作者：Flyingis
空間參考是GIS的基礎，失去了空間參考信息，地理空間內所有的信息也就失去了存在的意義，因為它們是不準確的或是錯誤的。關于ArcGIS坐標系統(tǒng)文件，可以看看這篇文章——《ArcGIS 坐標系統(tǒng)文件》。
刻畫Spatial Reference的精度
首先，我們主要討論的是ArcGIS中Spatial Reference的各種精度，看到resolution、tolerance、domain、scale factor、precision，是否很熟悉？為了保證表達的準確性，所有這些關鍵字使用英文表述，不再譯為中文。
Resolution和domain范圍決定了Geometry坐標的存儲方式，Resolution和關聯(lián)的坐標系使用相同的數(shù)量單位，如當空間參考使用以米為單位的投影參考時，XY resolution單位為米，默認情況下resolution=0.0001m，不管怎么樣resolution值至少應該小于數(shù)據精度的1/10。當定位某一坐標到坐標格網時，我們依據如下公式：
Persisted coordinate = Round((map coordinate - minimum domain extent) / resolution)
在ArcGIS 9.2之前，resolution=1/precision，ArcGIS 9.2認為resolution和precision幾乎相同，在9.2之前坐標的存儲精度是31位，9.2中為53位，對于上面的公式而言，當resolution很小時，坐標系統(tǒng)表達數(shù)據會更精確，但在9.2之前的ArcGIS中persisted coordinate會受到限制。例如在ArcGIS 9.2之前，當minimum domain value=0，resolution=1時，maximum domain value=231-2，resolution=0.0001時，maximum domain value=(231-2)*0.0001=214748.3647；在ArcGIS 9.2中，當minimum domain value=0，resolution=1時，maximum domain value=253-2，resolution=0.0001時，maximum domain value=(253-2)*0.0001，很顯然，ArcGIS 9.1中maximum domain value=214748.3647已經不能滿足UTM、State Plane等投影坐標系的要求，ArcGIS 9.2存儲的數(shù)據可以擁有更高精度的空間參考。
默認情況下，ArcGIS 9.2為整數(shù)坐標采用53位空間存儲，當然在編輯沒有升級空間參考的空間數(shù)據庫中的數(shù)據時，也可以保持向下兼容。新的COM接口已經可以用來判斷數(shù)據采用的是低精度的各種空間參考，還是高精度的，同時有新的接口可以在低精度空間參考和高精度空間參考之間轉換。
還是默認情況下，Tolerance=10*resolution，minimum tolerance=2*resolution=2.0/scale factor，tolerance決定在relational和topological操作中，兩個坐標之間的最小距離，當小于該距離時，認為這兩個坐標為相同的坐標。relational和topological操作采用不同的tolerance會得到不同的處理結果。9.2之前tolerance值受resolution的影響，9.2中必須要明確指定tolerance的值，在IRelationalOperator、ITopologicalOperator等對兩個幾何對象進行Geometry操作的接口中，使用第一個對象的tolerance來判斷兩幾何體點點之間的關系，如果空間參考是未定義，或沒有空間參考和幾何體關聯(lián)，將采用該格網所允許的最小tolerance取值。
如何編程？
使用SpatialReferenceEnvironment
private void PrintPreDefinedProjections()
{
ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
ISet projectionSet = spatialReferenceFactory.CreatePredefinedProjections();
System.Windows.Forms.MessageBox.Show("Number of predefined Projections = " + projectionSet.Count);
projectionSet.Reset();
for (int i = 0; i < projectionSet.Count; i++)
{
IProjection projection = projectionSet.Next() as IProjection;
System.Windows.Forms.MessageBox.Show(projection.Name);
}
}
CreatePredefinedProjections返回AE中所有預定義的投影坐標，一共59個。ISpatialReferenceFactory的CreateESRISpatialReferenceFromPRJFile方法可以從已定義的PRJ文件獲取坐標。
private IProjectedCoordinateSystem LoadProjectedCoordinateSystem()
{
ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
IProjectedCoordinateSystem projectedCoordinateSystem = spatialReferenceFactory.CreateESRISpatialReferenceFromPRJFile("C:\\Program Files\\ArcGIS\\Coordinate Systems\\Projected Coordinate Systems\\World\\Mollweide (world).prj") as IProjectedCoordinateSystem;
return projectedCoordinateSystem;
}
除了ISpatialReferenceFactory接口，AE還提供ISpatialReferenceFactory3接口實現(xiàn)了創(chuàng)建垂直坐標系、構建高精度坐標系和低精度坐標系的系列方法。
private void ConstructCoordinateSystem(bool highPrecision)
{
ISpatialReferenceFactory3 spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
ISpatialReference3 spatialReference = spatialReferenceFactory.CreateESRISpatialReferenceFromPRJFile("D:\\ArcGIS\\Coordinate Systems\\Geographic Coordinate Systems\\World\\WGS 1984.prj") as ISpatialReference3;
IControlPrecision2 controlPrecision = spatialReference as IControlPrecision2;
//Determines whether you are constructing a high or low.
