What Exactly is a Raster?
We get one raster file for each band within the Sentinel-2 data. The raster file contains cells ordered and arranged in a big grid. At the 10 meter resolution, a 10 by 10 meter square accounts for a single cell’s worth of information. At the 60 meter resolution, a 60 by 60 meter square area gives us a single cell’s worth of information.
The information within each cell is simply a measurement of amplitude. So if we open a 10 meter band 3 raster file from the Sentinel-2A sat and look at the value of the top left cell, it is telling us the MSI measured amplitude of the 559.8 (green) wavelength of light at a 10 by 10 meter spot on the Earth. Literally, it tells us how green that spot is.
Rasters are similar to images in many ways; digital images are simply visual representations of rasters. If we take the raster file for a single band, and map each cell to a 0 to 255 value of black, and save the result, we get a grey scale image where each pixel is representing one cell of a 10 by 10 meter spot on Earth.
That is pretty wildly blurry, and it is difficult to make out individual features, but we can still learn much from it.
If we combine rasters from bands 2 (blue), 3 (green), and 4 (red), we can make a ’natural color’ image (never mind that a pre-generated natural color image at each resolution is supplied within Sentinel products).
Managing Expectations in a Warzone
When we say Sentinel-2 has a “10-meter spatial resolution,” it is crucial to understand what that actually means on the ground, especially in the context of documenting a warzone.
A single pixel represents a 10x10 meter square. To put that in perspective:
- What you CAN see: A destroyed hospital, a leveled city block, a new trench system, the expansion of a displacement camp, or the burning of agricultural land.
- What you CANNOT see: Individual people, weapons, vehicles, or license plates.
Sentinel-2 is a macro-level observational tool. It is not a spy satellite. It cannot identify individuals or specific military hardware. However, its immense value lies in its ability to document systemic and structural changes over time. When an entire neighborhood turns from a grid of distinct buildings into a uniform field of rubble, that 10-meter resolution is more than enough to prove the scale of the destruction.
What’s Next?
Now that we understand the physical scale of a single pixel, we need to look at what that pixel is actually measuring. In the next section, we will explore the electromagnetic spectrum and how looking beyond visible light reveals hidden details about the ground below.