EXPOLRAS

Description

EXPOLRAS converts an input raster image to a binary image using a specified threshold value, extracts polygons from the resulting binary image, and then filters them based on area and compactness values. Finally, it outputs results to a vector layer.

Parameters

Name	Type	Length	Value range
Input: Input raster channel *	Raster port	1 - 1
Threshold type	String	10 - 1	Percentage | PixelValue Default: PixelValue
Threshold value *	Float	1 - 2
Area value	Float	0 - 2
Compactness value	Float	0 - 2
Output: Output vector segment *	Vector port	1 -

* Required parameter

Parameter descriptions

Input: Input raster channel

The input raster channel to use to extract the polygons. You can specify only one channel. To create polygons resulting from change-detection analysis, specify the channel that contains the change-detection results.

Threshold type

The threshold type to use to convert the input raster to a binary image.

The value attributed to the specified threshold type is defined with the Threshold value (TVAL) parameter.

Threshold value

The minimum and maximum threshold values, based on the value specified for the Threshold type (THRTYPE) parameter. If only one value is specified, pixels with values (or percentile value) equal to or above the specified value are retained.

When this parameter specifies a pixel value, that value should be within the dynamic range of the specified input channel (DBIC).

Area value

The minimum and maximum area values, in pixels, by which to filter extracted features based on their size.

All polygons with values within the defined range are retained. If only one value is specified, polygons with an area equal to or above the specified value are retained.

Compactness value

The minimum and maximum values of compactness for the target polygons.

All polygons with compactness values within the defined range are retained. If only one value is specified, polygons with compactness values equal to or above the specified value are retained.

Compactness is a ratio describing of the area of a polygon to its perimeter, defined as follows:

Compactness = 4*3.141592653589*Area / (Perimeter * Perimeter)

The value of compactness is between 0 and 1, where 1 represents a circle.

Output: Output vector segment

The output vector segment to receive the converted and filtered output.

Details

EXPOLRAS produces polygons for pixels for which the digital number (DN) or percentile value is higher than, or within the range of, the value specified as threshold (TVAL) and filters out those that do not fit within the specified area and compactness range of values. By default, only the threshold value is required; polygons will be derived using pixels above the specified threshold value. This function is typically used in conjunction with a change-detection image; for example, the output from CHDETOP or CCDINTEN.

When a percentage threshold is specified, it represents values of significance. The pixel value corresponding to the specified percentile is calculated from the input image, and only the pixels above the specified percentile (or within the percentile range) are retained. By specifying a threshold of 95 (95 percent) for processing a change-detection image, the pixels falling above the 95th percentile of change will be kept.

If you specify a minimum area value, only those polygons larger than the specified area value are retained, and the attribute table will contain the area value (for the remaining polygons) in pixel units in the "PixelCount" field. If you specify a compactness value or range, only those polygons above the defined compactness (or within the defined range) are retained, and their compactness value is written to the attribute table in the "Compactness" field.

The following fields are computed and written to the output attributes: Area, Perimeter, PixelCount, Compactness, Length and Orientation. Length is computed as the longest distance between any two vertices of each extracted polygon shape. Orientation is the slope of the line connecting those two vertices, in degrees. If you were extracting ships from imagery, the Length value could approximate the keel length while the Orientation could be the ship's heading.

If none of the polygons match the area or compactness criteria (for example, if the polygons are smaller than the specified area value) an error will occur.

Note that NoData pixels are excluded from consideration with one exception. When EXPOLRAS is run with Threshold type set to PIXELVALUE and threshold minimum and maximum are both specifed as the NoData number, then polygons will be extracted around those NoData pixels only. For example: THRTYPE=PIXELVALUE and TVAL=0,0 and NoData=0.