今天是2010的第一天,总想把它过得充实点,为我自己新的一年开个好头吧!首先,向关注我博客的网友道声:“元旦快乐!”,其次,接着跟大家分享一下我学习WW中DEM数据的加载和应用心得,希望大家从中有所收获!
DEM应用在WW的三维表现中占有很重要的位置,跟影像数据同等重要!幸好影像和DEM的加载和处理原理上几乎一致,对基于WW搞GIS三维开发来说是件好事,理解好任何一种,另一种触类旁通!前一篇,主要从功能上做了简单入门介绍,该篇将从代码级别分析WW内置的SRTM的DEM数据加载和应用,下一篇讲从二次开发角度上讲解如何处理、配置自己的影像和DEM数据。呵呵,因为DEM部分很重要,且是放假期间我也有时间,争取篇篇精彩!
两个缩写词介绍:因为这两个缩写词常出现,知道是什么缩写,就不觉得神秘啦!
SRTM:The Shuttle Radar Topography Mission (SRTM) obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth. SRTM consisted of a specially modified radar system that flew onboard the Space Shuttle Endeavour during an 11-day mission in February of 2000.
NLT:NASA Learning Technologies.
我从BMNG.cs为例入手研究DEM的使用,当然研究瓦片影像也该从此入手,但,今天影像不是我们关注的重点。现在正式步入主题,跟我一起分析和学习代码吧!
BMNG.cs类144行构造函数中代码,
imageStore.DataDirectory = null;
imageStore.LevelZeroTileSizeDegrees = 36.0;
imageStore.LevelCount = 5;
imageStore.ImageExtension = "jpg";
imageStore.CacheDirectory = String.Format("{0}\\BMNG\\{1}", m_WorldWindow.Cache.CacheDirectory, String.Format("BMNG (Shaded) Tiled - {0}.2004", i + 1));
ias = new WorldWind.ImageStore[1];
ias[0] = imageStore;
m_QuadTileLayers[0, i] = new WorldWind.Renderable.QuadTileSet(
String.Format("Tiled - {0}.2004", i + 1),
m_WorldWindow.CurrentWorld,
0,
90, -90, -180, 180,
true,
ias);
BMNG中的NltImageStore.cs、QuadTileSet类。这是我们关注的对象。
QuadTileSet继承自RenderableObject,是要绘制渲染的对象类。
关注它的562行Update()方法、517行Initialize()方法、 701行Render()方法。
Update()方法
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
{
if (!isInitialized)
Initialize(drawArgs);
if (m_effectPath != null && m_effect == null)
{
string errs = string.Empty;
m_effect = Effect.FromFile(DrawArgs.Device, m_effectPath, null, "", ShaderFlags.None, m_effectPool, out errs);
if (errs != null && errs != string.Empty)
{
Log.Write(Log.Levels.Warning, "Could not load effect " + m_effectPath + ": " + errs);
Log.Write(Log.Levels.Warning, "Effect has been disabled.");
m_effectPath = null;
m_effect = null;
}
}
if (ImageStores[0].LevelZeroTileSizeDegrees < 180)
{
// Check for layer outside view
double vrd = DrawArgs.Camera.ViewRange.Degrees;
double latitudeMax = DrawArgs.Camera.Latitude.Degrees + vrd;
double latitudeMin = DrawArgs.Camera.Latitude.Degrees - vrd;
double longitudeMax = DrawArgs.Camera.Longitude.Degrees + vrd;
double longitudeMin = DrawArgs.Camera.Longitude.Degrees - vrd;
if (latitudeMax < m_south || latitudeMin > m_north || longitudeMax < m_west || longitudeMin > m_east)
return;
}
if (DrawArgs.Camera.ViewRange * 0.5f >
Angle.FromDegrees(TileDrawDistance * ImageStores[0].LevelZeroTileSizeDegrees))
{
lock (m_topmostTiles.SyncRoot)
{
foreach (QuadTile qt in m_topmostTiles.Values)
qt.Dispose();
m_topmostTiles.Clear();
ClearDownloadRequests();
}
return;
}
//知识点,可以看看,如何计算不可见瓦片的算法。
RemoveInvisibleTiles(DrawArgs.Camera);
try
{
int middleRow = MathEngine.GetRowFromLatitude(DrawArgs.Camera.Latitude, ImageStores[0].LevelZeroTileSizeDegrees);
int middleCol = MathEngine.GetColFromLongitude(DrawArgs.Camera.Longitude, ImageStores[0].LevelZeroTileSizeDegrees);
//根据行列号,反推瓦片的四点对应的经度或纬度
double middleSouth = -90.0f + middleRow * ImageStores[0].LevelZeroTileSizeDegrees;
double middleNorth = -90.0f + middleRow * ImageStores[0].LevelZeroTileSizeDegrees + ImageStores[0].LevelZeroTileSizeDegrees;
double middleWest = -180.0f + middleCol * ImageStores[0].LevelZeroTileSizeDegrees;
double middleEast = -180.0f + middleCol * ImageStores[0].LevelZeroTileSizeDegrees + ImageStores[0].LevelZeroTileSizeDegrees;
double middleCenterLat = 0.5f * (middleNorth + middleSouth);
