User's Guide

CORRELATE

Performs tie point refinement via grey, edge, or phase correlation

Function:

Performs registration refinement based on the maximum value of either the gray-level (normalized cross) correlation function, the binary edge correlation function, or the phase correlation function computed over a set of relative offsets between a search subimage and a reference subimage. A fourth method, grey-edge, applies both the gray and edge correlation methods, choosing the better result on a point-by-point basis.

Parameters:

Subcommand -GREY:
Performs registration refinement based on the maximum value of the grey-level (normalized cross) correlation function.

IN("","")
Input image(s) (reference and search). The reference image name followed by the search image name. Both image names are optional, but must remain in appropriate array location (reference = 1, search = 2). If the reference image name is not given, it is assumed reference subimage chips are being supplied. If the search image is not given, it is assumed prerotated and/or scaled search subimage chips are being used. Refer to the User Notes section for more information on processing with subimage chips. Windowing is ignored.

OUTTL
Output tie point location file. The output tie point location file containing correlation results.

TVAL1(--)
Reference tie point line, sample values. Three to ten tie points in the first (reference) input image, used to determine the rotation, translation, and scale factors between the reference and search images. The number of values specified in TVAL1 and TVAL2 must be equal. These values are to be filled in as tiepoint1 line, tiepoint1 sample, tiepoint2 line, tiepoint2 sample...

TVAL2(--)
Search tie point line, sample values. Three to ten corresponding tie points in the second (search) input image. The number of values specified in TVAL1 and TVAL2 must be equal. These values are to be filled in as tiepoint1 line, tiepoint1 sample, tiepoint2 line, tiepoint2 sample...

INMT(--)
Input merged tie point file. One of four methods for entering the tie points to be correlated. The other methods are CUTLINE, GRIDSPEC, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

CUTLINE(--)
Input cutline file. The cutline file is an IBIS graphics file containing the coordinates for the cutline or polygonal contours of the reference image to be used as the correlation point set. The polygon must be closed. If an input cutline and point increment (PNTINC) are specified, the correlation point set is spaced along the cutline. One of four methods for entering the tie points to be correlated. The other methods are INMT, GRIDSPEC, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

PNTINC(200)
Point increment for cutline points. Spacing (in line, sample units) to be followed along the cutline. This is a required parameter when using an input cutline file (CUTLINE). If a cutline file is not specified, PNTINC is ignored.

GRIDSPEC(--)
Grid boundary specifications. Using line, sample coordinates, specify the upper-left (line, then sample) and lower-right corners of the grid in the reference image to be used as the correlation point set. One of four methods for entering the tie points to be correlated. The other methods are INMT, CUTLINE, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

NROWS(--)
Number of rows in the grid. This parameter is required when using the GRIDSPEC option.

NCOLS(--)
Number of columns in the grid. This parameter is required when using the GRIDSPEC option.

INTL(--)
Input tie point location file. If an input tie point location file is given, the contents of the file are copied to the output tie point location file before correlation begins.

MINCORR(6.0)
Minimum correlation. The minimum acceptable correlation strength.

MAXDIFF(-2.0)
Maximum difference. The maximum acceptable difference between the nominal tie point location and that given by correlation.

CONFLG(NO)
Confirmation flag.

  = YES: Confirm.  Full details of correlation are provided for
         each tie point, and the user is given an opportunity to
         reject the tie point (not include in the tie point
         location file).
  = NO:  Do not confirm.  Reports whether acceptable correlation
         results were obtained and the active flag field in tie
         point location file is set accordingly.

RETSPEC(--)
Retry specification. The retry specification allows more tries to be made near a location if a certain correlation threshold is not achieved. The first value specifies the number of retries (up to four). The second value is the correlation threshold. The third value is the distance to move in the line, sample direction from the current location. The retry pattern is to successively move to the corners of a square at this distance from the nominal location. The default value indicates that no retries will be attempted.

