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<EM>i.points</EM> is an
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<A HREF="imagery.html">imagery</A> function that enables
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the user to mark points on a (raster) image to be rectified
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and then input the geographic coordinates of each point for
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calculation of a coordinate transformation matrix.
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<EM>i.points</EM> must be followed by use of the GRASS
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program <EM><A HREF="i.rectify.html">i.rectify</A></EM>, which
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rectifies the image using the transformation matrix
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coefficients calculated by <EM>i.points</EM>.
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<EM>Rectification</EM> is the mapping (transformation) of
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an image from one coordinate system to another. The
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geometry of an image extracted into a GRASS LOCATION having
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an x,y coordinate system is not planimetric. To create a
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planimetric image, that is, to convert the x,y coordinate
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system into a standard coordinate system (for example, the
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UTM coordinate system or the State Plane coordinate
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system), points from a map having the standard coordinates
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must be associated with the same points on the image to be
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rectified. <EM>i.points</EM> enables the user to mark
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points on an image and input the standard coordinates for
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that point. <EM>i.points</EM> then calculates a least
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squares regression using the two coordinate systems (x,y
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and standard) for the marked points. A matrix containing
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transformation coefficients is the output file for
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During the process of marking points and entering map
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coordinates, the user can compute the RMS (root mean
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square) error for each point entered. <EM>i.points</EM>
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does this by calculating the transformation equation (the
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same one that is calculated in the GRASS program <EM>
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<A HREF="i.rectify.html">i.rectify</A></EM>), and then
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plugging these results into an equation for RMS error.
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<EM>i.points</EM> offers a zoom option to locate precisely
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the point to be marked on an image. This program also
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offers the user the option of acquiring standard
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coordinates for a marked point from a map layer in the
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<EM><A HREF="i.target.html">i.target</A></EM> must be
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run before running <EM>i.points</EM> to enable the PLOT
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RASTER option to be used and to identify a target data base
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LOCATION_NAME and MAPSET for the rectified image. To run
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<EM>i.points</EM>, a graphics monitor is required.
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The procedure for marking points, entering coordinates, and
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calculating RMS error is described below.
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The first prompt in the program asks the user for the
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<A HREF="i.group.html">imagery group</A> to be
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registered. Note that if
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<EM><A HREF="i.target.html">i.target</A></EM> is not run
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before <EM>i.points</EM>, the <EM>i.points</EM> program
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will display the following error message:
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ERROR: Target information for group [spot] missing
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Please run i.target for group [spot]
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After entering the group to be registered the terminal screen
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The graphics monitor displays the following screen:
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______________________________________________________________
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| imagery filename (mag) | target filename (mag) |
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|_________________________________|___________________________|
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|_________________________________|___________________________|
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|_________________________________|___________________________|
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| QUIT ZOOM PLOT RASTER ANALYZE| |
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|_________________________________|___________________________|
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A pop-down menu like that shown below will be superimposed on the
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left half of the screen:
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__________________________________
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| Double click on raster map layer|
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| Double click here to cancel |
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|_________________________________|
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|___________________|
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|__________|________|
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|__________|________|
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|__________|________|
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Any single raster map layer in the
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<A HREF="i.group.html">imagery group</A> may be used on
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which to mark points, and the user can mark points on more
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than one raster map layer in the
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<A HREF="i.group.html">imagery group</A> to accumulate the
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suggested minimum number of 12 points. Any raster map
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layer in the <A HREF="i.group.html">imagery group</A>
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can be rectified (using
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<EM><A HREF="i.rectify.html">i.rectify</A></EM>) based on the
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transformation matrix computed from these points.
