42           try {
43               var geom = feature.getGeometry();
44           } catch (e) {
45               return writeNextFeature();
46           }
47 
48           geom.transform(currentTransformation);
49           var geojson = geom.toJSON();
50           var fields = feature.fields.toJSON();
51           var featStr = ‘{"type": "Feature", "properties": ‘ + fields + ‘,"geometry": ‘ + geojson + ‘}‘;
52 
53           if (started) {
54               featStr = ‘,
‘ + featStr;
55           } else {
56               featStr = before + featStr;
57           }
58 
59           started = true;
60           out += featStr;
61 
62          countLimit++;  //要素读取数量
63                   
64          if(countLimit < 8000)
65             writeNextFeature();  //这个方法报错,栈溢出
66 
67       }
68 
69     })
70 
71     outStream.setReadable(layerStream);
72     outStream.end(after);
73 
74     return (out += after) ;  
75 }

View Code

三. 问题

这个包转为的速率并不理想，甚至很慢......

而且当要素数量过大时，会栈溢出，这和机器的内存有关......

当然我也束手无策，

所以，呵呵哒。