FITMETH(PARAB)
Fitting method. The correlation peak fitting method.

  = PARAB:   Elliptic paraboloid
  = GAUSS:   Elliptic Gaussian
  = RECIP:   Elliptic paraboloid fit to reciprocals of
             correlation values, after subtracting average
             background
  = INTEGER: No surface fit; nearest neighbor
Subcommand -EDGE:
Performs registration refinement based on the maximum value of the binary edge (cross) correlation function.

IN("","")
Input image(s) (reference and search). The reference image name followed by the search image name. Both image names are optional, but must remain in appropriate array location (reference = 1, search = 2). If the reference image name is not given, it is assumed reference subimage chips are being supplied. If the search image is not given, it is assumed prerotated and/or scaled search subimage chips are being used. Refer to the User Notes section for more information on processing with subimage chips. Windowing is ignored.

OUTTL
Output tie point location file. The output tie point location file containing correlation results.

TVAL1(--)
Reference tie point line, sample values. Three to ten tie points in the first (reference) input image, used to determine the rotation, translation, and scale factors between the reference and search images. The number of values specified in TVAL1 and TVAL2 must be equal. These values are to be filled in as tiepoint1 line, tiepoint1 sample, tiepoint2 line, tiepoint2 sample...

TVAL2(--)
Search tie point line, sample values. Three to ten corresponding tie points in the second (search) input image. The number of values specified in TVAL1 and TVAL2 must be equal. These values are to be filled in as tiepoint1 line, tiepoint1 sample, tiepoint2 line, tiepoint2 sample...

INMT(--)
Input merged tie point file. One of four methods for entering the tie points to be correlated. The other methods are CUTLINE, GRIDSPEC, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

CUTLINE(--)
Input cutline file. The cutline file is an IBIS graphics file containing the coordinates for the cutline or polygonal contours of the reference image to be used as the correlation point set. The polygon must be closed. If an input cutline and point increment (PNTINC) are specified, the correlation point set is spaced along the cutline. One of four methods for entering the tie points to be correlated. The other methods are INMT, GRIDSPEC, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

PNTINC(200)
Point increment for cutline points. Spacing (in line, sample units) to be followed along the cutline. This is a required parameter when using an input cutline file (CUTLINE). If a cutline file is not specified, PNTINC is ignored.

GRIDSPEC(--)
Grid boundary specifications. Using line, sample coordinates, specify the upper-left (line, then sample) and lower-right corners of the grid in the reference image to be used as the correlation point set. One of four methods for entering the tie points to be correlated. The other methods are INMT, CUTLINE, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

NROWS(--)
Number of rows in the grid. This parameter is required when using the GRIDSPEC option.

NCOLS(--)
Number of columns in the grid. This parameter is required when using the GRIDSPEC option.

INTL(--)
Input tie point location file. If an input tie point location file is given, the contents of the file are copied to the output tie point location file before correlation begins.

MINCORR(6.0)
Minimum correlation. The minimum acceptable correlation strength.

MAXDIFF(-2.0)
Maximum difference. The maximum acceptable difference between the nominal tie point location and that given by correlation.

CONFLG(NO)
Confirmation flag.

  = YES: Confirm.  Full details of correlation are provided for
         each tie point, and the user is given an opportunity to
         reject the tie point (not include in the tie point
         location file).
  = NO:  Do not confirm.  Reports whether acceptable correlation
         results were obtained and the active flag field in tie
         point location file is set accordingly.

RETSPEC(--)
Retry specification. The retry specification allows more tries to be made near a location if a certain correlation threshold is not achieved. The first value specifies the number of retries (up to four). The second value is the correlation threshold. The third value is the distance to move in the line, sample direction from the current location. The retry pattern is to successively move to the corners of a square at this distance from the nominal location. The default value indicates that no retries will be attempted.

FITMETH(PARAB)
Fitting method. The correlation peak fitting method.