double middleCenterLon = 0.5f * (middleWest + middleEast);
//这里存在一个算法,由中心瓦片框,向四周扩散地找相邻的瓦片矩形框。
int tileSpread = 4;
for (int i = 0; i < tileSpread; i++)
{
for (double j = middleCenterLat - i * ImageStores[0].LevelZeroTileSizeDegrees; j < middleCenterLat + i * ImageStores[0].LevelZeroTileSizeDegrees; j += ImageStores[0].LevelZeroTileSizeDegrees)
{
for (double k = middleCenterLon - i * ImageStores[0].LevelZeroTileSizeDegrees; k < middleCenterLon + i * ImageStores[0].LevelZeroTileSizeDegrees; k += ImageStores[0].LevelZeroTileSizeDegrees)
{
int curRow = MathEngine.GetRowFromLatitude(Angle.FromDegrees(j), ImageStores[0].LevelZeroTileSizeDegrees);
int curCol = MathEngine.GetColFromLongitude(Angle.FromDegrees(k), ImageStores[0].LevelZeroTileSizeDegrees);
long key = ((long)curRow << 32) + curCol;
//如果集合m_topmostTiles已经存在QuadTile,则更新QuadTile
QuadTile qt = (QuadTile)m_topmostTiles[key];
if (qt != null)
{
qt.Update(drawArgs);
continue;
}
// Check for tile outside layer boundaries,获取外边框四点经度或纬度坐标
double west = -180.0f + curCol * ImageStores[0].LevelZeroTileSizeDegrees;
if (west > m_east)
continue;
double east = west + ImageStores[0].LevelZeroTileSizeDegrees;
if (east < m_west)
continue;
double south = -90.0f + curRow * ImageStores[0].LevelZeroTileSizeDegrees;
if (south > m_north)
continue;
double north = south + ImageStores[0].LevelZeroTileSizeDegrees;
if (north < m_south)
continue;
//结合中不存在,创建新的QuadTile
qt = new QuadTile(south, north, west, east, 0, this);
//判断新的QuadTile是否在可视区域中。(可以关注一下:Intersects()方法判断矩形框相交)
{
lock (m_topmostTiles.SyncRoot)
m_topmostTiles.Add(key, qt);
}
}
}
}
}
catch (System.Threading.ThreadAbortException)
{
}
catch (Exception caught)
{
Log.Write(caught);
}
}
Render()方法的关键代码为:
device.VertexFormat = CustomVertex.PositionNormalTextured.Format;
foreach (QuadTile qt in m_topmostTiles.Values)
qt.Render(drawArgs);
从上面可以看出,QuadTileSet可看作是QuadTile的集合,真正实现更新和渲染的是QuadTile对象。里面有影像的加载和渲染绘制,也有DEM的渲染绘制。
我们先看看QuadTile.cs 中Update()方法:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
{
if (m_isResetingCache)
return;
try
{
double tileSize = North - South;
if (!isInitialized)
{
if (DrawArgs.Camera.ViewRange * 0.5f < Angle.FromDegrees(QuadTileSet.TileDrawDistance * tileSize)
&& MathEngine.SphericalDistance(CenterLatitude, CenterLongitude,
DrawArgs.Camera.Latitude, DrawArgs.Camera.Longitude) < Angle.FromDegrees(QuadTileSet.TileDrawSpread * tileSize * 1.25f)
&& DrawArgs.Camera.ViewFrustum.Intersects(BoundingBox)
)
Initialize();
}
if (isInitialized && World.Settings.VerticalExaggeration != verticalExaggeration || m_CurrentOpacity != QuadTileSet.Opacity ||
QuadTileSet.RenderStruts != renderStruts)
{
CreateTileMesh();
}
if (isInitialized)
{
if (DrawArgs.Camera.ViewRange < Angle.FromDegrees(QuadTileSet.TileDrawDistance * tileSize)
&& MathEngine.SphericalDistance(CenterLatitude, CenterLongitude,
DrawArgs.Camera.Latitude, DrawArgs.Camera.Longitude) < Angle.FromDegrees(QuadTileSet.TileDrawSpread * tileSize)
&& DrawArgs.Camera.ViewFrustum.Intersects(BoundingBox)
)
{
if (northEastChild == null || northWestChild == null || southEastChild == null || southWestChild == null)
{
ComputeChildren(drawArgs);
}
if (northEastChild != null)
{
northEastChild.Update(drawArgs);
}
if (northWestChild != null)
{
northWestChild.Update(drawArgs);
}
if (southEastChild != null)
{
southEastChild.Update(drawArgs);
}
if (southWestChild != null)
{
southWestChild.Update(drawArgs);
}
}
else
{
if (northWestChild != null)
{
northWestChild.Dispose();
northWestChild = null;
}
if (northEastChild != null)
