This text, revealed as a part of the Bellingcat Technical Writing Fellowship, is customized from a extra technically-detailed information on Agnes Cameron’s weblog.
Satellite tv for pc imagery has been used extensively in open-source investigative analysis: from monitoring world deforestation to documenting mass demolitions in Gaza.
After we view satellite tv for pc pictures on platforms like Google Earth, the world seems similar to the way it does with the bare eye – for instance, if you happen to have been trying down on the earth from an aeroplane window.
Nonetheless, satellite tv for pc pictures may also reveal issues that people can’t see. A standard instance of this type of imaging is evening imaginative and prescient, which makes use of infrared mild to light up a scene that’s not seen to the bare eye. In satellite tv for pc imaging, pictures made utilizing further varieties of mild are referred to as multispectral pictures.
Multispectral satellite tv for pc pictures can reveal helpful details about the world, such because the presence and high quality of water, varieties of vegetation, soil well being and extra. On this information, we’ll clarify the fundamentals of how multispectral satellite tv for pc imaging works, apply it to case research referring to mining and deforestation, and evaluation open-source instruments and sources for utilizing these strategies in apply.
Methods Of Seeing
After we have a look at an object, what we’re actually seeing is the completely different colors of sunshine which are mirrored by that object. After we have a look at crops, they seem inexperienced as a result of the chlorophyll of their leaves absorbs blue and pink mild, and displays inexperienced mild. In autumn, leaves flip orange as a result of the chlorophyll begins to interrupt right down to preserve power, and so they begin to replicate pink mild too.
A picture that displays how the human eye naturally sees issues is named a “true color picture”, consisting of three bands (or ranges) of seen mild – pink, inexperienced and blue – that may mix to make up any color. If an object displays all seen mild we see it as white. Conversely, if it absorbs all seen mild, it seems black.
See how this works by toggling the pink, inexperienced and blue channels within the picture under on and off:
Multispectral pictures will be helpful in investigations as they will spotlight modifications to the Earth’s floor that wouldn’t be seen to the bare eye, whether or not attributable to mining operations, deforestation, or air pollution. Mostly, these pictures use bands of sunshine from the infrared a part of the sunshine spectrum, along with these usually seen to the human eye.
We will search for indicators of various actions by evaluating completely different bands of sunshine (seen and invisible), highlighting contrasts that we usually wouldn’t be capable to see.
Within the instance under, a river is barely seen within the picture on the left, captured with a digital digicam, attributable to hazy situations. However it’s clearly seen within the picture on the proper, captured utilizing infrared mild. It is because the longer waves of the infrared a part of the spectrum are much less more likely to be scattered by small particles, such because the water droplets in fog or items of mud, than seen mild.
One other widespread use case of infrared mild is to spotlight vegetation, as wholesome timber and crops replicate loads of infrared mild compared to different supplies.
Since we can not usually see infrared mild, we’ve to map the information on the sunshine captured from these bands to seen ones – pink, inexperienced and blue – so as to detect several types of supplies. There are three predominant methods to do that, which we’ll cowl on this information.
1. Utilizing Band Combos to Create False Color Composites
Simply as seen mild could also be separated into pink, inexperienced and blue bands, the infrared mild that satellites can seize may also be damaged down into distinct bands, every representing a spread of wavelengths which are helpful for evaluation. Generally studied infrared bands embrace Close to Infrared (NIR), Shortwave Infrared (SWIR 1 and SWIR 2) and Thermal Infrared (TIR).
The best option to visualise these invisible bands of sunshine is to straight map them to a visual color channel (pink, inexperienced or blue) to create a “false color” picture. This mapping can spotlight variations between supplies that may look very related underneath seen mild, or, as within the instance above, permit us to see extra clearly in antagonistic situations.
There are a number of free and open-source satellite tv for pc imaging instruments that embrace options permitting customers to create false color composites utilizing completely different band combos, resembling EO Browser, Google Earth Engine and QGIS. To show the strategies used on this article, we additionally constructed a software known as the Multispectral Imagery Explorer that makes use of Google Earth Engine to indicate what false color pictures utilizing a number of band combos, band ratios and band indices (which might be lined later on this information) might appear like.
The case research under makes use of pictures from the Multispectral Imagery Explorer to show how band combos can spotlight particular supplies.
Case Examine: Discovering Clay Mines in Sandersville, Georgia, USA
Kaolin clay (often known as kaolinite) is a naturally occurring mineral that’s utilized in many industries, most notably the manufacturing of paper and ceramics. Like all sort of mining, extracting this materials can lead to environmental impacts such because the destruction of habitats and the contamination of close by water our bodies with mining waste merchandise.
Satellite tv for pc imagery evaluation may help researchers monitor areas with excessive deposits of kaolin clay, the place mining actions could also be happening.
The under true color satellite tv for pc picture exhibits mining pits and lakes excessive in kaolin clay deposits within the city of Sandersville in Georgia, USA, which is named the “Kaolin Capital of the World”. Greater than 8 million metric tons of kaolin clay, value an estimated US$1 billion, are extracted from massive open pit mines in Georgia state every year.