  = PARAB:   Elliptic paraboloid
  = GAUSS:   Elliptic Gaussian
  = RECIP:   Elliptic paraboloid fit to reciprocals of
             correlation values, after subtracting average
             background
  = INTEGER: No surface fit; nearest neighbor

EDGEVAL(0.15)
Edge value. The fraction of pixels to be classified as edges. Values outside the range 0.05 to 0.30 are unlikely to give good results.

THRVAL(255.0)
Threshold value. The threshold pixel value for screening out cloud image data. Any pixel value above THRVAL will not be used by the algorithm. This threshold value will be compared to both the reference and search images. See User Note 6.
Subcommand -GREDGE:
Performs registration by computing both grey-level and binary edge correlation and using the maximum of the two values.

IN("","")
Input image(s) (reference and search). The reference image name followed by the search image name. Both image names are optional, but must remain in appropriate array location (reference = 1, search = 2). If the reference image name is not given, it is assumed reference subimage chips are being supplied. If the search image is not given, it is assumed prerotated and/or scaled search subimage chips are being used. Refer to the User Notes section for more information on processing with subimage chips. Windowing is ignored.

OUTTL
Output tie point location file. The output tie point location file containing correlation results.

TVAL1(--)
Reference tie point line, sample values. Three to ten tie points in the first (reference) input image, used to determine the rotation, translation, and scale factors between the reference and search images. The number of values specified in TVAL1 and TVAL2 must be equal. These values are to be filled in as tiepoint1 line, tiepoint1 sample, tiepoint2 line, tiepoint2 sample...

TVAL2(--)
Search tie point line, sample values. Three to ten corresponding tie points in the second (search) input image. The number of values specified in TVAL1 and TVAL2 must be equal. These values are to be filled in as tiepoint1 line, tiepoint1 sample, tiepoint2 line, tiepoint2 sample...

INMT(--)
Input merged tie point file. One of four methods for entering the tie points to be correlated. The other methods are CUTLINE, GRIDSPEC, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

CUTLINE(--)
Input cutline file. The cutline file is an IBIS graphics file containing the coordinates for the cutline or polygonal contours of the reference image to be used as the correlation point set. The polygon must be closed. If an input cutline and point increment (PNTINC) are specified, the correlation point set is spaced along the cutline. One of four methods for entering the tie points to be correlated. The other methods are INMT, GRIDSPEC, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

PNTINC(200)
Point increment for cutline points. Spacing (in line, sample units) to be followed along the cutline. This is a required parameter when using an input cutline file (CUTLINE). If a cutline file is not specified, PNTINC is ignored.

GRIDSPEC(--)
Grid boundary specifications. Using line, sample coordinates, specify the upper-left (line, then sample) and lower-right corners of the grid in the reference image to be used as the correlation point set. One of four methods for entering the tie points to be correlated. The other methods are INMT, CUTLINE, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

NROWS(--)
Number of rows in the grid. This parameter is required when using the GRIDSPEC option.

NCOLS(--)
Number of columns in the grid. This parameter is required when using the GRIDSPEC option.

INTL(--)
Input tie point location file. If an input tie point location file is given, the contents of the file are copied to the output tie point location file before correlation begins.

MINCORR(6.0)
Minimum correlation. The minimum acceptable correlation strength.

MAXDIFF(-2.0)
Maximum difference. The maximum acceptable difference between the nominal tie point location and that given by correlation.

CONFLG(NO)
Confirmation flag.

  = YES: Confirm.  Full details of correlation are provided for
         each tie point, and the user is given an opportunity to
         reject the tie point (not include in the tie point
         location file).
  = NO:  Do not confirm.  Reports whether acceptable correlation
         results were obtained and the active flag field in tie
         point location file is set accordingly.

