AUTOGCP

Name	Type	Length	Value range
Input: Input raw image channel(s)	Raster port	0 - 1024
Source background options	String	0 -	Default: File Metadata
Source background values	Integer	0 - 1024	Default: 0
InputMMSeg: Math model segment number	BIN port	0 - 1
LOCLMASK	String	0 -	Default: NONE
OutputGCP: GCP segment	GCP port	0 - 1
InputRef: Reference image channel(s)	Raster port	0 - 1024	Default: 1
InputDEM: Elevation channel	DEM port	0 - 1	Default: 1
Elevation background value	Float	0 - 1
Elevation reference	String	0 -
Elevation units	String	0 - 1	METER \| FEET \| US_FEET
Elevation offset	Float	0 - 1	Default: 0.0
Elevation scale	Float	0 - 1	Default: 1.0
Search source method	String	0 -	SUSAN \| GRID \| DBREF Default: GRID:64
Number of GCPs per image	Integer	0 - 1024	Default: 64
Number of trials per point	Integer	0 - 1024	Default: 1
Edge margin distance	Integer	0 - 1024
Matching method	String	0 -	FFTP \| NCC Default: FFTP
Search radius	Integer	0 - 1	1 - Default: 100
Search radius units	String	0 - 1	PIXEL \| METER \| FEET \| US_FEET Default: PIXEL
Minimum acceptance score	Float	0 - 1	0.0 - 1.0 Default: 0.75
Approximate elevation	Integer	0 - 1
Report	String	0 - 192	See parameter description

Parameter descriptions

Input: Input raw image channel(s)

Specifies the channel(s) in the raw image file from which to extract the GCP samples. Multiple channels are averaged together.

If this parameter is not specified, all channels are processed by default.

Source background options

Specifies, potentially with Source Background Values, which pixels in the source image are to be considered background (NoData) pixels. In general, if a pixel is considered NoData, the application handles the pixel in a special manner.

Available options are:

File metadata: Reads the NoData value from the input file's NO_DATA_VALUE metadata. The file-level metadata tag is used as the default for every channel; channel-level tags, when available, override the default. When metadata does not exist, every pixel in the input is considered valid.
0: every pixel, in every channel, having a value of 0 is considered NoData
None: every pixel, in every channel, is considered valid
Specify values: every pixel, in every channel, having the value(s) specified in the Source Background Values parameter is considered NoData
File metadata, else specify values: Same as File Metadata, but uses the specified Source Background Values to define the NoData value for every channel, when metadata does not exist.

See the Source Background Values parameter for specific examples.

Source background values

Specifies the source background value(s) when the following options are specified for the Source Background Options parameter:

Specify values
File metadata, else specify values

Note: Specify multiple values in a comma-delimited list. The first value is applied to the first channel, the second value to the second channel, and so on. If fewer values are specified than the number of input channels, the last value is repeated for all remaining channels. If more values are specified than the number of input channels, the extra values are ignored.

For example:

File metadata, 0: uses file-level metadata. When metadata is unavailable, pixels that have a value of 0 are considered NoData.
255: every pixel, in every channel, that has a value of 255 is considered NoData

InputMMSeg: Math model segment number

Specifies the number of the segment in the input file that contains the math model that will be used to transform coordinates between the input and reference images. If this parameter is not specified, the function uses the highest-numbered math model segment that exists in the input file.

A special value of 0 instructs the function to use an affine math model based on the nominal georeferencing of the raw image.

A special value of -1 instructs the function to do the matching in pixel space exclusively; any and all georeferencing is ignored, meaning that the first pixel in the input image is assumed to correspond to the first pixel in the reference image. This method may be useful when trying to correct band-to-band misregistrations.

Note: If a segment other than 0 is specified but does not exist in the input file, this function raises an exception.

LOCLMASK

Specify whether to apply a local mask to prevent points from being collected in those locations. If no value is specified for this parameter (default), or you specify a value, but one or more of the source images do not contain a bitmap or vector segment, no local exclusion mask will be applied for those images.

The syntax for LOCLMASK is:

LOCLMASK = (NONE | BIT | VEC | <n>)

You can specify the value of LOCLMASK using the following:

NONE: exclude no pixels from the computation
BIT: use when there are bitmap segments in at least some of the source image files that define exclusion regions. When one of the source images contains multiple bitmaps, the last one is used. All image pixels covered by the bitmap are excluded from point collection.
VEC: use when there are vector segments in at least some of the source image files that define exclusion regions. When one of the source images contains multiple vector segments, the last one is used. This vector segment must be a polygon, and all image pixels enclosed by the polygon are excluded from point collection.
<n>: specifies a particular segment number that contains the desired mask segment to be used.

This parameter is optional.

OutputGCP: GCP segment

Specifies the GCP segment in the input file to receive the collected GCPs. If this parameter is not specified, a new GCP segment is created.

InputRef: Reference image channel(s)

Specifies the channel(s) in the input reference image to use as reference. Multiple channels will be averaged together.

See the Input (DBIC) parameter for additional details.

