User's Guide

MTPGRID

Creates a grid of tie points at user-specified intervals for correlation.

Function:

Creates a grid of tie points at user-specified intervals. This grid may then be supplied to CORRELATE to perform image registration refinement.

Parameters:

OUTMT

Output merged tie point file. This file will contain the generated tie point grid which can then be supplied to CORRELATE.

NL

Number of lines. Defines the space in the Y direction over which an image is to be correlated using the resulting tie point grid file.

NS

Number of samples. Defines the space in the X direction over which an image is to be correlated using the resulting tie point grid file.

LINEINC

Line increment. The spacing of the tie points in lines.

SAMPINC

Sample increment. The spacing of the tie points in samples.

PTID(GTP)

Point id prefix. The string prefix portion of the tie point ID. This string should consist only of alphabetic characters. The point ID consists of this string followed by an underscore followed by a unique number which distinguishes this tie point.

REFSIZ(32,32)

Reference size. The desired height and width of the reference image sub-area to be used by CORRELATE. Allowable range of values is 32 through 128.

SRCHOFF(16,16)

Search offsets. The desired horizontal and vertical search offsets to be used by CORRELATE. The alignments to be searched for a correlation maximum extend equal distances, to the left and right, as specified by the horizontal search offset parameter. Allowable range of values is 8 through 64. These values should typically be less than those specified for REFSIZ.

Example:

1. LAS> mtpgrid outmt=outgrid nl=512 ns=512 lineinc=4 sampinc=2

   Creates a Merged Tie Point file, using a sample increment of 2 and a line increment of 4 for the tie point spacing. In this case, the height and width of the reference image subarea defaults to (32,32). Similarly, the horizontal and vertical search offsets (SRCHOFF) default to (16,16). This run of MTPGRID will create the points located at:

   	(4, 2),  (4, 4),  (4, 6),   ... (4, 512)
   	(8, 2),  (8, 4),  (8, 6),   ... (8, 512)
   	  .        .        .        .    .
   	  .        .        .        .    .
   	  .        .        .        .    .
   	(512,2), (512,4), (512,6),  ... (512,512)
   

   When CORRELATE is given this tie point file, it will extract image subwindows at these locations.

Description/Algorithm:

MTPGRID is used mainly in image-to-image registrations as a preprocessing step for CORRELATE, which works with image subwindows. MTPGRID is used to generate a regular "grid" of tie points which CORRELATE can use to extract these subwindows.

Once OUTMT is opened successfully and all pertinent header information is assigned, the information is written to the MTP file, starting at (LINEINC,SAMPINC) and processing every (LINEINC,SAMPINC) pixel until NL and NS are reached.

Once the tie points are written to the MTP file for all pixels to the extent specified by NL and NS, the file is closed and processing ends.

Nonfatal Error Messages:

None.

Fatal Error Messages:

1. [mtpgrid-fatal] Fatal error encountered

   The error message that is displayed immediately preceding this message is the specific error encountered. Processing is terminated.

User Notes:

1. The output MTP file generated by MTPGRID is intended to be supplied to CORRELATE.

2. In an MTP file, there are two sets of points: the reference coordinates (true) and nominal coordinates (best guess). The nominal coordinates are refined by CORRELATE. MTPGRID sets the two equal. The initial assumption is that the two images being matched are registered, or very close to being registered.

3. MTPGRID and CORRELATE can be used "as is" for image-to-image verifies, or for image-to-image registrations when the systematic models are very good, that is, displacements are translational only and these displacements are less than (SRCHOFF / 2) - 2. If this is not the case, the Pyramid routines (GPYRAMID and EXPYRAMID) may be needed to reduce this translational displacement.