RETSPEC(--)
Retry specification. The retry specification allows more tries to be made near a location if a certain correlation threshold is not achieved. The first value specifies the number of retries (up to four). The second value is the correlation threshold. The third value is the distance to move in the line, sample direction from the current location. The retry pattern is to successively move to the corners of a square at this distance from the nominal location. The default value indicates that no retries will be attempted.

FITMETH(PARAB)
Fitting method. The correlation peak fitting method.

  = PARAB:   Elliptic paraboloid
  = GAUSS:   Elliptic Gaussian
  = RECIP:   Elliptic paraboloid fit to reciprocals of
             correlation values, after subtracting average
             background
  = INTEGER: No surface fit; nearest neighbor

EDGEVAL(0.15)
Edge value. The fraction of pixels to be classified as edges. Values outside the range 0.05 to 0.30 are unlikely to give good results.

THRVAL(255.0)
Threshold value. The threshold pixel value for screening out cloud image data. Any pixel value above THRVAL will not be used by the algorithm. This threshold value will be compared to both the reference and search images. See User Note 6.
Subcommand -PHASE:
Performs registration refinement based on the filtered phase correlation function.

IN("","")
Input image(s) (reference and search). The reference image name followed by the search image name. Both image names are optional, but must remain in appropriate array location (reference = 1, search = 2). If the reference image name is not given, it is assumed reference subimage chips are being supplied. If the search image is not given, it is assumed prerotated and/or scaled search subimage chips are being used. Refer to the User Notes section for more information on processing with subimage chips. Windowing is ignored.

OUTTL
Output tie point location file. The output tie point location file containing correlation results.

TVAL1(--)
Reference tie point line, sample values. Three to ten tie points in the first (reference) input image, used to determine the rotation, translation, and scale factors between the reference and search images. The number of values specified in TVAL1 and TVAL2 must be equal. These values are to be filled in as tiepoint1 line, tiepoint1 sample, tiepoint2 line, tiepoint2 sample...

TVAL2(--)
Search tie point line, sample values. Three to ten corresponding tie points in the second (search) input image. The number of values specified in TVAL1 and TVAL2 must be equal. These values are to be filled in as tiepoint1 line, tiepoint1 sample, tiepoint2 line, tiepoint2 sample...

INMT(--)
Input merged tie point file. One of four methods for entering the tie points to be correlated. The other methods are CUTLINE, GRIDSPEC, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

CUTLINE(--)
Input cutline file. The cutline file is an IBIS graphics file containing the coordinates for the cutline or polygonal contours of the reference image to be used as the correlation point set. The polygon must be closed. If an input cutline and point increment (PNTINC) are specified, the correlation point set is spaced along the cutline. One of four methods for entering the tie points to be correlated. The other methods are INMT, GRIDSPEC, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

PNTINC(200)
Point increment for cutline points. Spacing (in line, sample units) to be followed along the cutline. This is a required parameter when using an input cutline file (CUTLINE). If a cutline file is not specified, PNTINC is ignored.

GRIDSPEC(--)
Grid boundary specifications. Using line, sample coordinates, specify the upper-left (line, then sample) and lower-right corners of the grid in the reference image to be used as the correlation point set. One of four methods for entering the tie points to be correlated. The other methods are INMT, CUTLINE, and interactive. If no values are entered for INMT, CUTLINE, or GRIDSPEC, the user will be prompted for tie point data.

NROWS(--)
Number of rows in the grid. This parameter is required when using the GRIDSPEC option.

NCOLS(--)
Number of columns in the grid. This parameter is required when using the GRIDSPEC option.

INTL(--)
Input tie point location file. If an input tie point location file is given, the contents of the file are copied to the output tie point location file before correlation begins.

MINCORR(180.0)
Minimum correlation. The minimum acceptable correlation strength.

MAXDIFF(64.0)
Maximum difference. The maximum acceptable difference between the nominal tie point location and that given by correlation.

CONFLG(NO)
Confirmation flag.

