使用OGR创建弧形图形
很多时候我们需要在矢量文件中记录一些弧形图形，由于目前OGR中没有支持通过圆心半径等方式来保存矢量图形，所以对于这些矢量图形必须要计算出弧线上的点坐标来进行存储，如果自己按照圆弧的算法来创建这些点确实有些麻烦，OGR中提供了一个叫approximateArcAngles的函数来对圆弧点坐标进行计算，使用起来非常方便，下面是该函数的一个简单说明。函数原型为：
创建一段椭圆弧段。指定一个原点坐标、椭圆的长半轴长度、短半轴长度、旋转角度、起始角度和结束角度来生成一段弧段。参数dfMaxAngleStepSizeDegrees用来指定最大步长，可以使用配置选项OGR_ARC_STEPSIZE来进行设置默认值。生成弧段如图1所示，左侧和下侧两条白色线段分别为X轴和Y轴，交点为坐标原点，绿色椭圆的圆心坐标为(dfCenterX,dfCenterY,dfZ)，长半轴和短半轴长度分别如图中两条蓝色线段表示，起始角度和结束角度方向射线如图中紫色线段所示，图中红色的弧段即为生成的弧段。
图1 函数参数说明示意图
OGRGeometry * OGRGeometryFactory::approximateArcAngles( double dfCenterX, double dfCenterY, double dfZ, double dfPrimaryRadius, double dfSecondaryRadius, double dfRotation, double dfStartAngle, double dfEndAngle, double dfMaxAngleStepSizeDegrees )参数说明：
dfCenterX |
椭圆原点X坐标 |
dfCenterY |
椭圆原点Y坐标 |
dfZ |
椭圆原点Z坐标 |
dfPrimaryRadius |
椭圆长半轴长度 |
dfSecondaryRadius |
椭圆短半轴长度 |
dfRotation |
椭圆顺时针旋转角度 |
dfStartAngle |
弧段起始点角度（与X轴顺时针夹角） |
dfEndAngle |
弧段结束点角度（与X轴顺时针夹角） |
dfMaxAngleStepSizeDegrees |
沿着弧段最长步长对应的角度，设置为0按默认值计算 |
void WriteVectorFile1()
{
// 为了支持中文SHP路径，请添加下面这句代码
CPLSetConfigOption("GDAL_FILENAME_IS_UTF8", "NO");
#ifdef CREATSHP
// 为了使属性表字段支持中文，请添加下面这句，创建SHP
CPLSetConfigOption("SHAPE_ENCODING", "" );
const char* pszVectorFile = "E:\\TestArc.shp";
const char *pszDriverName = "ESRI Shapefile";
#else
// 创建DXF
CPLSetConfigOption("GDAL_DATA", "C:\\warmerda\\GDAL110\\data");
const char* pszVectorFile = "E:\\TestArc.dxf";
const char *pszDriverName = "DXF";
#endif
// 注册所有的驱动
OGRRegisterAll();
//创建数据
OGRSFDriver *poDriver = OGRSFDriverRegistrar::GetRegistrar()->GetDriverByName(pszDriverName );
if( poDriver == NULL )
{
printf( "%s 驱动不可用！\n", pszDriverName );
return;
}
// 创建数据源
OGRDataSource *poDS = poDriver->CreateDataSource(pszVectorFile, NULL );
if( poDS == NULL )
{
printf( "创建矢量文件【%s】失败！\n", pszVectorFile );
return;
}
// 创建图层，创建一个线图层，这里没有指定空间参考，如果需要的话，需要在这里进行指定
OGRLayer *poLayer = poDS->CreateLayer( "TestPolygon", NULL, wkbLineString, NULL );
if( poLayer == NULL )
{
printf( "图层创建失败！\n" );
OGRDataSource::DestroyDataSource( poDS );
return;
}
// 由于DXF不支持属性表，所以下面几句OGR会提示错误信息
// 下面创建属性表
// 先创建一个叫FieldID的整型属性
OGRFieldDefn oFieldID("FieldID", OFTInteger);
poLayer->CreateField(&oFieldID);
// 再创建一个叫FeatureName的字符型属性，字符长度为
OGRFieldDefn oFieldName("FieldName", OFTString);
oFieldName.SetWidth(100);
poLayer->CreateField(&oFieldName);
OGRFeatureDefn *poDefn = poLayer->GetLayerDefn();
// 创建两个坐标轴X和Y
OGRFeature *poFeatureXAxis = OGRFeature::CreateFeature(poDefn);
poFeatureXAxis->SetField(0, 0);
poFeatureXAxis->SetField(1, "X轴");
OGRLineString *pGeoXAxis = (OGRLineString*)OGRGeometryFactory::createGeometry(wkbLineString) ;
pGeoXAxis->setPoint(0, -100, 0);
pGeoXAxis->setPoint(1, 100, 0);
poFeatureXAxis->SetGeometry(pGeoXAxis);
poLayer->CreateFeature( poFeatureXAxis );
OGRFeature::DestroyFeature( poFeatureXAxis );
// 创建Y轴
OGRFeature *poFeatureYAxis = OGRFeature::CreateFeature(poDefn);
poFeatureYAxis->SetField(0, 1);
poFeatureYAxis->SetField(1, "Y轴");
OGRLineString *pGeoYAxis = (OGRLineString*) OGRGeometryFactory::createGeometry(wkbLineString) ;
pGeoYAxis->setPoint(0, 0, -100);
pGeoYAxis->setPoint(1, 0, 100);
poFeatureYAxis->SetGeometry(pGeoYAxis);
poLayer->CreateFeature( poFeatureYAxis );
OGRFeature::DestroyFeature( poFeatureYAxis );
// 创建圆形要素
OGRFeature *poFeatureCircle = OGRFeature::CreateFeature(poDefn);
poFeatureCircle->SetField(0, 2);
poFeatureCircle->SetField(1, "圆形");
OGRGeometry * pCircleLine = OGRGeometryFactory::approximateArcAngles(50.0,-10.0, 0.0, 10.0, 10.0, 0.0, 0.0, 360.0, 1.0 );
poFeatureCircle->SetGeometry(pCircleLine);
poLayer->CreateFeature( poFeatureCircle );
OGRFeature::DestroyFeature( poFeatureCircle );
// 创建圆形要素
OGRFeature *poFeatureEclipse = OGRFeature::CreateFeature(poDefn);
poFeatureEclipse->SetField(0, 3);
poFeatureEclipse->SetField(1, "椭圆");
OGRGeometry * pEclipseLine = OGRGeometryFactory::approximateArcAngles(50.0, 50.0, 0.0, 10.0, 20.0, 45.0, 0.0, 360.0, 1.0 );
poFeatureEclipse->SetGeometry(pEclipseLine);
poLayer->CreateFeature( poFeatureEclipse );
OGRFeature::DestroyFeature( poFeatureEclipse );
// 创建半圆形要素
OGRFeature *poFeatureArc = OGRFeature::CreateFeature(poDefn);
poFeatureArc->SetField(0, 4);
poFeatureArc->SetField(1, "半圆形");
OGRGeometry * pArcLine = OGRGeometryFactory::approximateArcAngles(-50.0,50.0, 0.0, 20.0, 20.0, 0.0, 45.0, 215.0, 1.0 );
poFeatureArc->SetGeometry(pArcLine);
poLayer->CreateFeature( poFeatureArc );
OGRFeature::DestroyFeature( poFeatureArc );
OGRDataSource::DestroyDataSource( poDS );
printf( "\n数据集创建完成！\n" );
}
图2 创建的Shp图形用ArcMap打开标记属性值
图3 创建的DXF图形用AutoCAD打开