Hi! I am working with MODIS images, I need to estimate EVI, to do it correctly, first I need to evaluate the quality assessment data layer and overlap it with the EVI layer to dismiss those pixels that are corrupted. Any idea of a tutorial to do this with qgis?
Omid Vakili Ch
Load the Data: Open QGIS and load both the EVI image and the corresponding QA layer into the project. Understand the QA Layer: Before proceeding, make sure you understand the information encoded in the QA layer. The QA layer typically contains flags or bits that represent the quality or validity of each pixel in the EVI image. Different bits might represent different data quality characteristics such as cloud cover, atmospheric conditions, sensor errors, etc. You need to know which bits indicate reliable data. Access the Layer Properties: Right-click on the EVI image layer in the QGIS Layers panel and select "Properties." Define Transparent/Invalid Pixels: In the Layer Properties window, navigate to the "Transparency" tab. Here, you can define which pixels should be considered invalid or unreliable based on the information stored in the QA layer. You can either set specific pixel values to be transparent or use the "Additional no data value" option to exclude unreliable values. Symbology and Visualization: Adjust the symbology settings of the EVI layer to better visualize the reliable pixels. You can use a color map or a gradient to highlight the areas with good data quality. Masking: If the QA layer contains specific bits representing unreliable data, you can use the "Raster Calculator" tool to create a mask that isolates only the reliable pixels. The Raster Calculator allows you to define an expression that filters the pixels based on the desired QA bits. For example, you can use a conditional statement to extract pixels with specific bit values indicating good data quality. Save the Reliable Pixels: If needed, you can save the masked EVI image as a new raster layer, containing only the reliable pixels. Further Analysis: With the reliable pixels isolated, you can now perform any further analysis or visualization using the clean EVI data.
you can follow these steps:
Keep in mind that the specific steps may vary based on the data format and the type of QA layer provided. It's crucial to refer to the data provider's documentation or metadata to understand the information stored in the QA layer and how to interpret and use it effectively to estimate reliable pixels in the EVI image.
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