  = YES: Confirm.  Full details of correlation are provided for
         each tie point, and the user is given an opportunity to
         reject the tie point (not include in the tie point
         location file).
  = NO:  Do not confirm.  Reports whether acceptable correlation
         results were obtained and the active flag field in tie
         point location file is set accordingly.

RETSPEC(--)
Retry specification. The retry specification allows more tries to be made near a location if a certain correlation threshold is not achieved. The first value specifies the number of retries (up to four). The second value is the correlation threshold. The third value is the distance to move in the line, sample direction from the current location. The retry pattern is to successively move to the corners of a square at this distance from the nominal location. The default value indicates that no retries will be attempted.

SIZE(128)
Search subimage size. The size of the search subimage window (N x N). Valid values are from 34 to 128.

STEP(12)
Stepping increment. The increment by which the 32 x 32 reference window is moved across the search subimage. Valid values are from 2 to 12.

EXPVAL(0.0)
Exponent value. The power to which correlation magnitude is raised. A value of 0.0 gives standard phase correlation. A value of 1.0 gives a cross correlation. Values between 0.0 and 1.0 give filtered phase correlation.

HPFFLG(YES)
High-pass filter flag. Allows the default high-pass filter in the correlation to be shut off.

  = YES: Apply high-pass filter.
  = NO:  Do not apply high-pass filter.

EDGEFLG(NO)
Edge enhancement flag. Allows a simple 3 x 3 edge enhancement to be applied to both reference and search subimages prior to correlation.

  = YES: Perform edge enhancement.
  = NO:  No edge enhancement.

Examples:

  1. LAS> correlate-grey in=(ref, srch) inmt=nominal outtl=refined

    CORRELATE obtains tie point information from the merged tie point file "nominal" and places its output in the tie point location file "refined". For each tie point in "nominal", CORRELATE locates a correlation peak for the gray levels from the respective neighborhoods of the tie point in the images "srch" and "ref". An elliptic paraboloid surface is fit to the correlation values in the vicinity of the peak. For each tie point, the strength of the correlation is compared against the default limits MINCORR=6 and MAXDIFF=-2.0, and the tie point active flag is set accordingly.

  2. LAS> correlate-edge in=(ref, srch) inmt=nominal outtl=refined edgeval=0.5 retspec=(4,4,50)

    CORRELATE obtains tie point information from the merged tie point file "nominal" and places its output in the tie point location file "refined". For each tie point, CORRELATE first performs a gradient operation to enhance edges. Next, the subimage is histogrammed. A thresholding operation is applied to create a binary edge image. Then a correlation peak is located between the binary edge subimages. An elliptic paraboloid surface is fit to the correlation values in the vicinity of the peak. For each tie point, the correlation strength is compared against the default limits MINCORR=6 and MAXDIFF=-2.0. If a successful correlation is not found, up to four additional attempts will be made using new reference and search locations offset 50 lines and 50 samples from the input location. For these new attempts, the minimum correlation magnitude is reduced to 4.

  3. LAS> correlate-gredge in=(ref, srch) outtl=refined gridspec=(200 200 1200 1200) nrows=6 ncols=6 tval1=(240,1360,400,450,1200,250) tval2=(680,1670,500,750,1170,290) mincorr=7 maxdiff=40 fitmeth=integer

    CORRELATE generates a 6 by 6 grid of points in the reference image and uses the reference/search tie point pairs (240,1360) (680,1670), (400,450)(500,750), and (1200,250)(1170,290) to compute nominal locations in the search image. The grid of points starts at (200,200) and spreads in steps of 200 in both the line and sample directions. Both gray and edge correlation are applied at each point, keeping the larger of the two strengths. The minimum acceptable correlation strength is 7 and the maximum correlation offset is 40 pixels. No subpixel correlation will be conducted.