InputDEM: Elevation channel

Specifies the channel that contains the digital elevation model (DEM) from which GCPs will be obtained.

Elevation background value

Optionally specifies the background elevation (NoData) value of the input DEM. Areas that are defined as NoData in the DEM are excluded from consideration while searching for GCPs.

If this parameter is not specified, the function checks for ELEVATION_BACKGROUND or NO_DATA_VALUE metadata tags, first at the channel level, then at the file level. If this value is not specified or found in the metadata, the function uses the default value of -32768.

Elevation reference

Optionally specifies the vertical reference for the elevation values contained in the source DEM, or for the constant ELFACTOR (Elevation Offset/Scale) value, if defined.

If this parameter is not specified, the metadata tag ELEVATION_DATUM is searched at the file level and the channel specified by DBEC. If not found, MSL is the default value.

Elevation units

Optionally specifies the units used to describe the elevation values of the input DEM file.

Acceptable values are:

METER (default value)
FEET
US_FEET

If elevation values are specified as FEET, the conversion factor to meters is 0.3048 (corresponding to International Feet); if US_FEET, the conversion factor is 1200/3937 (corresponding to U.S. Survey Feet).

If this parameter is not specified, the function searches for an ELEVATION_UNITS metadata tag at the file level, then again at the channel level. If this value is not specified through the parameter or found in the metadata, its value defaults to METER.

Elevation offset

Optionally specifies the elevation offset used by the DEM.

If this parameter is not specified, the function searches for an ELEVATION_OFFSET metadata tag at the file level, then again at the channel level. If this value is not specified through the parameter or found in the metadata, the offset defaults to 0.0, indicating that there is no offset.

Elevation scale

Optionally specifies the elevation scale used by the DEM.

If this parameter is not specified, the function searches for an ELEVATION_SCALE metadata tag at the file level, then again at the channel level. If this value is not specified through the parameter or found in the metadata, its value defaults to 1.0, indicating that the scale is 1:1.

Search source method

Specifies the source of sample points for GCP matching. GCPs may be automatically generated using the SUSAN or GRID option, or they may be explicitly specified in a sample file.

Available options are:

SUSAN: Sample points are generated automatically by using the SUSAN corner-detection algorithm.
GRID: Sample points are generated automatically by using sample points distributed evenly across the overlap region.

Available options are:

SUSAN: Sample points are generated automatically by using the SUSAN corner-detection algorithm.
GRID: Sample points are generated automatically by using sample points distributed evenly across the overlap region.

The SUSAN and GRID options determine how to find the initial candidate positions in one image–the source image–for collecting sample points. AUTOTIE builds a patch around each candidate position that it searches for in overlapping images.

The SUSAN option finds the candidates by running a corner-detection algorithm on the image, which looks for corner-like features to use as candidates.
The GRID option creates candidates in a grid-like pattern. Because this option does not preprocess the image, it tends to be faster, but it may find fewer matches because the grid point might fall on a featureless flat patch somewhere in the image that cannot be matched to anything in the overlapping images.

When collecting GCPs, the GRID option is preferred because the SUSAN option finds candidates on building corners that may not be represented in the DEM, leading to GCPs with higher residuals due to height errors.

Number of GCPs per image

Specifies the number of sample points to use for GCP matching in each image.

Number of trials per point

Specifies the number the total number of trials for each point before giving up on the match. The algorithm will try to match the primary sample point and if that fails to match, a secondary point will be chosen within the same grid cell as an additional sample. The default value is 1. Possible values are integer values between 1 and 500.

NOTE: Using the trials option will make the matching process slower as the system may try multiple times to match in each cell.

Edge margin distance

Only available if the Search source method is set to GRID.

If the margin option is used the value specified controls the minimum distance, in pixel units, between the edge of the image and the placement of the candidates.

When specified, the first point of the grid will be explicitly put at the value location. As well, when the margin option is used the points will be distributed starting at the edges of the overlap rather than in the center of the grid cells, which greatly improves the chances of finding matches at the edge(s) of the image

If margin is not specified the overlap region is divided into a grid and candidates, by default, are placed in the center of the grid cells.

Matching method

Specifies the algorithm used for automated GCP matching.

Supported methods are:

FFTP (Fast Fourier Transform Phase matching): When two images have a relative shift between them, the result is a phase difference in the Fourier domain. FFTP determines the shift between images using this phase difference.
NCC (Normalized Cross-Correlation matching): This method finds the relative shift between two images by finding the shift that produces the maximum cross-correlation coefficient of the gray values in the images.

If this parameter is not specified, FFTP is the default method.

When the two images being matched have similar gray values and appearances, NCC generally produces acceptable results. When there is a rotation or image size error in the initial math models, NCC may produce better matching results than FFTP. Because the template size that NCC uses is smaller than the one used by FFTP, this method also typically generates faster results.

For more consistently accurate results, FFTP is recommended. This method uses a larger template size than NCC and, because it works in the frequency domain, it looks at the patterns of details in the image rather than the gray values in a small neighborhood, which NCC uses. This makes FFTP more robust than NCC in cases where there is a large brightness difference between images or when a major land use change has occurred between the images and allows it to better match images of the same area from different sensors or spectral bands.

