Remote sensing specialists are standing by to capitalise on the launch
of the European Space Agency's (ESA) Sentinel-2 Earth observation
satellite in 2013, as that will, for the first time, enable them to
develop a nitrogen map for southern Africa.
Sentinel-2 is one of five Sentinel missions that ESA is developing for
Europe's global monitoring for environment and security programme, and
will provide full and open access to its data.
First in line is the CSIR's Dr Abel Ramoelo, who recently completed a
PhD at the University of Twente in the Netherlands, entitled ‘Savanna
grass quality: remote sensing estimation from local to regional scale'.
With this work he developed remote sensing methods to improve estimates
of grass quality over large areas. He explains, "The quality of grass is
determined by the concentrations of nitrogen and phosphorous in the
grass foliage. Foliar nitrogen concentration is known to relate to the
protein content. Protein is a major nutrient requirement for herbivores
(animals that only eat plants). Foliar phosphorous, on the other hand,
is crucial for reproduction and lactating animals."
Understanding how these elements are distributed is important for the
effective management of wildlife and livestock, and will help
ecologists, farmers and resource managers to understand the feeding
patterns, distribution and densities of herbivores in protected and
communal areas. When combined with remote sensing measurement and
analysis, managers can achieve a landscape-wide view of the nutrient
distribution and limitation of an area.
Developing a remote sensing method for estimating grass quality
For his PhD, and in anticipation of what Sentinel-2 will have to offer,
Dr Ramoelo had to go back to the basics of biochemistry, starting in the
CSIR's greenhouse where he sowed grass seeds into 90 pots.
He explains, "Light from the sun required for photosynthesis by plants
is measured in a red region of the spectrum, which is influenced by
chlorophyll absorption, and reflection on the green region of the
spectrum, which we observe as the ‘green' leaves and blades. At
different stages of growth, plants will have different levels of
‘green', each stage with its own unique footprint when measured with a
Over a period of nearly three months he measured the nitrogen content of
the growing grass in the greenhouse with a spectrometer. When the grass
reached maturity, it was dried and sent for chemical analysis in a
laboratory to retrieve leaf nitrogen and phosphorus, which will be used
to develop prediction models in combination with remote sensing data
using various modelling programmes.
At the same time, he also had to develop a method to work around the
problem of water in the leaves and the atmosphere, which ‘masks' the
actual nitrogen content of the leaves when using remote sensing
techniques. In a process called ‘spectral transformation' he factored
out the effect of water, and was then able to model his results and test
it out in the field, using data acquired by the spectrometer.
For the next part of his thesis Dr Ramoelo worked on eight different
experimental sites and over 49 plots of 900 m2 each in an area that
stretched from the Kruger National Park and an adjacent private game
reserve, to the rangelands in the surrounding communal area - covering
an area of about 35 000 hectares. During one of these field trips he
nearly lost all the data stored on his laptop when a buffalo charged
them. Retreating at first, he then ran back to retrieve the laptop, just
Lastly, he had to integrate remote sensing with environmental variables
such as temperature and rainfall into the model before he could declare
the remote sensing method ready to tackle the data that will become
available after the launch of Sentinel-2.
Currently, a nitrogen map for the Kruger National Park, Sabi Sands and
surrounding communal areas is available. However, derived using the red
edge band from the RapidEye spaceborne sensor at a cost of around R60
000 for a scene of about 5000 km², it is simply not feasible to do this
for the entire country.
Sentinel-2 - a dream come true
According to the ESA website, Sentinel-2 will carry a single multi-spectral instrument with 13 spectral channels in the visible, red edge, near infrared and shortwave infrared spectral range. With a resolution of 10m per pixel, and wide swath of 290 km, researchers will be able to estimate nitrogen for a wider area or even at country level. A ‘swath' is the area covered by one scene or picture as collected by Sentinel-2.
Add to that a repeat cycle of only five days and full and open access to the data, it will now be possible and cost-effective to map the nitrogen content of grasslands and forests over southern Africa, explains Dr Ramoelo.
"Traditional satellites such as LandSat - which have been around for 39 years - do not offer special spectral bands in the red and infrared range and also not at a resolution that is useful to us. We have been able to produce some maps with commercial satellites like RapidEye, but the data are prohibitively expensive and thus not sustainable in the long term."
Furthermore, while it takes Landsat 18 to 42 months to obtain imagery of the entire country, Sentinel-2 will require less than a year because of its more frequent coverage.
"And with Sentinel-2, we will have full and open access to data," he emphasises, literally a remote sensing scientist's dream come true.
For further reading or sources consulted http://www.esa.int/esaLP/SEMPCKQWJ1H_LPgmes_0.html
Dr Abel Ramoelo