{
northEastChild.Dispose();
northEastChild = null;
}
if (southEastChild != null)
{
southEastChild.Dispose();
southEastChild = null;
}
if (southWestChild != null)
{
southWestChild.Dispose();
southWestChild = null;
}
}
}
if (isInitialized)
{
if (DrawArgs.Camera.ViewRange / 2 > Angle.FromDegrees(QuadTileSet.TileDrawDistance * tileSize * 1.5f)
|| MathEngine.SphericalDistance(CenterLatitude, CenterLongitude, DrawArgs.Camera.Latitude, DrawArgs.Camera.Longitude) > Angle.FromDegrees(QuadTileSet.TileDrawSpread * tileSize * 1.5f))
{
if (Level != 0 || (Level == 0 && !QuadTileSet.AlwaysRenderBaseTiles))
this.Dispose();
}
}
}
catch
{
}
}
创建下一级的四个瓦片的方法:(可以被我们重用)
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
{
if (Level + 1 >= QuadTileSet.ImageStores[0].LevelCount)
return;
//计算瓦片的中点经纬度
double CenterLat = 0.5f * (South + North);
double CenterLon = 0.5f * (East + West);
if (northWestChild == null)
northWestChild = ComputeChild(CenterLat, North, West, CenterLon);
if (northEastChild == null)
northEastChild = ComputeChild(CenterLat, North, CenterLon, East);
if (southWestChild == null)
southWestChild = ComputeChild(South, CenterLat, West, CenterLon);
if (southEastChild == null)
southEastChild = ComputeChild(South, CenterLat, CenterLon, East);
}
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
/// Returns the QuadTile for specified location if available.
/// Tries to queue a download if not available.
/// </summary>
/// <returns>Initialized QuadTile if available locally, else null.</returns>
private QuadTile ComputeChild(double childSouth, double childNorth, double childWest, double childEast)
{
QuadTile child = new QuadTile(
childSouth,
childNorth,
childWest,
childEast,
this.Level + 1,
QuadTileSet);
return child;
}
QuadTile.cs 中的CreateTileMesh()方法用来创建瓦片格网的,分别在Initialize() 、Update()方法中调用。
410行 这里调用的CreateElevatedMesh();是添加DEM数据创建高程格网的。
/// Builds flat or terrain mesh for current tile
/// </summary>
public virtual void CreateTileMesh()
{
verticalExaggeration = World.Settings.VerticalExaggeration;
m_CurrentOpacity = QuadTileSet.Opacity;
renderStruts = QuadTileSet.RenderStruts;
if (QuadTileSet.TerrainMapped && Math.Abs(verticalExaggeration) > 1e-3)
CreateElevatedMesh();
else
CreateFlatMesh();
}
591行CreateElevatedMesh()
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
/// Build the elevated terrain mesh
/// </summary>
protected virtual void CreateElevatedMesh()
{
isDownloadingTerrain = true;
// Get height data with one extra sample around the tile
double degreePerSample = LatitudeSpan / vertexCountElevated;
TerrainTile tile = QuadTileSet.World.TerrainAccessor.GetElevationArray(North + degreePerSample, South - degreePerSample, West - degreePerSample, East + degreePerSample, vertexCountElevated + 3);
float[,] heightData = tile.ElevationData;
int vertexCountElevatedPlus3 = vertexCountElevated / 2 + 3;
int totalVertexCount = vertexCountElevatedPlus3 * vertexCountElevatedPlus3;
northWestVertices = new CustomVertex.PositionNormalTextured[totalVertexCount];
southWestVertices = new CustomVertex.PositionNormalTextured[totalVertexCount];
northEastVertices = new CustomVertex.PositionNormalTextured[totalVertexCount];
southEastVertices = new CustomVertex.PositionNormalTextured[totalVertexCount];
double layerRadius = (double)QuadTileSet.LayerRadius;
// Calculate mesh base radius (bottom vertices)
// Find minimum elevation to account for possible bathymetry
float minimumElevation = float.MaxValue;
float maximumElevation = float.MinValue;
