by Fred Snijders
and Yota Nicolarea
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The ARTEMIS system of the FAO Environment and Natural Resources Service (SDRN) routinely processes and disseminates data from a number of satellites. This concerns low-resolution images from the METEOSAT, NOAA and SPOT satellites that give an indication of rainfall conditions or vegetation development. The main use is in the field of early warning for food security.
By analyzing a sequence of images, which are available at 10-daily and monthly intervals, the status and progress of the crop growing season can be monitored over very large areas, as done routinely by the country analysts of the FAO Global Information and Early Warning System (GIEWS).
A number of software tools have been developed over the years to assist in the analysis. One of these is the METART web site (http://metart.fao.org), which provides a simple and effective access to the ARTEMIS data holdings.
In general, the data is analyzed in two ways. The first is by comparing current images with those from the past. This reveals areas that have received more or less rainfall, or that show better or worse vegetation development, but it does not provide the time sequence of these differences. The second method is by using statistics that are extracted from the images covering certain predefined zones or administrative areas and are visualized as a chart or graph. These show very well the behaviour in time and the differences with, for instance, the average. However, the spatial detail is limited to the extend of the zone used.
The METART site provides time-series-graphs for a large number of zones covering Africa. The supported products are "vegetation index", "estimated rainfall" and "cold cloud duration". After selecting a country, the user has to select the product it requires, the time period to chart, the zones for which the charts have to be prepared, and to select between "normal" and "cumulative". This selection is then interpreted by the METART server and the charts are prepared on-the-fly and send to the user. See Figure 1.1 for an example of a difference image and Fig 1.2 for a chart, both prepared by METART.
Figure 1.1 - Metart example, country image
Figure 1.2 - Metart example, charting of zones
Last year (2001), FAO started to support a new initiative to provide digital and multimedia content to users in Africa who do not have an Internet connection, RANET, an initiative from the African Centre for Meteorology Applied for Development (ACMAD).
The RANET information is transmitted over a satellite operated by the WorldSpace Corporation and can be received by a digital radio, connected to a computer. Currently, ARTEMIS produces routinely about 400 country-based web pages every 10-days for inclusion in this transmission. Click here to see the current FAO contribution to RANET.
An important difference between the RANET transmission and a normal Internet connection is that the former does not allow for any direct interaction with the web site. In other words, the user can not request a web site to provide something that was not included in the satellite transmission. So all ARTEMIS related information had to be "pre-cooked" and no interactive chart preparation was possible, as is normally the case with the METART site. This certainly limits the potential of the underlying data and this triggered the search for an alternative solution.
To improve on the situation of pre-cooked charts, a solution was found in which the underlying data was transmitted to the userīs browser (Netscape, Explorer, or other) and to provide a graphing capability to the web page. This capability is provided by a Java-based programme, a so-called Applet.
Originally, when developing this programme from directly available software modules or libraries, the result became too big. Therefore, the charting Applet was re-designed from scratch, trying to be as efficient in coding as possible. The resulting applet is indeed very small, around 110KB, which gives no problems in loading over the Internet. Adding the country data to the applet makes the whole somewhat bigger, bringing it to about 150K, which was considered fully acceptable. In fact when comparing this to the current RANET transmission, a net decrease in size of the resulting web pages was obtained, as instead of 3 different web pages, pre-cooked for the various products, only one page /country has to be prepared.
The charting capabilities of the applet are extremely flexible. The x-axes can be set to cover any period as required, be it one season, or many years. One can display various years covering one zone, or various zones covering one year. Both "normal" charts and "cumulative" charts are supported. Furthermore, once a suitable chart has been obtained, it can be "cloned". This is, copied to a new, separate window, which allows for easy inter-comparison of a number of charts. See Figure 1.3.
Figure 1.3 - Example of the Charting Applet, with cloned charts
Fred Snijders, Roberto Giaccio, Jeroen Ticheler and Yota Nicolarea: Bringing Africa data to the desktop: the ARTEMIS Charting Applet, Page 1.