OGR对shp文件的操作

本文介绍如何使用GDAL/OGR 库对shapefile文件进行简单的操作，包括读取、创建、修改等.在GDAL官网上有读写shp文件的介绍，主要是针对点要素的操作例子：点击打开链接

1.读取shp文件

void ExtracInfo::ReadShapeFile()
{
	OGRRegisterAll();

	OGRDataSource *poDS;
	poDS = OGRSFDriverRegistrar::Open("F:/test.shp", FALSE);
	if (poDS == NULL)
	{
	//	printf("Open failed \n");

	}

	OGRLayer *poLayer;
	poLayer = poDS->GetLayerByName("test");
	int layercount = poDS->GetLayerCount();
//	printf("layer count is : %d\n", layercount);

	OGRFeature *poFeature;
	poLayer->ResetReading();
	while((poFeature = poLayer->GetNextFeature()) != NULL)
	{
		//printf("hello!\n");
		//get field
		OGRFeatureDefn *poFDefn = poLayer->GetLayerDefn();
		int iField;
		int fieldcount = poFDefn->GetFieldCount();
//		printf("Field count is : %d\n", poFDefn->GetFieldCount());
		int fieldv1;
		double fieldv2;
		string feildv3;
		for (iField = 0; iField < poFDefn->GetFieldCount(); iField ++)
		{
			OGRFieldDefn * poFieldDefn = poFDefn->GetFieldDefn(iField);
			if (poFieldDefn->GetType() == OFTInteger)
			{
				fieldv1 = poFeature->GetFieldAsInteger(iField);
//				printf("%d ", poFeature->GetFieldAsInteger(iField));
			}
			else if (poFieldDefn->GetType() == OFTReal)
			{
				fieldv2 = poFeature->GetFieldAsDouble(iField);
//				printf("%3f ", poFeature->GetFieldAsDouble(iField));
			}
			else if (poFieldDefn->GetType() == OFTString)
			{
				feildv3 = poFeature->GetFieldAsString(iField);
//				printf("%s ", poFeature->GetFieldAsString(iField));
			}
//			else
//				printf("%s", poFeature->GetFieldAsString(iField));
		}
//以上操作与gdal官网一样
		//get Geometry获取polygon的信息
		OGRGeometry *poGeometry;
		poGeometry = poFeature->GetGeometryRef();
		double x, y;double area;
		if (poGeometry != NULL && wkbFlatten(poGeometry->getGeometryType()) == wkbPolygon)
		{
			OGRPolygon *poPolygon = (OGRPolygon * )poGeometry;
			area = poPolygon->get_Area();
			OGRPoint point;
			OGRLinearRing *pOGRLinearRing = poPolygon->getExteriorRing();
			pOGRLinearRing->getPoint(0,&point);//得到第一个点的坐标
			x = point.getX();
			y = point.getY();
//			printf("%.6f, %.6f\n", poPoint->getX(), poPoint->getY());
		}
		else
//			printf("no point geometry\n");

		
		OGRFeature::DestroyFeature(poFeature);
	}
	

	OGRDataSource::DestroyDataSource(poDS);


}

2.shp文件添加字段

有时需要对已经存在的shp文件进行添加字段操作。代码如下：

	OGRDataSource *poDS;
	poDS = OGRSFDriverRegistrar::Open("F:/test.shp", TRUE);//第二个参数表示只读还是可读写
	if (poDS == NULL)
	{
		//	printf("Open failed \n");

	}
	OGRLayer *poLayer;
	poLayer = poDS->GetLayerByName("test");
	int layercount = poDS->GetLayerCount();

	OGRFieldDefn poField("NewField", OFTReal);
//	poField.SetWidth(1); //设置的是格式化输出时的位数
	poField.SetPrecision(8);//设置精度，除了real类型，其他的一般为0
	if (poLayer->CreateField(&poField) != OGRERR_NONE)
	{
// 		printf("Creating Name Field Failed\n");
// 		exit(1);
	}
	OGRFeature *poFeature;
 	poLayer->ResetReading();//设置从第0个要素开始读

	int featureNum = 0;
	while((poFeature = poLayer->GetNextFeature()) != NULL)
	{
		OGRFeatureDefn *poFDefn = poLayer->GetLayerDefn();
		int fieldcount = poFDefn->GetFieldCount();

		poFeature->SetField(fieldcount - 1,  featureNum);

		poLayer->SetFeature(poFeature);
		OGRFeature::DestroyFeature(poFeature);
		featureNum ++;
	}
	OGRDataSource::DestroyDataSource(poDS);

刚开始老是添加不成功，最后发现是open函数里第二个参数，如果默认设置为FALSE，则表示只读；需要把它设置为TRUE才可读写。

3.创建shp文件

下面主要说说创建多边形shp文件。创建的方法：使用的WKT格式的字符串来进行创建。也可以使用其他的方式进行创建（参考大神博客：点击打开链接）。但是它是wkt方法的C#、python代码，c++代码对应的函数没查到。所以用另一种方法创建如下：

	const char *pszDriverName = "ESRI Shapefile";
	OGRSFDriver *poDriver;

	OGRRegisterAll();

	poDriver = OGRSFDriverRegistrar::GetRegistrar()->GetDriverByName(pszDriverName);
	if (poDriver == NULL)
	{
//		printf("%s driver not available \n", pszDriverName);
	}

	OGRDataSource *poDS;
	poDS = poDriver->CreateDataSource("F:/zjq/testProject/ogrtest/polygon.shp", NULL);
	if (poDS == NULL)
	{
// 		printf("Create shapefile failed\n");
// 		exit(1);
	}

	OGRLayer *poLayer;
	poLayer = poDS->CreateLayer("polygon1", NULL, wkbPolygon, NULL);
	if (poLayer == NULL)
	{
// 		printf("Layer create failed\n");
// 		exit(1);
	}
	//下面创建属性表
	OGRFieldDefn poFieldID("ID", OFTInteger);//创建ID字段
	poLayer->CreateField(&poFieldID);

	OGRFieldDefn poFieldname("Name", OFTString);//创建Name字段
	poFieldname.SetWidth(32);
	poLayer->CreateField(&poFieldname);

	double x = 0.0, y = 0.0;
	for (int i  = 0; i < 10; i++)
	{
		string szname = "a_" + QString::number(i).toStdString();
		OGRFeature *poFeature;
		poFeature = OGRFeature::CreateFeature( poLayer->GetLayerDefn() );
		poFeature->SetField(0, i);
		poFeature->SetField(1, szname.c_str());

		OGRLinearRing ring;

		ring.addPoint(0 + 10 * i, 0 + 10 *i);
		ring.addPoint(0 +10 * i, 10 + 10 *i);
		ring.addPoint(10 + 10 *i, 10 + 10 *i);
		ring.addPoint(10 + 10 *i, 0 + 10*i);

		ring.closeRings();//首尾点重合形成闭合环
		
		OGRPolygon polygon;
		polygon.addRing(&ring);
		poFeature->SetGeometry(&polygon);

		if ( poLayer->CreateFeature( poFeature) != OGRERR_NONE)
		{
// 			printf("Failed to create feature in shapefile \n");
// 			exit(1);
		}
		OGRFeature::DestroyFeature(poFeature);
	}

	OGRDataSource::DestroyDataSource(poDS);

以上创建代码的结果如下图所示：（QGIS显示）