  4. LAS> correlate-phase in=(ref, srch) cutline=polycut pntinc=100 outtl=refined intl=precorr tval1=(240,1360,400,450,1200,250) tval2=(680,1670,500,750,1170,290)

    CORRELATE reads the cutline file "polycut" and generates the reference space tie points at 100-pixel intervals along the cutline. The values of TVAL1 and TVAL2 are used to compute the nominal search space points. The phase correlation is computed after applying the default edge and high-pass filters to both the reference and search chips. The default values for MINCORR (180) and MAXDIFF (64) are enforced.

Description/Algorithm:

CORRELATE extracts tie point neighborhood subimages from an image that is to be geometrically rectified and uses gray-level (normalized) cross correlation, binary edge-image cross correlation, or filtered phase correlation to refine tie point locations. A fourth "method" consists of applying both gray and edge correlation to each point and choosing the result with the better correlation strength. Reference subimages are extracted from a single reference image or from multiple chip images. Likewise, either a single image or multiple chip images may be specified for search subimage extraction. CORRELATE accomplishes tie point refinement using the following steps.

The first step determines the search and reference images used, the tie point location file to be created, and additional correlation parameters. If an input tie point location file is not used, correlation parameters are obtained from the user via TAE parameters. If an input tie point location file from a previous correlation process is given, most of the parameters are obtained from the header of the input tie point location file.

The next step extracts the tie point neighborhood subimages from the reference and search images as each tie point is processed. Tie point specific parameters define the tie point neighborhoods and subimage sizes. The tie point parameters may be entered interactively before each tie point is correlated, or the parameters may be read from a merged tie point file as generated by TIEMERGE-PRECORR. In the case of cutline and grid generated tie points, the chip sizes default to 64 x 64 (reference) and 96 x 96 (search).

The third step is performed only for edge correlation. In this step, an edge extraction filter converts each grey-level subimage into a binary edge image. First a gradient filter is used to enhance image edges. Then a histogram of edge values is compiled, and an edge threshold is chosen so that the fraction of all pixels that are classified as edges can be as close to the EDGEVAL parameter as possible. An array of edge density values for each alignment of the reference subimage, relative to the input subimage, is also compiled.

The fourth step performs the actual correlation and determines the exact translational alignment of the reference subimage relative to the search subimage. This step also yields estimates of the strength of the correlation and the probable root mean square errors in the final alignment values. For the PHASE subcommand, a 32 x 32 reference images window is passed over the search subimage. The correlation is calculated at that point, then the window is moved forward by STEP pixels, and the correlation is recalculated. This continues until the entire subimage has been covered.

Each search and reference subimage is subjected to a complex-valued Discrete Fourier Transform (DFT) via the Fast Fourier Transform (FFT) algorithm. Each element in the transformed search subimage is multiplied by the corresponding element in the conjugated reference subimage. With the PHASE subcommand, the result is normalized to unity by dividing by the absolute value to give phase correlation. Next, an inverse FFT is applied to these results.

For the GREY and EDGE (and GREDGE) subcommands, the unnormalized sums of the pixel-by-pixel products (GREY) or the edge-pixel coincidences (EDGE) for every possible alignment within the specified search offsets are computed. These results are used to compute the normalized cross-correlation values.

Next, the alignment at which the correlation value is maximum is found. The overall strength is computed by first finding the mean and standard deviation of the correlation values for all alignments outside a 9 x 9 array centered on the main correlation peak. The difference between the main peak (maximum correlation value) and background (mean correlation value) and the difference between the main peak and the subsidiary peak (largest correlation value outside a 7 x 7 array centered on the main peak) are determined. These differences are then converted to units of standard deviations by dividing them by the standard deviation as previously computed. The overall correlation strength is then defined as the sum of these two heights plus 0.2 times the number of alignments in the neighborhood of the main peak whose correlation values exceed the value for the strongest subsidiary peak.

Finally, a quadratic surface is fit to the correlation values in the neighborhood of the main peak, and the fractional pixel coordinates of the peak and estimates of the root mean square errors in these coordinates are computed.

