The Fantastic World of Multispectral Satellite Imagery
Multispectral imagery contains data over many different bands of the electromagnetic spectrum, including visible light. This extra information has a multitude of amazing, different uses — everything from measuring the health of an area of crops, to Wildfire management, monitoring chemical seepages, detecting oils slicks and hydrocarbons, and measuring climate risks.
Landsat is a satellite program created by NASA, launching its first satellite, Landsat-1 in 1972. The first satellite produced by this program, Landsat 1, was launched into orbit in 1972. The latest satellite is LANDSAT-8. Landsat is in the public domain and is freely accessible. Imagine — almost every square kilometer of earth mapped across the majority of the electromagnetic spectrum, freely available for us to tinker with! Let’s take a look at the different bands Landsat-8 captures.
Coastal Aerosol. It captures the deep blues and violets. It has two uses: capturing shallow water and tracking particles like dust or smoke.
Band 2, 3, 4:
Bands Blue, Green, and Red. These bands capture visible light. Blue can be used for bathymetric mapping, Green for assessing plant vigor, and Red for “Discriminating Vegetation slopes”.
This band consists of Near-Infrared. This is used in ecology and agriculture since healthy plants reflect them. It also emphasizes shorelines.
Bands 6 and 7:
These bands consist of Short-wave Infrared. We can use these bands to map wildfires, determine the ‘wetness’ of soil, and compare different sections of rock between each other.
The Panchromatic band. It takes all the visible colors and combines them into one image. Due to this, it has the sharpest resolution. A technique called pan-sharpening fuses this band with a lower-resolution dataset. This increases the spatial resolution and improves quality.
It catches the light reflected by clouds. It was specifically created to capture Cirrus clouds, and all clouds are visible.
Bands 10 and 11:
It is the thermal infrared band. It measures the temperature of the ground itself. You can use this to estimate the soil moisture, and for managing water consumption.
Now that we have understood the various bands Landsat can capture, let’s download some multispectral imagery for us to play with.
Downloading Landsat-8 Data
1.Create a new account on the USGS (United States Geological Survey) website here: https://ers.cr.usgs.gov/register. We will be using their ‘EarthExplorer’ tool.
2. After you’ve signed in, go to https://earthexplorer.usgs.gov/.
3. Click on the map to add markers. They define the bounds of the area you want to capture.
4. After you have defined the boundaries, click on ‘Data Sets’. Scroll down and choose Landsat → Landsat Collection 2 Level-2 → Landsat 8 OLI/TIRS C2 L2
5. After you have selected Landsat 8 OLI/TIRS C2 L2, click on ‘Results’, and Click on ‘Download options’ from the first result.
6. You can then select all the items you want. You can either download all the bands separately or download everything in one bundle.
Great! You now have the GeoTiff files, which you can view in GIS software like ArcGiff. In my next article, I’ll be showing how to use this downloaded data, in a real world application.
©Rahul Prabhu 2021