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撒哈拉沙漠是什么颜色,从轨道?

2015 - 05 - 04 - t12:40:04z

在地球的图像从飞船(或合成图像),颜色呈现似乎很不一致。

Examples

(Source: NASA, https://visibleearth.nasa.gov/images/57735/the-blue-marble-land-surface-ocean-color-sea-ice-and-clouds)

This is one of NASA's composite images. Here, the Sahara is yellow.

(Source: NASA, https://visibleearth.nasa.gov/images/55418/the-blue-marble-from-apollo-17)

This is a photo from an Apollo mission. Here, the Sahara is an orange/red ochre. This is a very different color.

I realize that there are annual variations to be expected in the color of the landscape, but the above differences seem to be too large to be accounted for by that effect.

Is the Sahara ochre or yellow?

Jezibelle回答的是什么颜色的撒哈拉沙漠,从轨道?

2015 - 05 - 04 - t16:36:44z

记得从学校当你得知光实际上是光子在特定频率振荡,我们认为这些频率和特定的颜色吗?从技术上讲,在光谱波段卫星测量反射光,每个乐队实际上是一个范围的波长或频率。的原始数据发送回地球的卫星由测量的观测到的光量在每个乐队在每个像素。一些乐队在大多数卫星发生对应的频率,我们通常认为特定颜色(红、绿、蓝色是最常见的),和这些颜色通常选择可视化数据在这些乐队。(参见:http://earthobservatory.nasa.gov/blogs/elegantfigures/2013/10/22/how-to-make-a-true-color-landsat-8-image/ < a href = " http://earthobservatory.nasa.gov/blogs/elegantfigures/2013/10/22/how-to-make-a-true-color-landsat-8-image/ " > < / >更好的解释的复合图像是怎样制成的)所以,原因有很多合成图像之间的颜色会改变,包括: < ol > <李>创建不同的合成图像的人选择不同的色调为红色,绿色和蓝色来代表这些值对应于近似颜色在地上。李李< / > < >这两个仪器(MODIS或陆地卫星与阿波罗)收集的数据的一般乐队在一天中的不同时间和/或在不同范围的实际光波长和/或使用不同的算法来计算反映在每个波段的光量的测量值的数量反映了红色、绿色和蓝色光在不同的图像不同< /李> <李>的东西改变了沙子或大气的光谱属性(例如下雨或者有尘暴)前后的图像拍摄实际反射率值在不同的图像不同< /李> < / ol > 简短的回答你的问题可能是,它随着时间和空间变化和仪器捕捉图像,平均而言,是介于黄色和赭石。

罗伯特·西蒙回答的是什么颜色的撒哈拉沙漠,从轨道?

2015 - 07 - 18 - t04:20:02z

。撒哈拉沙漠是异质的。黄色,红色,橙色,棕色,黑色,绿色,甚至在某些地方,在每年的特定时间。 稍微不那么厚颜无耻的回答:没有。这两个例子都是近似人类的视野,显示在电脑屏幕上,可以高度可变。2002年版本的蓝色大理石复合atmospherically-corrected 4个月的数据在狭窄的波长。复合数据比例近似人类的视觉的非线性响应。特别明亮的区域图像剪一些乐队,导致不正确的色调和损失的对比。

For all the gory details, here’s the user’s guide for the MODIS land surface reflectance product (which has always looked slightly greenish to me): http://modis-sr.ltdri.org/guide/MOD09_UserGuide_v1.4.pdf

The Apollo 17 Blue Marble is a color-corrected scan of 40-year-old film (stored in a freezer at NASA’s Johnson Space Center) and is also not a particularly good representation of what an astronaut would see (as Pont commented earlier). The best description of the appearance of the Earth from space is from astronaut Piers Sellers, who said it’s “brilliant blue, and too bright to look at directly—like sunlight reflected off snow while skiing at high altitude”.

Brutally honest answer: neither. To paraphrase Mapbox’s Charlie Loyd, there’s an irreducible subjectivity to color. How we perceive color is affected by ambient light, adjacent colors, overall amount of light, individual physiology, our age, and probably mood and what we had to eat for breakfast. Viewed from space there are even more variables: particularly atmospheric conditions (which influence both incident and reflected light) and viewing angle.

Current state-of-the-art is probably NOAA View, http://www.nnvl.noaa.gov/view/#TRUE which uses a multi-band algorithm to simulate true color with Visible Infrared Imaging Radiometer Suite (VIIRS) data. According to that algorithm, the Sahara is more red than yellow.

For the record, I’ve worked on color-correcting MODIS imagery, the Blue Marble datasets, and astronaut photography on the NASA Earth Observatory & Visible Earth sites (including the referenced version of AS17-148-22727—unfortunately I can’t find the original TIFF.) Perhaps that means I think the Sahara is both ochre and yellow. I also wrote the guide to color-correcting Landsat imagery linked by Jezibelle (thank you).