moezlabiadh / gee_snowexplorer

Snow Explorer - GEE based App

SnowExplorer

Description of the tool

SnowExplorer is a Google Earth Engine based application for mapping snow on the ground. The tool is intended to help BCTS staff to plan their field visits by providing information on the Snow Cover Extent at specific dates and location.

The tool is accessible at this address: https://labiadhmoez.users.earthengine.app/view/snowexplorer

Please note that the tool doesn’t work on Internet Explorer. Use Google Chrome or Firefox to access the tool.

Snow Detection Algorithm

The algorithm is inspired and adapted from:

Gascoin, S., Grizonnet, M., Bouchet, M., Salgues, G., and Hagolle, O.: Theia Snow collection: high-resolution operational snow cover maps from Sentinel-2 and Landsat-8 data, Earth Syst. Sci. Data, 11, 493–514, https://doi.org/10.5194/essd-11-493-2019, 2019. https://www.earth-syst-sci-data.net/11/493/2019/

The Algorithm is based on Sentinel-2 Level-2A Surface Reflectance product. The algorithm for Cloud and Snow detection are set as follow:

• Cloud Mask: [MSK_CLDPRB >= 90%] OR [(MSK_CLDPRB >= 50%) AND (B8 >= 0.3)] OR [(MSK_CLDPRB >= 20%) AND (B8 >= 0.9)]

with “MSK_CLDPRB” is the Cloud Probability band and “B8” is the Reflectance in the Near-infrared band (at 0.86 μm).

• Snow Mask: (NDSI >= 0.3) AND (B4 >= 0.14)

with “NDSI” is the Normalized Difference Snow Index and “B4” is Reflectance in the Red band (at 0.66 μm) . NDSI = (B3 – B11) / (B3 + B11) with “B3” is the Reflectance in Green band (at 0.55 μm) and “B11” is the Reflectance in the Shortwave infrared band (at 1.61 μm).

Assumptions & Considerations:

• Sentinel-2 has a spatial resolution of 10 m which not suited to detect very fine details. The tool is intended to give an idea on the general Snow condition.
• Sentinel-2 provides an image every 6 days BUT the tool will filter out images with more than 30% Cloud coverage and use the most recent and least cloudy image available.
• If no images found at the selected dates the tool will throw the following message:
• If your ROI covers more than one Sentinel-2, the tool will make a mosaic of the most recent images. This might result in a strange border effect.
• Multiple ROIs cannot be selected on the tool. Users need to reset the App (press F5) and start again.