After converting to coordinates relative to the full search and reference images, the final step records the results of the correlation in the output tie point location file, one tie point at a time.

Nonfatal Error Messages:

    Self-documented.

Fatal Error Messages:

    Self-documented.

User Notes:

  1. The following is a possible scenario involving the use of CORRELATE.

    a. Given a reference image, "ref.img", and a search image, "srch.img", taken at the same location but at a different times (image-to-image registration), run TIEPTS to select a set of tie points. The output will consist of two tie point selection files, for example "ref.tps" for the reference image and "srch.tps" for the search image.

    b. Run TIEMERGE-PRECORR, using "ref.tps" and "srch.tps" as inputs, to create a merged tie point file named "tie.mtp".

    c. Run CORRELATE IN=(ref, srch) INMT=tie OUTTL=tie to generate the new tie point location file, "tie.tpl".

    d. Run POLYFIT (or EDITCORR, AUTOFIT) with "tie.tpl" as input to generate a geometric mapping grid file, "geomprm.grid".

    e. Run RESAMPLE with "srch.img" as the input image and "geomprm.grid" as the geometric mapping grid file to produce an image that is registered to the original image (ref.img).

  2. The results of the correlation, which include the estimated horizontal and vertical root mean square errors, the tie point active flag, the strength of correlation and the displacement of the nominal tie point location to that obtained from correlation, are stored in the output tie point location file. Depending upon the active flag, certain fields in the tie point location file will not be meaningful; for example, if the tie point is rejected because the correlation peak is too near the edge of the search image (active flag = 2), the correlation strength, the root mean square error, etc. are not computed and these fields must be ignored.

  3. If an input tie point location file is used and the input search image name does not agree with the image name in the tie point location file header, the user is warned and the name of the input search image is replaced by the image name contained in the header. If an input tie point location file is not given and the search image name from the header of the merged tie point file given does not match IN, the user is warned and the user-entered search image name is used. The search image name is copied to the header of output tie point location file in this case.

  4. An image "chip" is nothing more than a small (typically 64 x 64) subimage centered about a tie point of interest. Three types of subimage chips may be used: reference chips for image-to-image registration, reference chips for geographic registration, and preprocessed search chips.

    To process with reference chips, enter a null ("") reference image name. To process with search chips, enter a null search image name. Each tie point record in the input merged tie point file must have the name of the chip image file or that tie point will not be processed. Tie point coordinates in the merged tie point file are relative to the subimage chip, not the full image. Each type of chip has a different way to reference coordinates back to the full image.

    Reference chips for image-to-image registration must have a valid master line and master sample field in the chip image's DDR to compute full image coordinates of the tie point. Reference chips for geographic registration must have valid projection and location information in the chip image's DDR. Furthermore, this information must match the projection and location information contained in the header of the merged tie point file. Geographic or projection coordinates of the tie point are given in the geo_coord field of the merged tie point file. This coordinate is converted to reference image coordinate space (the reference image might not exist) using the projection and location information. Preprocessed search chips are used when it is not feasible to process an entire search (input) image or when the search and reference images differ greatly in scale or distortions. Search chip images must have valid projection and location information in the DDR file. The "projection" is a user-defined space relating the subimage chip back to the full search image.

  5. The PHASE subcommand performs a filtered phase correlation which is implemented somewhat differently than the GREY and EDGE subcommands. PHASE sometimes gives unreliable results. Tests at EDC show that the GREY and EDGE subcommands give more reliable results for Landsat MSS and TM, AVHRR, and ERS-1 SAR and SEASAT imagery. The PHASE subcommand generally gives good results with small search images, but the "goodness-of-fit" is more difficult to determine.

  6. The THRVAL parameter in the EDGE subcommand should be used with care, as it is used in screening out pixels in both the search and reference images (or chips). For example, if reference chips contain line work with a pixel value of 255 and THRVAL is set to 200, all linework will be masked out and no correlations will be successful.