foreach (float height in heightData)
{
if (height < minimumElevation)
minimumElevation = height;
if (height > maximumElevation)
maximumElevation = height;
}
minimumElevation *= verticalExaggeration;
maximumElevation *= verticalExaggeration;
if (minimumElevation > maximumElevation)
{
// Compensate for negative vertical exaggeration
minimumElevation = maximumElevation;
maximumElevation = minimumElevation;
}
double overlap = 500 * verticalExaggeration; // 500m high tiles
// Radius of mesh bottom grid
meshBaseRadius = layerRadius + minimumElevation - overlap;
CreateElevatedMesh(ChildLocation.NorthWest, northWestVertices, meshBaseRadius, heightData);
CreateElevatedMesh(ChildLocation.SouthWest, southWestVertices, meshBaseRadius, heightData);
CreateElevatedMesh(ChildLocation.NorthEast, northEastVertices, meshBaseRadius, heightData);
CreateElevatedMesh(ChildLocation.SouthEast, southEastVertices, meshBaseRadius, heightData);
BoundingBox = new BoundingBox((float)South, (float)North, (float)West, (float)East,
(float)layerRadius, (float)layerRadius + 10000 * this.verticalExaggeration);
QuadTileSet.IsDownloadingElevation = false;
// Build common set of indexes for the 4 child meshes
int vertexCountElevatedPlus2 = vertexCountElevated / 2 + 2;
vertexIndexes = new short[2 * vertexCountElevatedPlus2 * vertexCountElevatedPlus2 * 3];
int elevated_idx = 0;
for (int i = 0; i < vertexCountElevatedPlus2; i++)
{
for (int j = 0; j < vertexCountElevatedPlus2; j++)
{
vertexIndexes[elevated_idx++] = (short)(i * vertexCountElevatedPlus3 + j);
vertexIndexes[elevated_idx++] = (short)((i + 1) * vertexCountElevatedPlus3 + j);
vertexIndexes[elevated_idx++] = (short)(i * vertexCountElevatedPlus3 + j + 1);
vertexIndexes[elevated_idx++] = (short)(i * vertexCountElevatedPlus3 + j + 1);
vertexIndexes[elevated_idx++] = (short)((i + 1) * vertexCountElevatedPlus3 + j);
vertexIndexes[elevated_idx++] = (short)((i + 1) * vertexCountElevatedPlus3 + j + 1);
}
}
calculate_normals(ref northWestVertices, vertexIndexes);
calculate_normals(ref southWestVertices, vertexIndexes);
calculate_normals(ref northEastVertices, vertexIndexes);
calculate_normals(ref southEastVertices, vertexIndexes);
isDownloadingTerrain = false;
}
596行TerrainTile tile = QuadTileSet.World.TerrainAccessor.GetElevationArray(North + degreePerSample, South - degreePerSample, West - degreePerSample, East + degreePerSample, vertexCountElevated + 3);
获取样本点的高程值数组。
使用了TerrainAccessor.cs类120行代码
{
TerrainTile res = null;
res = new TerrainTile(null);
res.North = north;
res.South = south;
res.West = west;
res.East = east;
res.SamplesPerTile = samples;
res.IsInitialized = true;
res.IsValid = true;
double latrange = Math.Abs(north - south);
double lonrange = Math.Abs(east - west);
float[,] data = new float[samples,samples];
float scaleFactor = (float)1/(samples - 1);
for(int x = 0; x < samples; x++)
{
for(int y = 0; y < samples; y++)
{
double curLat = north - scaleFactor * latrange * x;
double curLon = west + scaleFactor * lonrange * y;
//关键,获取瓦片所有样本点的高程值
data[x,y] = GetElevationAt(curLat, curLon, 0);
}
}
res.ElevationData = data;
return res;
}
关键代码:data[x,y] = GetElevationAt(curLat, curLon, 0);
GetElevationAt();在TerrainAccessor.cs是抽象方法。真正实现是在TerrainAccessor的子类NltTerrainAccessor中重载实现的。
120行 public override float GetElevationAt(double latitude, double longitude)
{
return GetElevationAt(latitude, longitude, m_terrainTileService.SamplesPerTile / m_terrainTileService.LevelZeroTileSizeDegrees);
}
TerrainAccessor对象哪里来的(即:在哪完成初始化和传入的?)
