How to Read Satellite Imagery: A Guide to Free Satellite Sources
Satellite images are one of the most powerful ways to see what a place looks like and how it changes over time. Fires, floods, construction, or whether an event really happened where it is claimed to have happened, can be checked with satellite imagery. And most of the time you do not have to pay for it.
Two kinds of source: basemaps and current imagery
The high-resolution images you see in Google Maps or Google Earth are basemaps made by stitching together many images taken on different dates. They are very detailed but usually not current, and it is not always clear when each part was captured.
For current imagery, you use the public images that satellites such as Sentinel and Landsat capture regularly. They have lower resolution, but their dates are known and they image the same place often.
Free sources
- Sentinel-2 (Europe’s Copernicus programme): 10 metre resolution in the visible bands. Together, the two satellites image the same area every few days. It is the most useful free source for change detection.
- Sentinel-1: A radar (SAR) satellite. It can “see” through clouds and at night; it is especially useful for flood and maritime research, but it is harder to read.
- Landsat 8 and 9 (USA): 30 metre resolution. The archive goes back decades; ideal for seeing long-term change.
- NASA Worldview: Low resolution but daily global imagery. Suitable for wildfire smoke, dust storms and large-scale events.
- Google Earth Pro (desktop): With the historical imagery feature you can compare high-resolution images of the same place over the years.
The basics of reading
Understand resolution
10 metre resolution means each pixel in the image represents a 10x10 metre area on the ground. At this resolution you can see buildings, roads, fields and large burn scars, but not cars or people. Knowing what you can see keeps you from drawing the wrong conclusion.
Always note the date
If you are going to use a satellite image as evidence, always record the capture date. Saying “there is no building in the satellite image” only means something if you know which date the image is from.
Use shadows and direction
Satellite images are usually shown with north at the top. The direction of shadows helps estimate the sun’s position, and their length helps estimate the height of objects. When matching a ground photo to a satellite image, roof shapes, road curves and open spaces are the most reliable references.
False colour images
Satellites record not only the colours our eyes see but also bands such as infrared. In “false colour” images built from these bands, vegetation stands out clearly, and burn scars and water bodies look sharper. EO Browser and Copernicus Browser offer these combinations ready-made.
Compare change
The most powerful use of satellite imagery is putting the same place on two different dates side by side. A new road, a demolished building, floodwater or the trace of a fire shows up in this comparison. Filter out cloudy days and pick the clearest dates.
Common mistakes
- Mistaking a basemap for current imagery.
- Mistaking a cloud shadow for a burn scar or water.
- Commenting on details the resolution cannot show.
- Relying on a single image. Where possible, verify with images from different dates and different sources.
To use satellite imagery together with ground photos, see our guide to finding a photo’s location.