CalculateCloudMaskForProduct Task

This task uses the Fmask algorithm to calculate a cloud mask for the following sensors:

Landsat 4-5 TM
Landsat 7 ETM+
Landsat 8
Sentinel-2 Level-1C
NPP VIIRS Moderate-resolution (750 m) reflectance and brightness temperature SDR data downloaded from NOAA CLASS (HDF5 format)

The input image must contain multispectral bands within the following wavelength ranges:

Blue: 0.45 - 0.523 µm
Green: 0.52 - 0.605 µm
Red: 0.63 - 0.69 µm
NIR: 0.75 - 0.9 µm
SWIR1: 1.55 - 1.75 µm
SWIR2: 2.05 - 2.380 µm

If the image has thermal and cirrus bands, these will improve the accuracy of the cloud mask result. The thermal band must range from 10.4 to 12.5 µm. Landsat 8 provides a separate band group with two thermal bands. When both are present, ENVI uses Thermal Infrared (TIR) Band 1 (10.9 µm) for cloud mask generation. If you provide a spectral subset with only one thermal band, it will use that band for cloud mask generation. Landsat 8 OLI and Sentinel-2A sensors also have cirrus bands. The cirrus band must range from 1.36 to 1.385 µm.

ENVI will automatically calibrate the multispectral and cirrus bands (if available) to top-of-atmosphere reflectance while generating the cloud mask. It will calibrate the thermal band (if available) to brightness temperatures. You do not need to perform radiometric or thermal calibration separately as you do with the CalculateCloudMaskUsingFmask task.

ENVI uses the Fmask algorithm cited in the following references:

Zhu, Z., S. Wang, and C. E. Woodcock. "Improvement and Expansion of the Fmask Algorithm: Cloud, Cloud Shadow, and Snow Detection for Landsats 4-7, 8, and Sentinel 2 Images." Remote Sensing of Environment 159 (2015): 269-277, doi:10.1016/j.rse.2014.12.014 (paper for Fmask version 3.2).

Zhu, Z., and C. E. Woodcock. "Object-Based Cloud and Cloud Shadow Detection in Landsat Imagery." Remote Sensing of Environment 118 (2012): 83-94, doi:10.1016/j.rse.2011.10.028 (paper for Fmask version 1.6).

Example

This example calculates a cloud mask for a Landsat-8 file. Update the filename and location with your own data.

; Start the application

e = ENVI()

; Open a Landsat 8 raster

File = 'LC80410302013213LGN00_MTL.txt'

Rasters = e.OpenRaster(File)

; Get the task from the catalog of ENVITasks

Task = ENVITask('CalculateCloudMaskForProduct')

; Define inputs

Task.INPUT_RASTERS = Rasters

; Run the task

Task.Execute

; Get the collection of data objects currently available in the Data Manager

dataColl = e.Data

; Add the output to the Data Manager

dataColl.Add, Task.OUTPUT_RASTER

; Display the result

View = e.GetView()

Layer = View.CreateLayer(Task.OUTPUT_RASTER)

View.Zoom, /FULL_EXTENT

The result of this process is a binary image where cloud (masked) pixels have values of 0 and non-cloud (non-masked) pixels have values of 1. To apply the mask to the input reflectance raster, add the following steps. The masked pixels will have NoData values.

OLIRaster = Rasters[0]

maskedRaster = ENVIMaskRaster( $

  OLIRaster, Task.OUTPUT_RASTER)

dataColl.Add, maskedRaster

Layer2 = View.CreateLayer(maskedRaster)

; Export the masked raster to disk

outFile = e.GetTemporaryFilename()

maskedRaster.Export, outFile, 'ENVI'

You can use maskedRaster for any subsequent image processing. If you will perform spectral analysis on the data, you should correct for atmospheric effects next by using the QUAC task.

Syntax

Result = ENVITask('CalculateCloudMaskForProduct')

Input parameters (Set, Get): CLOUD_THRESHOLD, INPUT_RASTERS, KERNEL_SIZE, OUTPUT_RASTER_URI

Output parameters (Get only): OUTPUT_RASTER

Parameters marked as "Set" are those that you can set to specific values. You can also retrieve their current values any time. Parameters marked as "Get" are those whose values you can retrieve but not set.

Input Parameters

CLOUD_THRESHOLD (optional)

Specify a floating-point threshold for cloud probability over clear-land and clear-water regions. The default value is 22.5.

INPUT_RASTERS (required)

Specify an array of rasters containing multispectral, thermal (optional), and cirrus (optional) datasets. For Landsat, select the *_MTL.txt file. For Sentinel-2, select the S2A_*.xml file.

KERNEL_SIZE (optional)

Specify the number of pixels along one side of a square kernel. The default value is 7 (to create a 7 x 7 array). Cloud regions are dilated by this kernel size before output.

OUTPUT_RASTER_URI (optional)

Specify a string with the fully qualified filename and path of the associated OUTPUT_RASTER.

If you do not specify this parameter, or set it to an exclamation symbol (!), ENVI creates a temporary file.
If you set it to the hash symbol (#), ENVI creates a file in the temporary directory, but this file will not be deleted when ENVI closes.