CHDETOP

Description

CHDETOP detects differences between two input raster images ('new' and 'reference'). Typically, CHDETOP is used on a pair of images from the same optical satellite sensor and, depending on the sensor, tasseled caps can be used to determine the changes. The output raster reveals the areas of change that occurred between two defined time periods. The output file may be used as input to EXPOLRAS, which extracts and refines polygons from raster to detect very specific changes in features of interest.

Parameters

Name	Type	Length	Value range
InputNew: Input "new" image channels	Raster port	0 - 1024
InputRef: Input reference image channels	Raster port	0 - 1024
Algorithm	String	0 - 1	COMBINE | MAGDIFF | CHANDIFF | INTENRATIO | TCGNDIFF | TCSBDIFF | AVGDIFF | AVGRATIO Default: MAGDIFF
Difference detection algorithm	String	0 - 1	POS | NEG | ABS Default: POS
Satellite sensor	String	0 - 1	See Description.
InputMask: Area mask	Any port	0 - 4

Parameter descriptions

InputNew: Input "new" image channels

Specifies the input image channels representing the area after change occurs.

If this parameter is not explicitly specified, the behavior depends on the ALGO (Algorithm) parameter setting.

InputRef: Input reference image channels

Specifies the input image channels representing the area before change occurs.

If this parameter is not explicitly specified, the behavior depends on the ALGO (Algorithm) parameter setting.

Algorithm

Specifies the algorithm used for automated change detection.

Difference detection algorithm

Specifies the direction of change to be detected, unless ALGO (Algorithm) is set to COMBINE, in which case this parameter is ignored.

Satellite sensor

Specifies the name of the sensor used to capture the input images.

This parameter is ignored unless Algorithm is Tasseled Cap Greenness Difference or Tasseled Cap Brightness Difference.

To use either of the tasseled cap based change detection algorithms, the sensor must be known. Identifying information can either exist in metadata or be specified explicitly using this parameter.

Change detection is typically done between two images from the same sensor. It is possible, however, to use images from two sensors, provided the images are spatially comparable. You may specify two sensors by separating the names with a comma and space; for example, "Landsat-5 MSS, Landsat-4 MSS". When two sensors are specified, the first entry is applied to the new image; the second is applied to the reference image.

If this parameter is not specified, the function checks for CollectorType, PlatformName, PlatformNumber, and SensorModelName metadata tags at the file level.

When sensor information is required, if this parameter is not specified and the necessary tags are not found in the metadata, an error is generated.

InputMask: Area mask

Optionally, specifies the areas of the data to be processed. If this parameter is specified, only pixels within the mask area are processed; all other pixels in the output will be set to -3.4E38 and defined as NoData. If the Algorithm parameter is specified as Combine, however, the NoData values in the input channels are preserved. If this parameter is not specified, the entire overlapping region is processed.

You can define these areas using two types of mask boundaries: bitmap and vector.

For a bitmap mask, specify the number of the bitmap segment that you want to use. All the pixels within the specified segment having a pixel value of 1 define the area where input data must exist to be processed.

For a vector mask, specify the number of the vector segment that you want to use as a mask. If the segment is a polygon layer, the input data is clipped along the polygon boundary. If the segment is not a polygon layer, the extents of the shapes in the segment are used to define the mask area.

Details

CHDETOP automatically detects changes between two specified raster images (new and reference). The two input rasters must overlap, align at the pixel level and have the same spatial resolution. They can have different extents and the output will cover the spatial intersection of the two input rasters. The two files should be co-registered and radiometrically normalized.

The same number of channels must be processed from both the input new and reference images. When channels are explicitly specified for DBIC (InputNew), the corresponding channels must be explicitly specified through DBIC_REF (InputRef) or vice versa. When input channels are not explicitly specified, the automatic behavior depends on the setting for the ALGO (Algorithm) parameter. If ALGO (Algorithm) is specified as COMBINE, MAGDIFF (Magnitude Difference), CHANDIFF (Channel Difference), or INTENRATIO (Intensity Ratio), all the channels in the input files are automatically used unless specifically defined. When using either of the tasseled cap based algorithms, however, the appropriate input channels are automatically determined based on the metadata of the input files. In particular, the input files must contain MINWAVELENGTH and MAXWAVELENGTH metadata tags at the channel level to properly associate each channel with the correct band. If this metadata is not present (and the channels are not explicitly specified and ALGO is TCGNDIFF or TCSBDIFF), an error is generated.

When using TCGNDIFF (Tassel Cap Greenness Difference) or TCSBDIFF (Tassel Cap Brightness Difference), there must be four input channels for Landsat-4/5 MSS, IKONOS, and QUICKBIRD data, and six channels for Landsat-4/5 TM, Landsat-7 ETM+ data. Furthermore, for the tasseled cap calculations to have meaning when specifying channels explicitly, you must specify them in increasing band order.