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关于Google Earth中的比例尺、图像精度和eye alt的问题(转)  

2011-12-20 09:52:19|  分类: 空间分析 |  标签: |举报 |字号 订阅

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谷球,也就是谷歌地球,google earth,堪称划时代的创新,不是因为它提供了最好的卫星图(显然不是),而是因为
1 其卫星图可以极度方便的免费获得,前所未有的方便。
2 其卫星图的整合界面极为简单,也就是一个地球的形状
3 有完整的全球信息,虽然有些地方精度不足,但总体细节够多。
可以说,谷球改变了大众对地球的基本视角。必须足够简单方便,影响足够广泛,细节足够精细,才称得上“全人类的基本的”视角。史上第一次,普通人可以真正有效的以全球的、深入细致的角度去观察地球。无论对比传统实体地球仪,还是厚度惊人的地图册,或者昂贵的商业图,谷球显然开创性的做到了这一点。
从此我们观念中的世界图像,将是真正完整的,丰富深入细致立体的,一个非常接近真实地球的虚拟地球。这只有谷球这样对公众开放的海量数据系统才能实现。这东西对于学术研究真是太好了。

最近正在通过谷球调研全国汉地主要佛寺,为了能在统一比例尺下对比研究,我关心了一下谷球的比例和精度问题,有如下结论:
1 首先要记得在view菜单里打开比例尺。
2 同样比例尺数值的图,全屏和非全屏,显示的内容宽度相同,高度随比例变化,像素随放缩情况变化,这显然是谷球程序的预设。同样相对高度的视点下,屏幕所显示的地图内容的宽度是不变的,无论是否全屏,如何缩放。而比例尺的长度固定等于图像宽度的四分之一。
3 右下角的eye alt的意思,官方解释为: The Eye Alt figure at the bottom of the 3D viewer refers to eye altitude (the elevation of your viewpoint).显然,这一海拔高度是那个虚拟相机的高度(眼球海拔),其视角应当是固定的。需要注意的是,此标高需减去图像所在标高才能得到相对地面的视点高度。通过在海平面测试,眼球海拔/屏幕宽度=0.86左右。由此推得,谷球的相机的视角大概是60度。不出所料,果然是这个经典角度。
4 平移图像时,比例尺会变化。应该是由于海拔变化引起的。如果从低海拔平移至高海拔,视点不能低于海拔,所以可能会强行升高视点。
5 要微调比例尺,可以用alt键加上数字键盘的+或者-键。
6 如果不严格控制截图比例,依靠比例尺后期缩放也是很方便的。
7 如果要统一比例尺截图,最好是在定点是设定其range,也就是眼球的相对高度。但是比例尺和屏幕宽度不是此高度的整倍数。此数值无法批量调整(很遗憾,建议改进)。图面比例尺为100M时,range为346米。
8 地球经纬度,每0度0分1秒的球面距离大概是30米。谷球的精度能到15米,也就是半秒。据说新卫星能到3米,似乎目前已经上天了,详情我不知道。。。
9 谷球据说有20级精度,放大到最大精度以后,再放大就只是插值显示了。我不能确定谷球原始图的分辨率,但通过比较不同比例尺输出图像,我觉得大概比例尺在100米时,基本上谷球显示的是1:1的图像,此时截图应该效率最高。谷球大概在比例尺300M时就已经在使用最高图像,不过是缩小显示的。我也觉得似乎50米时的截图细节会略丰富一点点,但50米时显然已经是插值放大了。所以也可能是在比例尺80多米,也就是眼球高度300米的时候为1比1显示。当然也可能谷球是用的英制,那就不好说了。总之,谷球1:1显示图像时的比例尺肯定在150-50之间,按照100M截图差不多。注意,此精度和显示器分辨率有关。我的显示器是1280*1024。输出谷球图宽是1280像素,比例尺100米。其他分辨率的显示器可以类推,也就是说,400米宽的图,像素是1280宽,真实图像精度是 3像素/米。你的显示器像素越多,你的最佳截屏比例尺越长。当然这只是估算。实际谷球卫星图的精度,只有它自己知道。。。另外,谷球软件的输出默认设置似乎对图像精度也有影响,详情未知。
11 据附后的wiki文档指出,一般全球图的精度是15M/像素,高度的精度最高10M,不过这里要注意,图像的分辨率精度和谷球的矢量定位精度是不同的。具体两者的实际精度,应该有谷球的官方白皮书说明才对,可惜我没有。如果谁找到了欢迎提供。
12 以上为本人瞎猜的,欢迎讨论。转载请注明出处。
http://blog.sina.com.cn/yuanmu96
这样有讨论就可以反馈给我以便改进。谢谢。
————
以下wiki的文章:感谢水木敢达版的萝莉控FaYuri同学提供资料并与我讨论。

