How Teagasc uses satellites and aircraft to research agriculture in Ireland
By Stuart Green, Remote Sensing Specialist, Teagasc, Agriculture and Food Development Authority
Many of us are familiar with Google Earth, looking down on any location and seeing where you live, spotting your house or farm. But, perhaps few realise that these images are taken from space by satellite.
We see satellite images every day on weather forecasts. These images come from geostationary satellites that are always orbiting over the same point on the equator, whereas the satellites we use in Teagasc are orbiting much closer to earth in a polar orbit, flying overhead every few days or so.
Right: Near Infra-Red image of Co Cork
This large red image is a false colour NIR image of part of Co Cork (Cork Airport is visible top middle) taken by the French SPOT satellite. The various shades of red are recording the amount of NIR reflected back to the satellite from the fields – the different shades can tell us what crop is growing and at what stage of ripening it is at – for grass fields the shades of red tell us how much grass is on the field. The many blue fields you can see have no vegetation on them – they have been harvested and are only bare soil.
The sensors on board the satellites are quite similar to everyday digital cameras that we use but they do have some tricks. The images we’re used to seeing are made up of the red, green and blue colour components of sunlight reflected off the object we are looking at, but sunlight contains more than just visible light, it also has a “near infra-red” (NIR) component which we cannot see but satellites can.
White light is made up of equal parts blue, green and red – if white light falls on a leaf it looks green to us because the leaf reflects back twice as much green light as blue or red but it also reflects ten times as much NIR light as red, this means that vegetation is easily observed from space if you have a NIR sensor.
The amount of NIR that’s detected depends on many things; the amount of plants in a given area, how well they are growing, what species are growing and how healthy they are.
The satellites that Teagasc uses measure the NIR components very precisely, enabling us to relate the amount of NIR reflected from a field with productivity (how much of a crop is growing). They can also tell us the type of crop or grass. This information is useful by itself but we can also use it with other information about topography (hills, slopes and valleys) and geology to make a prediction on the type of soil the plant is growing in.
Teagasc (funded through the Environmental Protection Agency) has previously used this approach to produce the National Indicative Soil & Subsoil map. Because satellites can see over a very large area at any one time (170km by 180km) they are very useful for mapping the whole country and, using these data from LANDSAT satellites, we created a national habitat map.
Some satellites are designed to see very small objects, as small as 50cm across. This allows us to make accurate maps of species types and distributions within a field and small variations in growing potential in a field. We can also see the shape of individual trees and this can help identify the species.
The use of satellites and aircraft to make observations of the earth below is the science of Remote Sensing. Remote Sensing allows us to see things we normally can’t, to gain view-access and insights to even isolated parts of the country and can help us ensure that Irish agriculture is productive and sustainable.
Stuart Green is one of the Discover Science & Engineering Science Ambassadors on the My Science Career website.
Comparing RGB with NIR
The two detailed images above were taken in 2009 by the commercially operated Quickbird satellite of an area around French park in Co Roscommon. The familiar first (green) image is how things look to our eyes (imagine being in an aeroplane and looking down). The second image is what’s known as a false-colour-image – it’s how the satellite sees the land when measuring the near Infra-red (NIR). You can see that there is much variation in the shade of red on the NIR images – telling us about how well the grass is growing and how much is visible. Also, as drainage affects grass growth and buried objects affect drainage, in the NIR image we can see clearly old field marks, post holes and field boundaries that are buried underground. The marks are associated with Rathcroghan archaeological complex.