Free Satellite Data

Would you like to know where to get free satellite data?

I spend a lot of time looking at satellite images. I’ve been looking at satellite images since 1993 when I took my first course in remote sensing. Thinking about it like that makes me feel quite old.

Swedish glaciers imaged by Landsat

Most often I use Synthetic Aperture Radar (SAR) data. SARs can obtain images through cloud cover, day and night, and are therefore particularly useful in the Arctic and tropics. There are other interesting properties of SAR data but I’ll save that for another post. 

Most satellite data, though, measures reflected solar radiation (sunlight) in the visible to infrared parts of the electromagnetic spectrum. This is the type of data used in weather forecasting and presented on television weather broadcasts (along with ground based radar measurements). [Thermal] Infrared data measures emitted heat at the surface; for example lava from a volcano. Together visible-infrared data are very powerful tools for mapping the Earth’s surface. Much of this data is available for free, if you know where to look. 

NASA provides data from instruments such as MODIS (250- 500m pixels*), ASTER (15-30m pixels), covering the whole globe. You don’t need to register and the search tool is relatively easy to use.

The USGS offers ASTER and Landsat (15-30m resolution*) amongst others cover the globe. Access is via a Google Maps based tool that is simple to use.

Also in the US is the Alaska Satellite Facility (ASF). The ASF offers a lot of data but much of it you will need to submit a proposal for. Their User Services Office though, are incredibly helpful; any question? Just send them an email. ASF also offers software tools to help users process their data.

In Europe the best source of data is almost undoubtedly the European Space Agency (ESA). ESA develops it’s own range of satellites and instruments and leads the World in some technologies. ESAs next generation of satellites, the Sentinel Missions, are the most comprehensive range of satellites, designed to suit the needs of a wide range of users and applications. ESA data is almost always only available through a peer-reviewed proposal system. Fear Not, though, the system is online and proposals are expedited quickly. ESA satellite sensors include MERIS (300m resolution), AATSR-2 (1 km), and SARs (12.5m). In the future ESA will provide a range of data from high resolution (~10m) to regional coverage (100s of metres resolution).

If Sweden is your place of interest the National Satellite Database is the place to start. Anyone can access data from the database, known as saccess. Most of the data is high resolution (15m) and the database is updated every summer (but doesn’t include winter images).

Finally, if you want to look at vegetation on a regional scale, why not look at the Global Land Cover Facility (GLCF) at the University of Maryland. The nice people there have created a number of datasets, of which the GIMMS Normalised Difference Vegetation Index (NDVI) is perhaps the most useful. I’ve used their data to look at vegetation change in Africa (see the earlier post on Darfur). This data represents the healthiness or greeness of vegetation and spans 1981-2006. It is an incredibly useful time series and has global cover at 8 km resolution.

In short, there is a lot of free data out there. It can be used to map ecological changes, or sea ice extent; you can look at local changes, or the ‘big picture’; daily or yearly images are available. So next time you need to create a map for a paper, or want to look at an ecotone, why not forget Google Earth for once, and try to do it yourself?

There are lots of free tools out there to help you, ESA has been particularly good at developing software for end users, I may even get round to posting some links on freeware.  If you get stuck in the meantime, send me an email and I’ll see if I can help.

ian.brown [at] natgeo.su.se

*resolution/pixel size, describes the size of the ground area covered by one pixel.

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