MapKit 通过 MKOverlay 添加栅格地图奇怪的渲染问题

我已将 NOAA 雷达图像添加到我的天气应用程序中。经过一些测试后，我注意到图像没有在地图上正确绘制。图像的角点似乎接近正确，但当您远离角点时，它会倾斜。下面是相关代码。

Overlay:

@implementation FMRadarOverlay

- (id)initWithImage:(UIImage*)radarImage withLowerLeftCoordinate:(CLLocationCoordinate2D)lowerLeftCoordinate withUpperRightCoordinate:(CLLocationCoordinate2D)upperRightCoordinate{
    self.radarImage = radarImage;

    MKMapPoint lowerLeft = MKMapPointForCoordinate(lowerLeftCoordinate);
    MKMapPoint upperRight = MKMapPointForCoordinate(upperRightCoordinate);
    mapRect = MKMapRectMake(lowerLeft.x, upperRight.y, upperRight.x - lowerLeft.x, lowerLeft.y - upperRight.y);
    return self;
}

- (CLLocationCoordinate2D)coordinate{
    return MKCoordinateForMapPoint(MKMapPointMake(MKMapRectGetMidX(mapRect), MKMapRectGetMidY(mapRect)));
}

- (MKMapRect)boundingMapRect{
    return mapRect;
}

@end

渲染器：

@implementation FMRadarOverlayRenderer

- (void)drawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale inContext:(CGContextRef)context {
    FMRadarOverlay *radarOverlay = (FMRadarOverlay*)self.overlay;
    CGImageRef imageReference = radarOverlay.radarImage.CGImage;

    MKMapRect theMapRect = [radarOverlay boundingMapRect];
    CGRect theRect = [self rectForMapRect:theMapRect];

    CGContextScaleCTM(context, 1.0, -1.0);
    CGContextTranslateCTM(context, 0.0, -theRect.size.height);
    CGContextSetAlpha(context, 0.6);
    CGContextDrawImage(context, theRect, imageReference);
}

@end

以及创建图层的示例：

//using url with map and drawing transparent to check accuracy of rendering
//url for radar data only is http://radar.weather.gov/Conus/RadarImg/latest_radaronly.gif   
NSURL *radarUrl = [NSURL URLWithString:@"http://radar.weather.gov/Conus/RadarImg/latest.gif"];
NSData *radarData = [NSData dataWithContentsOfURL:radarUrl];
UIImage *rawImage = [UIImage imageWithData:radarData];
FMRadarOverlay *radarOverlay = [[FMRadarOverlay alloc] initWithImage:rawImage withLowerLeftCoordinate:CLLocationCoordinate2DMake(21.652538062803, -127.620375523875420) withUpperRightCoordinate:CLLocationCoordinate2DMake(50.406626367301044, -66.517937876818)];

如果您拍摄相同的图像和坐标并使用 kml 文件将它们覆盖在 google 地球上，它们会正确渲染。就好像它没有补偿地球的曲率或其他东西。我已经尝试使用不同的地理参考栅格图层进行此操作，它具有类似的效果。有任何想法吗？

KML 示例：

<?xml version="1.0" encoding="UTF-8"?><kml xmlns="http://earth.google.com/kml/2.0"><Document><name>NWS Radar Images</name><open>1</open><Folder>  <name>National Weather Service</name> <ScreenOverlay>  <name>National Weather Service</name>   <description>National Weather Service Doppler Radar RIDGE Imagery http://radar.weather.gov</description>   <visibility>1</visibility> <Icon>  <href>http://radar.weather.gov/ridge/graphics/nws_google.gif</href>  </Icon>  <overlayXY x="0" y="1" xunits="fraction" yunits="fraction" />  <screenXY x="0" y="1" xunits="fraction" yunits="fraction" />  <rotationXY x="0" y="0" xunits="fraction" yunits="fraction" />  <size x="0" y="0" xunits="fraction" yunits="fraction" />  </ScreenOverlay><ScreenOverlay>  <name>NOAA</name>   <description>National Oceanic and Atomospheric Administration  http://www.noaa.gov</description>   <visibility>1</visibility> <Icon>  <href>http://radar.weather.gov/ridge/graphics/noaa_google.gif</href>  </Icon>  <overlayXY x=".2" y="1" xunits="fraction" yunits="fraction" />  <screenXY x=".2" y="1" xunits="fraction" yunits="fraction" />  <rotationXY x="0" y="0" xunits="fraction" yunits="fraction" />  <size x="0" y="0" xunits="fraction" yunits="fraction" />  </ScreenOverlay></Folder><Folder><name>LATEST_RADARONLY</name><Folder><name>Mosaic</name><ScreenOverlay><name>Legend</name><visibility>1</visibility><Icon><href></href><refreshMode>onInterval</refreshMode><refreshInterval>120</refreshInterval></Icon><overlayXY x="1.0" y="0.5" xunits="fraction" yunits="fraction"/><screenXY x="1.0" y="0.5" xunits="fraction" yunits="fraction"/><rotationXY x="0" y="0" xunits="fraction" yunits="fraction"/></ScreenOverlay><GroundOverlay><name>Mosaic</name><Icon><href>http://radar.weather.gov/ridge/Conus/RadarImg/latest.gif</href><refreshMode>onInterval</refreshMode><refreshInterval>120</refreshInterval></Icon><visibility>1</visibility><LatLonBox><north>50.406626367301044</north><south>21.652538062803</south><east>-66.517937876818</east><west>-127.620375523875420</west></LatLonBox></GroundOverlay></Folder></Folder></Document></kml>

Images of bad rendering:

问题在于图像的空间投影。我最终使用了一个名为 gdalwarp 的工具来重新投影墨卡托投影中的图像，就像 MapKit 使用的那样。我在 MapKit 中看到过很多地面覆盖的例子，但它们都忽略了这个重要信息。我认为大多数地理栅格地图都是针对 Google Earth 或 GIS 应用程序等应用程序进行投影的。