Resolution and accuracy
The Isles of Scilly, showing the very low resolution of some islands. The islands (green area) are about 10 km across.49°56′10.81″N 6°19′22.88″W / 49.9363361, -6.3230222 (Low resolution Isles of Scilly)
The Isles of Scilly, showing the very low resolution of some islands. The islands (green area) are about 10 km across. [show location on an interactive map] 49°56′10.81″N 6°19′22.88″W / 49.9363361, -6.3230222 (Low resolution Isles of Scilly)
The west side of Gibraltar, tilted view showing the sea rising up the Rock of Gibraltar - claimed altitude of the sea just off the beach at Elliots Memorial, 252 m. 36°6′59.6″N 5°21′5.2″W / 36.116556, -5.351444 (Water altitude problem in Google Earth)
The west side of Gibraltar, tilted view showing the sea rising up the Rock of Gibraltar - claimed altitude of the sea just off the beach at Elliots Memorial, 252 m. [show location on an interactive map] 36°6′59.6″N 5°21′5.2″W / 36.116556, -5.351444 (Water altitude problem in Google Earth)

Most land areas are covered in satellite imagery with a resolution of about 15 m per pixel. Some population centers are also covered by aircraft imagery (orthophotography) with several pixels per meter. Oceans are covered at a much lower resolution, as are a number of islands; most notably, Tórshavn, the capital of the Faroe Islands, and the Isles of Scilly off southwest England, are at a resolution of about 500 m or less. These pictures are provided by Terrametrics.

Google has resolved many inaccuracies in the vector mapping since the original public release of the software, without requiring an update to the program itself. An example of this was the absence from Google Earth's map boundaries of the Nunavut territory in Canada, a territory that had been created on April 1, 1999; this mistake was corrected by one of the data updates in early 2006. Recent updates have also increased the coverage of detailed aerial photography, particularly in certain areas of western Europe, though not including Ireland where imagery remains extremely limited.

The images are not all taken at the same time, but are generally current to within three years. Image sets are sometimes not correctly stitched together. Updates to the photographic database can occasionally be noticed when drastic changes take place in the appearance of the landscape, like for example Google Earth's incomplete updates of New Orleans following Hurricane Katrina, or when placemarks appear to shift unexpectedly across the Earth's surface. Though the placemarks have not in fact moved, the imagery is composed and stitched differently. Such an update to London's photography in early 2006 created shifts of 15-20 metres in many areas, noticeable because the resolution is so high.

Place name and road detail vary greatly from place to place. They are most accurate in North America and Europe, but regular mapping updates are improving coverage elsewhere.

Errors sometimes occur due to the technology used to measure the height of terrain; for example, tall buildings in Adelaide cause one part of the city to be rendered as a small mountain, when it is in fact flat. The height of the Eiffel Tower creates a similar effect in the rendering of Paris. Also, elevations below sea level are presented as sea level; i.e. Salton City, California; Death Valley; and the Dead Sea are all listed as 0 ft when Salton City is approx 200 ft; Death Valley is 286 ft; and the Dead Sea is 1,378 ft.

Where no 3 arc second digital elevation data was available, the three dimensional images covering some areas of high relief are not at all accurate, but most mountain areas are now well mapped. The underlying digital elevation model has been placed 3 arc seconds too far north and up to 3 arc seconds too far west. This means that some steep mountain ridges incorrectly appear to have shadows extending over onto their south facing sides. Some high resolution images have also been misplaced, an example is the image covering Annapurna, which is misplaced by about 12 arc seconds. Elevation data was recently updated to 10-meter (1/3-arc-second) resolution for much of the United States from the previous 30-meter (1-arc-second) resolution.

The "Measure" function shows that the length of equator is about 40,030.24 km, giving an error of 0.112% compared with the actual value of 40,075.02 km Earth; for the meridional circumference, it shows a length of about 39,963.13 km, also giving an error of 0.112% compared with the actual value of 40,007.86 km.

The Arctic polar ice caps are completely absent from the current version of Google Earth, as are waves in the oceans. The geographic North Pole is found hovering over the Arctic Ocean. There is very low resolution coverage of the Antarctic continent (1m resolution images of some parts of Antarctica were added in June 2007 for the first time). The tiling system produces artifacts near the poles as the tiles become 'infinitely' small and rounding errors accumulate.

Cloud cover and shadows can make it difficult or impossible to see details in some land areas, including the shadow side of mountains.


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