Low Spatial Resolution
MODIS sensor features
MODIS sensor (Moderate Resolution Imaging Spectroradiometer) is found on board Terra and Aqua satellites which are part of NASA’s EOS mission (Earth Observing System) It provides a high radiometric resolution: 12 bits, in 36 bands of the electromagnetic spectrum that comprise wave lengths from 0.4 to 14.4 µm. Two of these bands have a nominal spatial resolution (in nadir) of 250 metres, five have 500 metres and the remaining 29 bands have 1 kilometre. A double faced rotating mirror provides a FOV (Field Of View) of ±55°.
The Terra orbit is designed to pass the equator from North to South in the morning whereas the Aqua goes across from South to North in the evening. Both are positioned some 705 km altitude and provide a view width of 2330 km. this allows the observation of the earth surface every one or two days. Therefore their features are most adequate to improve the knowledge about the processes and the global dynamics that our planet undergoes.
For more information about MODIS sensor: http://modis.gsfc.nasa.gov/
Three daily images will be shown in this space with a one day delay. They will be about the whole Iberia Peninsula and will be captured by MODIS sensor. They are images generated by NASA and they are freely available.
They are two images generated parting from compositions from the original bands registered by the sensor. Three is also a third image representing one of the vegetation indexes obtained by operating with two of these spectral bands.
Composition in true colour (143):
In order to generate this image, sensor bands 1, 4 and 3 are assigned to red, green and blue image channels. They are called images in true colour or in natural colour because this combination of wave lengths is similar to what the human eye would see.
Their main application is the daily watching of features of land, ocean and atmosphere. That is why they are the best-suited ones for the immediate detection of phenomenon and sudden trend changes like fire detection, for instance.
Composition in false colour (721):
In this case the image is generated assigning bands 7, 2 and 1 to red, green and blue from the digital image.
They are very adequate images to differentiate zones like:
- Very active vegetation, represented in intense green shades.
- Bare, barren soil, like deserts, that show a pinky colour.
- Areas affected by fires will be represented in different shades of red. The most intense red will represent the most badly burned area.
- Water will be shown black and marine blue if it has silt. One of the advantages of these images is that they highlight flooded areas.
Normalized Difference Vegetation Index (NDVI):
As we could see in the previous section the NDVI image represents quantity and strength of the vegetation. NDVI values observed in each zone of the peninsula are closely linked with the type of vegetation and climatic conditions as well as prevailing pattern for land use.
CHG works with MODIS sensors
Tasks carried-out with images from MODIS sensor consist of a continuous vegetation analysis and follow-up (temporal series) from NDVI images.
Currently CHG maintains two temporal series of images:
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NDVI representative images from each fortnight.
Are images coming from one of the products generated by NASA, “MOD13Q1”. These NDVI images are obtained by the temporal composition of the 16 daily images. It is carried-out by taking the maximum value from the vegetation index obtained in the period for every pixel. Holben (1986) proved that the compositions executed with maximum in a certain period of time reduce clouds influence as well as the effect of the different vision angles and the greater or lesser existence of aerosol suspension in the atmosphere.
One of the most immediate analysis with these images is the comparison of the fortnight received with all corresponding fortnights within the available historical series. This allows the measuring of the vegetable layer stress degree in relation to what is usual at a given date.
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NDVI accumulated images by hydrologic year.
These images are obtained by adding to each pixel the addition of all previous values in the year for such pixel, being the first fortnightly image of the year identical to the first accumulated image. The last one is equivalent to the yearly total. The use of this algorithm allows the integration of the hydrologic year grouping and provides information about the “accumulated stress” amount.
NDVI values obtained are in accordance with the vegetation type, with climatic factors (rains, evaporation/transpiration…) and other external factors like irrigation allocation. These images provide environmental information about the Guadalquivir demarcation in a continuous way.
Once a temporal series of images has been obtained, there are many and very different statistic analysis that can be carried-out using them to obtain additional information. On following we show the evolution of the average NDVI for the basin in these 9 years.
Below is the evolution of NDVI in different land uses that exist in the basin:
The first diagram represents the development of the photosynthesis activity for average NDVI of the whole basin in each one of the fortnights, as well as the next one that represents each hydrological year in a different diagram so that it is more intuitive for analysis and inter-annual comparisons:
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In the analysis carried-out by CHG special importance is given to draughts. For these studies the use of NDVI is not the best suited as the geographic diversity in land uses makes comparison difficult in both temporal and spatial sides. In very heterogeneous areas NDVI is higher in zones with more favourable geographic conditions (climate, soil, etc.) than in areas having worse conditions.
That is why in this study we have used VCI (Vegetation Condition Index, Kogan, 1990). This index establishes a proportion in relation to NDVI maximum and minimum observed for each fortnight of the year. If they are calculated with a sufficient number of years where different climate conditions have occurred they can be used as criteria to quantify extreme conditions undergone by the vegetation.
The NDVI climate component is emphasized in this index. For this purpose NDVI values of each pixel and period of time (fortnights) are lineally scaled between 0 (minimum NDVI) and 100 (maximum NDVI), thus obtaining the VCI:
Where NDVI is the vegetation index in a pixel in a given moment (fortnightly in this case), NDVImin is the historic minimum for such fortnight and NDVImax is the maximum for the same fortnight of the year. This indicator approaches the related component to the climatic conditions, from extremely unfavourable vegetation conditions to optimal conditions.
Every fifteen days VCI indexes corresponding to the two available historic series are processed:
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Individual fortnights: this image is obtained parting from fortnightly NDVI images. Each fortnight is compared with the same one in the other years of the series.
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Accumulated fortnights: This image is obtained parting from NDVI accumulated images by hydrologic year, comparing them with those accumulated from previous years.
Hydrological years works
Access to images by means of OGC services
The use of these services will permit the access to any of the NDVI fortnightly images belonging to CHG temporal series. They will be available at:
- WMS “ Web Map Service”: NDVI images from the demarcation portion in colour, JPG format. They will be useful to visualize.
- URL for access is: http://idechg.chguadalquivir.es/ogc/wms?
- WCS “Web Coverage Service”: Here are the original images reprojected and trimmed. Therefore they will show in grey shades maintaining the values of attributes associated to the pixels. They are in binary format.
- URL for access is: http://idechg.chguadalquivir.es/ogc/wcs?
Legal note
Accessibility
Interest links
Confederación Hidrográfica del Guadalquivir,
Plaza de España, Sector II y Sector III
C.P 41071 Sevilla [España]- Tel. 955926000