ConfigurationLoader.cs的Load()方法的97行代码:
TerrainAccessor[] terrainAccessor = getTerrainAccessorsFromXPathNodeIterator(worldIter.Current.Select("TerrainAccessor"),
System.IO.Path.Combine(cache.CacheDirectory, worldName));
World newWorld = new World(
worldName,
new Microsoft.DirectX.Vector3(0, 0, 0),
new Microsoft.DirectX.Quaternion(0, 0, 0, 0),
equatorialRadius,
cache.CacheDirectory,
(terrainAccessor != null ? terrainAccessor[0] : null)//TODO: Oops, World should be able to handle an array of terrainAccessors
);
然后通过World对象传入到QuadTileSet类中的。
我们看看getTerrainAccessorsFromXPathNodeIterator方法如何完成完成TerrainAccessor对象。
注意:该方法返回值为TerrainAccessor[],是个数组,为什么呢??(请关注我下一篇文章)
2078行
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
{
System.Collections.ArrayList terrainAccessorList = new System.Collections.ArrayList();
{
string terrainAccessorName = iter.Current.GetAttribute("Name", "");
if(terrainAccessorName == null)
{
// TODO: Throw exception?
continue;
}
XPathNodeIterator latLonBoxIter = iter.Current.Select("LatLonBoundingBox");
if(latLonBoxIter.Count != 1)
{
// TODO: Throw exception?
continue;
}
double north = 0;
double south = 0;
double west = 0;
double east = 0;
latLonBoxIter.MoveNext();
north = ParseDouble(getInnerTextFromFirstChild(latLonBoxIter.Current.Select("North")));
south = ParseDouble(getInnerTextFromFirstChild(latLonBoxIter.Current.Select("South")));
west = ParseDouble(getInnerTextFromFirstChild(latLonBoxIter.Current.Select("West")));
east = ParseDouble(getInnerTextFromFirstChild(latLonBoxIter.Current.Select("East")));
TerrainAccessor[] higerResolutionSubsets = getTerrainAccessorsFromXPathNodeIterator(
iter.Current.Select("HigherResolutionSubsets"),
Path.Combine(cacheDirectory, terrainAccessorName));
XPathNodeIterator tileServiceIter = iter.Current.Select("TerrainTileService");
if(tileServiceIter.Count == 1)
{
string serverUrl = null;
string dataSetName = null;
double levelZeroTileSizeDegrees = double.NaN;
uint numberLevels = 0;
uint samplesPerTile = 0;
string dataFormat = null;
string fileExtension = null;
string compressionType = null;
tileServiceIter.MoveNext();
serverUrl = getInnerTextFromFirstChild(tileServiceIter.Current.Select("ServerUrl"));
dataSetName = getInnerTextFromFirstChild(tileServiceIter.Current.Select("DataSetName"));
levelZeroTileSizeDegrees = ParseDouble(getInnerTextFromFirstChild(tileServiceIter.Current.Select("LevelZeroTileSizeDegrees")));
numberLevels = uint.Parse(getInnerTextFromFirstChild(tileServiceIter.Current.Select("NumberLevels")));
samplesPerTile = uint.Parse(getInnerTextFromFirstChild(tileServiceIter.Current.Select("SamplesPerTile")));
dataFormat = getInnerTextFromFirstChild(tileServiceIter.Current.Select("DataFormat"));
fileExtension = getInnerTextFromFirstChild(tileServiceIter.Current.Select("FileExtension"));
compressionType = getInnerTextFromFirstChild(tileServiceIter.Current.Select("CompressonType"));
//根据配置信息创建TerrainTileService对象和TerrainAccessor对象
TerrainTileService tts = new TerrainTileService(
serverUrl,
dataSetName,
levelZeroTileSizeDegrees,
(int)samplesPerTile,
fileExtension,
(int)numberLevels,
Path.Combine(cacheDirectory, terrainAccessorName),
World.Settings.TerrainTileRetryInterval,
dataFormat);
TerrainAccessor newTerrainAccessor = new NltTerrainAccessor(
terrainAccessorName,
west,
south,
east,
north,
