ThematicChange Task

This task takes two classification images of the same scene taken at different times and identifies differences between them. The resulting classification image shows class transitions, for example, from class 1 to class 2. Thematic change detection can be used to analyze land use, land cover change, deforestation, urbanization, agricultural expansion, water variability, and more.

Example

This example creates a thematic change classification image that shows deforestation in an area of the Amazon rainforest between 1984 and 2013, using Landsat imagery. The two images are available from our ENVI Tutorials web page. Click the Landsat Case Studies link to download the .zip file to your machine, then unzip the files. The files you will use are located in the Deforestation folder of the download. Update the file references in the example with the correct locations.

; Start the application

e = ENVI()

; Open the before-and-after rasters

Time1File = 'LandsatAmazon1984.dat'

Time1Raster = e.OpenRaster(Time1File)

Time2File = 'LandsatAmazon2013.dat'

Time2Raster = e.OpenRaster(Time2File)

; Create a spectral subset of the SWIR band only since it

; delineates vegetation from non-vegetation features.

; Bands are zero-based.

Time1Subset = ENVISubsetRaster(Time1Raster, BANDS=[5])

Time2Subset = ENVISubsetRaster(Time2Raster, BANDS=[6])

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

DataColl = e.Data

; Use the image intersection task to get the

; overlapping area between the two images

IntTask = ENVITask('ImageIntersection')

IntTask.INPUT_RASTER1 = Time1Subset

IntTask.INPUT_RASTER2 = Time2Subset

IntTask.Execute

; Get the classification task from the catalog of ENVITasks

ISODATA1Task = ENVITask('ISODATAClassification')

; Define parameters for Time 1 raster

ISODATA1Task.INPUT_RASTER = IntTask.OUTPUT_RASTER1

ISODATA1Task.NUMBER_OF_CLASSES = 2

ISODATA1Task.Execute

Time1ClassRaster = ISODATA1Task.OUTPUT_RASTER

; Change class names and colors

Time1Metadata = Time1ClassRaster.METADATA

Time1Metadata.UpdateItem, 'CLASS LOOKUP', $

  [[0,0,0], [0,128,0], [200,200,200]]

Time1Metadata.UpdateItem, 'CLASS NAMES', $

  ['Unclassified','Vegetation','No Vegetation']

; Add the result to the Data Manager

DataColl.Add, Time1ClassRaster

; Define parameters for Time 2 raster

ISODATA2Task = ENVITask('ISODATAClassification')

ISODATA2Task.INPUT_RASTER = IntTask.OUTPUT_RASTER2

ISODATA2Task.NUMBER_OF_CLASSES = 2

ISODATA2Task.Execute

Time2ClassRaster = ISODATA2Task.OUTPUT_RASTER

; Change class names and colors

Time2Metadata = Time2ClassRaster.METADATA

Time2Metadata.UpdateItem, 'CLASS LOOKUP', $

  [[0,0,0], [0,128,0], [200,200,200]]

Time2Metadata.UpdateItem, 'CLASS NAMES', $

  ['Unclassified','Vegetation','No Vegetation']

; Add the result to the Data Manager

DataColl.Add, Time2ClassRaster

; Perform thematic change detection

Task = ENVITask('ThematicChange')

Task.INPUT_RASTER1 = Time1ClassRaster

Task.INPUT_RASTER2 = Time2ClassRaster

Task.MERGE_NO_CHANGE = 'True'

Task.OUTPUT_RASTER_URI = e.GetTemporaryFilename()

Task.Execute

DataColl.Add, Task.OUTPUT_RASTER

; Display the result

View = e.GetView()

Layer = View.CreateLayer(Task.OUTPUT_RASTER)

View.Zoom, /FULL_EXTENT

Syntax

Result = ENVITask('ThematicChange')

Input parameters (Set, Get): INPUT_RASTER1, INPUT_RASTER2, MERGE_NO_CHANGE, 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

INPUT_RASTER1 (required)

Specify a classification raster representing the earliest time.

INPUT_RASTER2 (required)

Specify a second classification raster representing the latest time.

MERGE_NO_CHANGE (optional)

If the two input classification images have the same number of classes that use the same class name, set this parameter to true to group them into a class named "no change" if there was no difference detected during processing. Set this parameter to false if the output is a new classification image that includes all class transition information. The default value is true.

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.