The lakes are seen as spots of inexperienced on this picture, however it’s tough to inform the place we must always look to seek out mining operations. Though kaolinite is white in color, it can’t be distinguished from different white supplies in a real color picture as a result of these usually have related reflectance properties underneath seen mild.
To assist us spotlight variations in these areas, we’ll study the reflectance spectrum of kaolinite, which exhibits how the fabric displays mild throughout each seen and infrared wavelengths.
We examine them with the reflectance spectra of eight completely different bands of sunshine captured by Landsat 8, a satellite tv for pc launched by NASA in 2013 which continues to be in orbit and accumulating one of many world’s longest-running satellite tv for pc imagery datasets. Whereas multispectral pictures captured by different satellites, resembling Landsat 9 and Landsat 7 (which ceased operation earlier this 12 months), document knowledge on related bands of sunshine, we might be utilizing the Landsat 8 bands for all of the examples on this article.
The black line under represents the reflectance spectrum of kaolinite, whereas the vertical bars present the wavelengths of the Landsat 8 bands.
To assist us spot attainable kaolin clay, we will use the bands by which the fabric displays a excessive quantity of sunshine, alongside these by which it largely absorbs mild, to create a false color composite. The completely different bands are mapped to seen colors in our false color picture as follows:
- SWIR 2 band (7) → Crimson
- SWIR 1 band (6) → Inexperienced
- Crimson band (4) → Blue
Which means that if the fabric proven within the picture displays a excessive quantity of sunshine within the SWIR 2 band, it might present up as pink pixels within the false color picture, mild within the SWIR 1 band exhibits up as inexperienced pixels, and light-weight within the pink band exhibits up as blue pixels.
Since kaolinite displays extra mild in bands 4 and 6, and fewer mild in band 7, areas with a focus of kaolin clay ought to seem cyan, a mixture of the colors they’re mapped to – inexperienced and blue respectively. In comparison with the above RGB picture, it’s now attainable to see a lot clearer delineations between elements of the picture that seem blended above to establish attainable kaolinite mines – though these nonetheless must be confirmed with different sources, since different supplies might have related reflectance properties.
2. Band Ratios
Band combos are helpful if the fabric we’re searching for will be noticed with comparisons of as much as three bands (one band per color channel). Nonetheless, these is probably not as exact as we want as a result of band combos could also be affected by variations in shadows and light-weight, atmospheric situations resembling haze or supplies with overlapping reflectance properties – an space that appears cyan may very well replicate an space that accommodates supplies that replicate loads of mild individually within the blue and inexperienced channels, for instance. By combining completely different bands collectively utilizing a band ratio, we will construct much more highly effective visualisations to distinguish options of curiosity.
Band ratios spotlight the distinction between completely different bands by dividing the values of pixels in a single band by these in one other. When figuring out which ratio to make use of to spotlight a given materials, we search for the elements of the sunshine spectrum the place the fabric displays probably the most and least mild. By dividing one by the opposite, it’s attainable to pinpoint very particular supplies.
Bellingcat contributor Ollie Ballinger has a complete information to utilizing Google Earth Engine for satellite tv for pc imagery, which may very well be helpful if you want to discover these strategies additional. The code for the Multispectral Imagery Explorer can be obtainable on GitHub.
The case research under, with pictures from the Multispectral Imagery Explorer, exhibits how band ratios will be helpful for investigations involving mining exercise and environmental modifications.
Case Examine: Bauxite Mining in West Kalimantan
Bauxite is a rock with a excessive focus of aluminium minerals, and is the supply of over 99 % of worldwide aluminium manufacturing. Surging demand for aluminium in recent times has pushed large expansions of bauxite mining operations. More and more, bauxite mining operations are extending into indigenous land, and are related to deforestation, modifications to water sources and the displacement of communities.
Bauxite is extracted utilizing massive open-pit mines, and a key byproduct of bauxite extraction is “pink mud”, a polluting slurry that’s often saved in massive, poisonous “tailings ponds” near processing amenities. As a result of its affect on native populations and quickly altering relationship to the worldwide financial system, tracing these mines can present essential insights into the political ecology of a area.
The aluminium in bauxite is tough to establish straight, however we will search for the signatures of different minerals that make up bauxite and pink mud: ferric iron oxides (contained in minerals like goethite and haematite), and kaolinite. A 3-part band ratio comparability may help establish the presence of those completely different minerals.
On this instance, ferric iron oxides ought to seem yellow (a mix of pink and inexperienced), kaolinite clay ought to seem blue, and areas the place there’s each kaolinite clay and ferric iron oxides ought to seem white.
- Ferric iron oxides ratio 1 (4/2) → Crimson
- Ferric iron oxides ratio 2 (4/3) → Inexperienced
- Clay (together with kaolinite) band ratio (6/7) → Blue
If we examine an RGB picture of the area to at least one with the three-part ferric iron oxides/kaolinite mixture, we will rather more clearly see areas with potential bauxite mines highlighted in vibrant yellow. There are additionally seen spots of yellow the place the river meets the ocean, indicating attainable traces of water air pollution from these mines.