tts,
higerResolutionSubsets);
terrainAccessorList.Add(newTerrainAccessor);
}
//TODO: Add Floating point terrain Accessor code
//TODO: Add WMSAccessor code and make it work in TerrainAccessor (which it currently doesn't)
}
if(terrainAccessorList.Count > 0)
{
return (TerrainAccessor[])terrainAccessorList.ToArray(typeof(TerrainAccessor));
}
else
{
return null;
}
}
再来看看DEM的配置在哪里和XML内容吧
路径:
C:\Program Files\NASA\World Wind 1.4\Config\Earth.xml
配置内容:
<World Name="Earth" EquatorialRadius="6378137.0" LayerDirectory="Earth" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="WorldXmlDescriptor.xsd">
<TerrainAccessor Name="SRTM"> //黄色之间的XML就是一个TerrainAccessor配置
<TerrainTileService>
<ServerUrl>http://worldwind25.arc.nasa.gov/wwelevation/wwelevation.aspx</ServerUrl>
<DataSetName>srtm30pluszip</DataSetName>
<LevelZeroTileSizeDegrees>20.0</LevelZeroTileSizeDegrees>
<NumberLevels>12</NumberLevels>
<SamplesPerTile>150</SamplesPerTile>
<DataFormat>Int16</DataFormat>
<FileExtension>bil</FileExtension>
<CompressonType>zip</CompressonType>
</TerrainTileService>
<LatLonBoundingBox>
<North>
<Value>90.0</Value>
</North>
<South>
<Value>-90.0</Value>
</South>
<West>
<Value>-180.0</Value>
</West>
<East>
<Value>180.0</Value>
</East>
</LatLonBoundingBox>
</TerrainAccessor>
</World>
接着上面的讲NltTerrainAccessor类76行代码GetElevationAt(double latitude, double longitude, double targetSamplesPerDegree)方法。
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
/// Get terrain elevation at specified location.
/// </summary>
/// <param name="latitude">Latitude in decimal degrees.</param>
/// <param name="longitude">Longitude in decimal degrees.</param>
/// <param name="targetSamplesPerDegree"></param>
/// <returns>Returns 0 if the tile is not available on disk.</returns>
public override float GetElevationAt(double latitude, double longitude, double targetSamplesPerDegree)
{
try
{
if (m_terrainTileService == null || targetSamplesPerDegree < World.Settings.MinSamplesPerDegree)
return 0;
if (m_higherResolutionSubsets != null)
{
foreach (TerrainAccessor higherResSub in m_higherResolutionSubsets)
{
if (latitude > higherResSub.South && latitude < higherResSub.North &&
longitude > higherResSub.West && longitude < higherResSub.East)
{
return higherResSub.GetElevationAt(latitude, longitude, targetSamplesPerDegree);
}
}
}
//自己可以看看如何完成TerrainTile的初始化构建的
TerrainTile tt = m_terrainTileService.GetTerrainTile(latitude, longitude, targetSamplesPerDegree);
TerrainTileCacheEntry ttce = (TerrainTileCacheEntry)m_tileCache[tt.TerrainTileFilePath];
if (ttce == null)
{
ttce = new TerrainTileCacheEntry(tt);
AddToCache(ttce);
}
if (!ttce.TerrainTile.IsInitialized)
ttce.TerrainTile.Initialize();
ttce.LastAccess = DateTime.Now;
}
catch (Exception)
{
}
return 0;
}
上面获取高程值的关键:TerrainTile类的330行的GetElevationAt(double latitude, double longitude)方法
{
try
{
double deltaLat = North - latitude;
double deltaLon = longitude - West;
//TileSizeDegrees为当前级别下瓦片的度数大小
double df2 = (SamplesPerTile-1) / TileSizeDegrees;
float lat_pixel = (float)(deltaLat * df2);
float lon_pixel = (float)(deltaLon * df2);
//这里是将点,近似成包含点的最小正方形(经纬度取整)
int lat_min = (int)lat_pixel;
int lat_max = (int)Math.Ceiling(lat_pixel);
int lon_min = (int)lon_pixel;
int lon_max = (int)Math.Ceiling(lon_pixel);
if(lat_min >= SamplesPerTile)
lat_min = SamplesPerTile - 1;