Search radius

Optionally specifies the distance in the X and Y directions from a starting location on the reference image over which the search for the best match with a fixed point on the input image will be conducted. The units for this value are controlled by the SEARCHUN (Search Radius Units) parameter.

The search radius is an estimation of error with the raw image's positional information and the DEM accuracy. If you know that your image is accurate to 80 meters and your DEM is accurate to 200 meters, set the search radius to 280 meters. A larger search radius will require more processing time, because more locations are evaluated to determine the best match for a GCP.

If this parameter is not specified, the function uses a search radius of 100 pixels.

Search radius units

Optionally specifies the units of the search radius value, as specified by the SEARCHR (Search Radius) parameter.

Acceptable values are:

PIXEL: search radius in pixels (default)
METER: search radius in meters
FEET: search radius in feet
US_FEET: search radius in U.S. Survey feet

When using PIXEL, the search radius value is interpreted to mean the number of pixels in the reference image. If the specified unit is METER, FEET, or US_FEET, the search radius represents meters or feet in the reference image.

Minimum acceptance score

Optionally specifies a threshold value that controls whether a candidate GCP is accepted as a GCP or rejected. This parameter specifies the minimum match quality that is considered an acceptable match, with 1.0 indicating a perfect match.

When using the FFTP algorithm, this value is converted internally to a minimum acceptable phase shift peak value.

When using the NCC matching method, this value specifies the minimum match score value required to accept a local match between the input and reference images as a GCP. The default value is 0.75.

Approximate elevation

Optionally specifies, in meters, an approximated average elevation value.

If no DEM is provided, this option is enabled to enter an approximate average elevation value.

If no DEM is provided and no value is entered for the approximate average elevation value, an approximated elevation value is calculated from the file gmted2010.pix stored in the etc subdirectory of the CATALYST Professional installation folder.

Report

Specifies where to direct the generated report.

Available options are:

LOG: generates a report on the terminal (default)
<modulename.RPT>: appends a report to the module's RPT file
DISK: generates a report on file "IMPRPT.LST"

Details

AUTOGCP collects ground control points (GCPs) from an input image using a reference image. A digital elevation model (DEM) may be used to extract the elevation for each GCP.

AUTOGCP is typically used to:

Co-register two referenced images

This is used for applications such as pansharpening, where the multispectral and the panchromatic images are slightly misaligned. You can use this function to collect GCPs between the input and reference files, then correct the input file using a resampling program such as REG.
Collect GCPs between an unreferenced image and a referenced file

This is used for orthorectifying satellite images. You can use this function to collect GCPs and a DEM to extract the elevation for each GCP. The resulting GCPs are then used to compute the geometric model (or math model). Use AUTOGCP to remove the bottleneck of manual GCP collection from the image rectification workflow.

You may specifiy a threshold value that controls whether a candidate GCP is accepted as a GCP or rejected. The MINSCORE (Minimun Acceptance Level) parameter specifies the minimum match quality that is considered an acceptable match, with 1.0 indicating a perfect match.

When using the FFTP algorithm, this value is converted internally to a minimum acceptable phase shift peak value.

When using the NCC matching method, this value specifies the minimum match score value required to accept a local match between the input and reference images as a GCP. The default value is 0.75.

You may also change the Search Radius (SEARCHR) for matching each GCP, and the units in which the radius is defined (SEARCHUN); the default radius is 100 pixels. If the approximate positions between the input and reference files are not accurate, increase the SEARCHR value.

Upon completion, AUTOGCP populates the NUMGCPS variable with the number of extracted GCPs.

If no GCPs were collected, there are several settings that can be adjusted to increase the channces of finding matches. Increasing the search radius will cause the matcher to search a larger area for each point and more matches may be found. If possible, measure the error between the raw image's model or georeferencing and the reference image and set the search radius to a value larger than it. Increasing the number of trials may also help with collecting more matches as it tries additional points near every point that failed to match. Increasing the number of samples may also result in more matches as it increases the number of locations where matches are attempted. Ensuring that the input channel and the reference channel have as similar spectral content as possible will also help the matcher find successful matches.

Working with radar data

When the input data contains image layers written as complex values, the total power in decibels is computed temporarily and used for matching. When possible, it is also recommended that you calibrate the data in sigma, beta or gamma naught. FFTP matching is also recommended when working with SAR data.

DEM index file

The FILEDEM parameter may specify the name of a file, a directory, or a text file. If a directory is specified, that directory is searched for a file named index.txt which must match the DEM index format.

If a text file is specified, the file's format must match the DEM index file format. The DEM index file format is treated as a single virtual DEM, eliminating the need to merge the DEM tiles into a single file.

Environments	PYTHON :: EASI :: MODELER
Batch Mode	Yes
Quick links	Description :: Parameters :: Parameter descriptions :: Details

AUTOGCP

Enhanced automatic GCP collection

Description

Parameters

Parameter descriptions

Details