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<p><span class="cf0">A <strong>composite</strong> is an image created by combining multiple observations over time. For example, DE Africa offers cloud-free composites (</span><span class="cf0">GeoMAD</span><span class="cf0">) that merge all cloud-free pixels in a year or season. </span></p>
<p><span class="cf0">The advantage is a clearer image: clouds and noise are removed by taking a median or other statistic over time. </span></p>
<p><span class="cf0">Composites are useful for long-term analysis (e.g., land cover, topography) because they reduce random artifacts. You will encounter composites like annual median reflectance in later sessions; for now, understand that a composite merges many raw images into one representative image.</span></p>
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<p><span class="cf0">A <strong>spectral index</strong> combines two or more spectral bands to highlight specific surface features. For example, the <strong>Normalized Difference Vegetation Index (NDVI)</strong> uses the near-infrared (NIR) and red bands:</span></p>
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<p></p>
<p class="pf0"><span class="cf0">NDVI values range from </span><span class="cf1">–1 to +1, where higher values (closer to +1) indicate dense green vegetation, and lower values indicate sparse or no vegetation. Another common index is the <strong>Normalized Difference Water Index (NDWI)</strong>, which uses NIR and green bands to highlight water bodies. NDWI is high for water and low for land, enabling the mapping of rivers and lakes.</span></p>
<p class="pf0"><span class="cf1"><span class="cf0">Indices are useful because they simplify analysis (e.g., one band for vegetation) and can reveal features that are not obvious in the raw bands. In future sessions, you will learn to compute indices (NDVI, NDWI, etc.) using DE Africa data. For now, recall that an index is a ratio of bands designed to emphasise something (water, vegetation, snow, built-up, etc.).</span></span></p>
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<ul>
<li><em><span style="font-size: 1em;">What is the difference between WMS, WMTS, and WCS services? (Hint: image vs. raw data vs. tiled images).</span></em></li>
<li><em><span class="cf0">Why might one use a composite image instead of a single satellite scene?</span></em></li>
<li><em><span class="cf0">Explain in your own words how NDVI is calculated and what it represents.</span></em></li>
</ul>
<p class="pf0"><span class="cf0">Review the session content to answer these before taking the quiz below.</span></p>
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