一. 前言

作為編程菜雞，我有事沒事都會上Github上抄代碼。

基本上想寫點東西時，都會先在Github上看有沒有現成的......

又因為寫的代碼往往和GIS相關，GIS又是個炒雞冷門的方向，所以往往搜索結果慘淡，拉下來能用的代碼更是寥寥無幾呀。

這時候就只能在別人代碼基礎上修改了，愁......

該Github項目的地址

https://github.com/substack/shp2json

二. 修改

當初是想實現一個將shapefile文件轉為json的功能，

在Github上類似的代碼還算多。

但是，作為菜雞，那麽多的代碼裏，我能運行的也就那麽一兩個，再從中間挑一個評分高的，那就選中shp2json了。

作為菜雞，真的心累。

在運行完shp2json，發現json數據能正確顯示在控制臺裏。

那麽呵呵呵，怎麽把控制臺的所謂的“流”變成json字符串呢？

這對連json是什麽都不知道的我真是頭大呀。

當時各種搜索，各種關鍵詞都一無所獲，

最後走投無路時只能打開源碼。

其實我連源碼怎麽看都不知道，

但還好，所以就挑了個最長最大的index.js開始瞎幾把改。

最後，就莫名其妙的成功了。

呵呵呵~

下面是代碼修改：

原來的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": [\n‘;
 7     var after = ‘\n]}\n‘;
 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 = ‘,\n‘ + 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 }

修改之後的代碼：

 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;//要素讀取數量限制
 8     
 9     var before = ‘{"type": "FeatureCollection","features": [\n‘;
10     var after = ‘\n]}\n‘;
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 = ‘,\n‘ + featStr;
55           } else {
56               featStr = before + featStr;
57           }
58 
59           started = true;
60           out += featStr;
61 
62          countLimit++;  //要素讀取數量
63                   
64          if(countLimit < 8000)
65             writeNextFeature();  //這個方法報錯,棧溢出
66 
67       }
68 
69     })
70 
71     outStream.setReadable(layerStream);
72     outStream.end(after);
73 
74     return (out += after) ;  
75 }

三. 問題

這個包轉為的速率並不理想，甚至很慢......

而且當要素數量過大時，會棧溢出，這和機器的內存有關......

當然我也束手無策，

所以，呵呵噠。

[Github]shapefile轉json——對Github上shp2json項目的修改