controlPrecision.IsHighPrecision = highPrecision;
ISpatialReferenceResolution spatialReferenceResolution = spatialReference as ISpatialReferenceResolution;
//These three methods are the keys, construct horizon, then set the default x,y resolution and tolerance.
spatialReferenceResolution.ConstructFromHorizon();
//Set the default x,y resolution value.
spatialReferenceResolution.SetDefaultXYResolution();
//Set the default x,y tolerance value.
ISpatialReferenceTolerance spatialReferenceTolerance = spatialReference as ISpatialReferenceTolerance;
spatialReferenceTolerance.SetDefaultXYTolerance();
double xMin;
double xMax;
double yMin;
double yMax;
spatialReference.GetDomain(out xMin, out xMax, out yMin, out yMax);
System.Windows.Forms.MessageBox.Show("Domain : " + xMin + ", " + xMax + ", " + yMin + ", " + yMax);
}
IControlPrecision2.IsHighPrecision用來判斷是否對數(shù)據采用高精度坐標，后面的設置空間參考的方法將根據這個判斷來決定各種參數(shù)的精確程度。highPrecision等于true或false時，返回的Domain分別是：
highPrecision=true -400 9006799.25474099 -400 9006799.25474099
highPrecision=false -400 793.04646944444426 -400 793.04646944444426
可以看出兩者之間精度差別的大小。
我們可以創(chuàng)建一個新的投影坐標系并保存為prj文件，同時可以利用這個prj生成一個新的投影坐標系。
private void ImportExportSR_Example()
{
//Instantiate a predefined spatial reference and set its coordinate grid information.
ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
IProjectedCoordinateSystem projectedCoordinateSystem = spatialReferenceFactory.CreateProjectedCoordinateSystem((int)esriSRProjCSType.esriSRProjCS_WGS1984UTM_10N);
ISpatialReferenceResolution spatialReferenceResolution = projectedCoordinateSystem as ISpatialReferenceResolution;
ISpatialReferenceTolerance spatialReferenceTolerance = projectedCoordinateSystem as ISpatialReferenceTolerance;
spatialReferenceResolution.ConstructFromHorizon();
spatialReferenceTolerance.SetDefaultXYTolerance();
//Export the PCS to a .prj file.
String fileName = "c:\\temp\\utm10.prj";
spatialReferenceFactory.ExportESRISpatialReferenceToPRJFile(fileName, projectedCoordinateSystem);
//Rehydrate it as a new spatial reference object.
ISpatialReference projectedCoordinateSystem2 = spatialReferenceFactory.CreateESRISpatialReferenceFromPRJFile(fileName);
//See if they are equal.
IClone comparison = projectedCoordinateSystem as IClone;
//Should be true, but coordinate grid information has not been checked.