if(lat_max >= SamplesPerTile)
lat_max = SamplesPerTile - 1;
if(lon_min >= SamplesPerTile)
lon_min = SamplesPerTile - 1;
if(lon_max >= SamplesPerTile)
lon_max = SamplesPerTile - 1;
if(lat_min < 0)
lat_min = 0;
if(lat_max < 0)
lat_max = 0;
if(lon_min < 0)
lon_min = 0;
if(lon_max < 0)
lon_max = 0;
float delta = lat_pixel - lat_min;
float westElevation =
ElevationData[lon_min, lat_min]*(1-delta) +
ElevationData[lon_min, lat_max]*delta;
float eastElevation =
ElevationData[lon_max, lat_min]*(1-delta) +
ElevationData[lon_max, lat_max]*delta;
//利用插值计算点的高程值
delta = lon_pixel - lon_min;
float interpolatedElevation =
westElevation*(1-delta) +
eastElevation*delta;
return interpolatedElevation;
}
catch
{
}
return 0;
}
public float[,] ElevationData就是存放当前瓦片所有样本点高程值的数值。这是通过Initialize()中读取DEM(.bil)文件来获取的。
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
/// This method initializes the terrain tile add switches to
/// Initialize floating point/int 16 tiles
/// </summary>
public void Initialize()
{
if(IsInitialized)
return;
if(!File.Exists(TerrainTileFilePath))
{
// Download elevation
if(request==null)
{
using( request = new TerrainDownloadRequest(this, m_owner, Row, Col, TargetLevel) )
{
request.SaveFilePath = TerrainTileFilePath;
request.DownloadInForeground();
}
}
}
if(ElevationData==null)
ElevationData = new float[SamplesPerTile, SamplesPerTile];
if(File.Exists(TerrainTileFilePath))
{
// Load elevation file
try
{
// TerrainDownloadRequest's FlagBadTile() creates empty files
// as a way to flag "bad" terrain tiles.
// Remove the empty 'flag' files after preset time.
try
{
FileInfo tileInfo = new FileInfo(TerrainTileFilePath);
if(tileInfo.Length == 0)
{
TimeSpan age = DateTime.Now.Subtract( tileInfo.LastWriteTime );
if(age < m_owner.TerrainTileRetryInterval)
{
// This tile is still flagged bad
IsInitialized = true;
}
else
{
// remove the empty 'flag' file
File.Delete(TerrainTileFilePath);
}
return;
}
}
catch
{
// Ignore any errors in the above block, and continue.
// For example, if someone had the empty 'flag' file
// open, the delete would fail.
}
//读取BIL文件数据的关键代码,可以被我们借鉴使用
using( Stream s = File.OpenRead(TerrainTileFilePath))
{
BinaryReader reader = new BinaryReader(s);
if(m_owner.DataType=="Int16")
{
for(int y = 0; y < SamplesPerTile; y++)
for(int x = 0; x < SamplesPerTile; x++)
ElevationData[x,y] = reader.ReadInt16();
}
if(m_owner.DataType=="Float32")
{
for(int y = 0; y < SamplesPerTile; y++)
for(int x = 0; x < SamplesPerTile; x++)
{
ElevationData[x,y] = reader.ReadSingle();
}
}
IsInitialized = true;
IsValid = true;
}
return;
}
catch(IOException)
{
// If there is an IO exception when reading the terrain tile,
// then either something is wrong with the file, or with
// access to the file, so try and remove it.
try
{
File.Delete(TerrainTileFilePath);
}
catch(Exception ex)
{
throw new ApplicationException(String.Format("Error while trying to delete corrupt terrain tile {0}", TerrainTileFilePath), ex);
}
}
catch(Exception ex)
{
// Some other type of error when reading the terrain tile.
throw new ApplicationException(String.Format("Error while trying to read terrain tile {0}", TerrainTileFilePath), ex);
}
}
}
另注:SRTM的高程数据存放路径如下图:(DEM跟影像也是分级存放的,存放方式一致)
至此,如何加载DEM数据创建网格的过程已经分析完了。
接下来,继续分析QuadTile.cs中CreateElevatedMesh()和Render方法,内容主要是DirectX编程,稍后添加……