We will additionally examine a number of false color pictures taken over time to visualise the results of coverage modifications on the area.
If we take a timelapse of an space across the Kapuas river (the world within the above picture with a focus of yellow areas) between 2013 and 2024, we will see that there was barely any enlargement of the yellow spots indicating attainable bauxite mines between 2014 and 2018. This coincides with a 2014 ban by the Indonesian authorities on sure uncooked supplies together with bauxite. Nonetheless, the yellow spots proceed to broaden once more from 2018, after the Indonesian authorities relaxed the export ban on bauxite in 2017.
This timelapse mirrors the USGS mineral commodity summaries figures for Indonesia’s bauxite manufacturing throughout that timeframe: a big drop following the export ban in 2014, adopted by a rise from 2018.
3. Band Indices and the NDVI
An extension of the concept of the band ratio used above is the satellite tv for pc imaging index. These indices are extra complicated combos of various bands, and are used extensively to trace particular phenomena, resembling modifications in vegetation, the presence and high quality of water, soil well being and tree protection.
The Index DataBase by Verena Henricht and Ktharina Bruser is a helpful useful resource that means that you can discover indices related to completely different supplies. Sentinel Hub’s EO Browser additionally permits customers to make use of satellite tv for pc imaging indices and customized scripts to create false color pictures that spotlight particular supplies or phenomena resembling algae and wildfires.
The case research under exhibits how the Normalised Distinction Vegetation Index (NDVI), a preferred index for detecting the presence of wholesome vegetation, was used to research the affect of the Yemeni civil battle on meals safety within the agricultural Tihama area of the nation.
Case Examine: Date Palms in Yemen
In a 2020 evaluation for Bellingcat, researcher Wim Zwijnenburg used the EO Browser – primarily based primarily on satellite tv for pc pictures collected by the European House Company’s Copernicus Programme – with the NDVI to visualise vegetation loss in areas identified for date palm plantations.
The evaluation confirmed that a lot of the vegetation loss was within the space close to the ocean. This supplied a place to begin to concentrate on and, with the assistance of different open-source and distant sensing instruments, made it attainable to watch modifications in vegetation for the interval and clarify why the battle had led to tree-cover loss.
Sensible Suggestions for Researchers
Because the above case research show, multispectral satellite tv for pc imaging offers broad and different details about the Earth’s floor, together with a lot that’s invisible to the bare eye.
These satellite tv for pc pictures present open supply researchers with a set of lenses that allow us see a panorama otherwise, highlighting floor modifications such because the enlargement of mines, the unfold of pollution, and modifications to vegetation. By amplifying delicate variations in how supplies replicate several types of mild, we will make fairly subtle inferences a couple of panorama’s ecological and geological properties.
Nonetheless, it is usually essential to know a number of the limitations of multispectral satellite tv for pc pictures so as to keep away from misinterpreting them. Under are some suggestions for what to look out for.
Tip 1: Complicated Clouds
Clouds will be significantly deceptive when utilizing false color pictures as they usually look completely different from the pure panorama, and will be tough to distinguish from different supplies of curiosity.
Swap to the true color picture to test that what you’re seeing is of curiosity, and never only a cloud.
Tip 2: Laborious (To Spot) Rock
Fascinating options (like mining-related minerals) will be hidden by vegetation and different coverings. Areas which are lined with tree canopies are significantly liable to this. Moreover, many minerals are mined underground, quite than on the floor, making satellite tv for pc imaging instruments unsuitable for this evaluation.
Think about these pictures of a bauxite mine in Kuantan, Malaysia, from the Multispectral Imagery Explorer, earlier than and after the land within the space was lined with topsoil following the 2015-16 Kuantan Bauxite mining catastrophe. Within the first picture in 2015, the mine is clearly seen – within the second picture of the identical space in 2023, the mine is now hidden from view.
Tip 3: Conclude fastidiously
Lastly, it may be tempting to attract untimely conclusions in regards to the that means of various options on a false color picture. However in apply, geologists use a number of extra complicated strategies, alongside contextual surveys of the area, to attract conclusions in regards to the mineral composition of an space. There are various supplies that may look related in false color pictures, so it’s essential to test with different sources of knowledge earlier than drawing conclusions.
For the case research on clay and bauxite mining on this information, we cross-referenced the false color pictures in opposition to the USGS Mineral Sources map, which maps government-issued mining licenses worldwide. We additionally referred to a detailed interactive map of mining concessions within the area issued by the Indonesian authorities to verify our observations for the case research on bauxite mining in West Kalimantan.
This information and the Multispectral Imagery Explorer have been created by Agnes Cameron as a part of the Bellingcat Technical Writing Fellowship. Agnes’ full weblog put up features a extra detailed dialogue of instruments and attention-grabbing initiatives that use satellite tv for pc imagery.
Galen Reich contributed analysis to this piece.
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