System.Windows.Forms.MessageBox.Show((comparison.IsEqual(projectedCoordinateSystem2 as IClone)).ToString());
ISpatialReference2 comparePrecisions = projectedCoordinateSystem as ISpatialReference2;
//Should be false, PRJ files do not persist coordinate grid information.
System.Windows.Forms.MessageBox.Show((comparePrecisions.IsXYPrecisionEqual(projectedCoordinateSystem2)).ToString());
}
兩個空間參考是否相等不能簡單通過一個方法來確定，IClone.isEqual只能比較坐標系相同，而不能確定坐標格網信息是否相同，因此最后一行代碼比較的結果是false(IsXYPrecisionEqual是ISpatialReference2接口中的方法)。最后，看看如何自定義一個完整的投影坐標參考系，地理坐標系和垂直坐標系定義方法類似。
private void SetProjectionParameters()
{
//Create a factory.
ISpatialReferenceFactory2 spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
//Create a projection, GeographicCoordinateSystem, and unit using the factory.
IProjectionGEN projection = spatialReferenceFactory.CreateProjection((int)esriSRProjectionType.esriSRProjection_Sinusoidal) as IProjectionGEN;
IGeographicCoordinateSystem geographicCoordinateSystem = spatialReferenceFactory.CreateGeographicCoordinateSystem((int)
esriSRGeoCSType.esriSRGeoCS_WGS1984);
ILinearUnit unit = spatialReferenceFactory.CreateUnit((int)esriSRUnitType.esriSRUnit_Meter) as ILinearUnit;
//Get the default parameters from the projection.
IParameter[] parameters = projection.GetDefaultParameters();
//Iterate through the parameters and print out their name and value.
for (int i = 0; i < parameters.Length; i++)
{
IParameter currentParameter = parameters[i];
if (currentParameter != null)
{
System.Windows.Forms.MessageBox.Show(currentParameter.Name + ", " + currentParameter.Index + ", " + currentParameter.Value);
}
}
//Reset one of the parameter values.
IParameter parameter = parameters[2];
parameter.Value = 45;
//Create a projected coordinate system using the Define method.
IProjectedCoordinateSystemEdit projectedCoordinateSystemEdit = new ProjectedCoordinateSystemClass();
object name = "Newfoundland";
object alias = "NF_LAB";
object abbreviation = "NF";
object remarks = "Most Eastern Province in Canada";
object usage = "When making maps of Newfoundland";
object geographicCoordinateSystemObject = geographicCoordinateSystem as object;
object unitObject = unit as object;
object projectionObject = projection as object;
object parametersObject = parameters as object;
projectedCoordinateSystemEdit.Define(ref name,
ref alias,
ref abbreviation,
ref remarks,
ref usage,
ref geographicCoordinateSystemObject,
ref unitObject,
ref projectionObject,
ref parametersObject);
IProjectedCoordinateSystem4GEN projectedCoordinateSystem = projectedCoordinateSystemEdit as IProjectedCoordinateSystem4GEN;
//Get the parameters from your new projected coordinate system and verify
//that the parameter value was changed.
//Create an array of IParameters with 16 elements.
IParameter[] newParameters = new IParameter[16];
//Get the parameters.
projectedCoordinateSystem.GetParameters(ref newParameters);
//Iterate through the parameters and print out their name and value.
for (int i = 0; i < newParameters.Length; i++)
{
IParameter currentNewParameter = newParameters[i];
if (currentNewParameter != null)
{
System.Windows.Forms.MessageBox.Show(currentNewParameter.Name + ", " + currentNewParameter.Index + ", " + currentNewParameter.Value);
}
}
}
具體的可以設置每一個參數(shù)，精確定義空間參考。
可以看到，關于Spatial Reference編程與實現(xiàn)無非就是在幾個接口之間實現(xiàn)，了解了這幾個接口及它們之間的聯(lián)系（參考GeometryObjectModel.pdf文檔），也就了解了Spatial Reference編程的方法，當然基礎是地理空間參考的基本理論知識。