Area calculation

3 months ago
26 April 2024
4 replies
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SentinelHub User
4181 posts replied to

Hi,

I am working with Landsat 5 TM L2 product, I want to calculate the area in Km2 of a particular ROI where I have the number of pixels that cover this ROI. How can I do it? Is there any example?
Thank you!

4 replies

Hi,

The easiest way to measure areas is using an GIS application, e.g. QGIS. You can find how to do it with their documentation.

Hi,

You could also calculate the area quite easily in your current workflow. Are you requesting the data over your ROI at the original resolution of Landsat 5: 30 meters? If so, you can obtain your area in km² by multiplying your number of pixels by 0,0009 (0.03*0.03).

SentinelHub User
Author
4181 posts replied to
3 months ago
26 April 2024

Hi,

Thanks for your responses! I understand that Landsat 5 have a resolution of 30 meters (pixel size= 30mx30m) but actually I am not sure that I am requesting the data over my ROI at the original resolution.

How can I check or configure this variable?

At this moment I have traced a polygon over a lake of approximately 6500 Km2, then I detect water using NDWI idex and try to count the number of pixels of NDWI > 0.5 using an histogram. Then the area calculated dont represent the actual area when I multiply: q_px * 0.03 * 0.03

Is there any example similar to this maybe? It has any relation with the ResX and ResY in CRS units? If so, How can I know the correct values to fill this fields according to the resolution of Landsat5 and the ROI coordinates? I am not sure if this is correct…

Thanks for the support!

Hi,

To check the pixels’ resolution of your file, you can either use gdalinfo or open it with a GIS application and check its properties. You can find the resolution under the Pixel size attribute. After having the resolution of your imagery, the way you calculate the area (q_px * pixel_size_x * pixel_size_y) should represent the actual area of the lake.

If you are interested in requesting data with a specific resolution, you can replace the height/width parameters by resx/resy: please note that in this case the resolution is in the same units as the bounding box / polygon coordinates specified (e.g. if you are using EPSG:4326, the units are in degrees), so you may want to specify your inputs in UTM or a similar reference system. Our API reference describes the inputs in more detail, or if you are using Python, you can refer to the package documentation.