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The imagery file chosen by
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the user is displayed in the upper left quadrant of the
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To magnify the displayed file, the user must
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place the mouse cross hairs on the word ZOOM. The following menu
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will then be displayed at the bottom of the screen:
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____________________________________________
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| Cancel| Box| Point| Select type of ZOOM|
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|_______|_____|_______|_____________________|
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The user has the option of identifying the zoom region
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either by using the mouse to make a box, or by using the
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mouse to mark the two diagonal points of the desired
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region. The terminal screen will display a mouse button
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menu to guide the user in identifying the corner points of
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<H3>MARKING POINTS</H3>
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To mark the points on the image that correspond to the
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points on a standard coordinate system map, the user must
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place the mouse cross hairs on the corresponding location
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on the image to be marked and press the left hand button on
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the mouse. A diamond shaped symbol will be marked on the
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image. The user's terminal will display the following
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___________________________________________________________
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| Point 1 marked on the image at| |
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|__________________________________________________________|
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| Enter coordinates as east north: |
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|__________________________________________________________|
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The user then enters the easting and northing (separated by
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a space) for the point marked on the image. If the user
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wishes not to enter a coordinate, he or she may simply hit
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RETURN to return control to the mouse; the marked point
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In addition to acquiring reference points from a standard
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map, the user has the option of acquiring the reference
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points from a raster map layer in the target data base
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LOCATION_NAME. The data base raster map layer is displayed
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by placing the mouse cross hairs on the words PLOT RASTER.
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The following line is then displayed at the bottom of the
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________________________________________________
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| Cancel| Indicate which side should be plotted|
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|_______|_______________________________________|
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Which side of the graphics monitor is to be plotted
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is indicated by placing the mouse cross hairs on the
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half of the graphics monitor screen that the user would like to use,
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and pressing the left mouse button. The following pop-down
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menu will be superimposed on the half of the screen that was chosen:
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_________________________________________
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| Double click on raster (cell) map layer|
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| Double click here to cancel |
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|________________________________________|
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______________________________
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|_____________________________|
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|______________|______________|
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|_____________________________|
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|_____________________________|
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| elevation | geology |
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|______________|______________|
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|______________|______________|
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|______________|______________|
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|______________|______________|
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After the raster map layer is displayed the following message appears
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at the bottom of the graphics monitor:
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______________________________________
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| input method -->| keyboard| screen|
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|_________________|__________|________|
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If the user wishes to use the plotted raster map layer only
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as a comparative reference, then the keyboard can be chosen
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as the means to input coordinates corresponding to the
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marked points on the image. This is done by placing the
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mouse cross hairs on the word KEYBOARD and pressing the
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left button on the mouse.
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If the user selects the SCREEN option, then points marked
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on the image will automatically be associated with the
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coordinates from the corresponding points on the target
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data base map layer. In this option, when the user marks a
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point on the image, the following menu is displayed at the
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___________________________________________________________
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| Point 5 marked on the image at| |
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| Point located at | |
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| East: 679132.57 | |
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| North: 4351080.67 | |
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|__________________________________________________________|
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|__________________________________________________________|
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The user then uses the mouse to mark a corresponding point
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on the displayed image from the target data base. The
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coordinates for the target data base map layer are
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automatically saved as the coordinates corresponding to the
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marked point on the image.
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After a number of points have been marked (4 to 7), the
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user can check the RMS error of the points marked on the
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image. This is done by placing the mouse cross hairs on
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the word ANALYZE at the bottom of the graphics monitor. An
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error report resembling that shown below is superimposed on
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the graphics monitor:
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______________________________________________________________________
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| error image target |
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|# row col target east north east north |
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|____________________________________________________________________|
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|1 0.0 -0.9 1.0 1048.5 -144.8 679132.5 4351080.6|
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|2 0.4 1.0 1.3 2153.1 -567.2 684314.7 4399001.4|
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|3 -1.2 -0.5 .6 1452.8 -476.5 567841.4 3457682.8|
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|4 1.1 0.5 1.3 1034.0 -109.2 677573.8 4352626.4|
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|5 -2.7 14.0 14.2 1048.6 -144.9 679132.6 4351080.7|
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|____________________________________________________________________|
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| overall rms error: 4.46 |
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|____________________________________________________________________|
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The following menu then appears at the bottom of the graphics monitor:
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____________________________________________________________________
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| DONE| PRINT FILE| Double click on point to be included/excluded|
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|_____|____________|________________________________________________|
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The RMS error for the image is given under the column
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TITLEd "error" and subTITLEd "row" and "col". In the above
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report, point number 1 is 0.0 rows and -0.9 columns from
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the predicted location calculated from the transformation
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equation. The RMS error for the target raster map layer is
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listed under the heading "target". This is the RMS error
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for the east and north coordinates of the target map layer,
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but it is presented in the table using one general value.
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The overall RMS error is displayed at the bottom of the
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screen in meters. Points that create high RMS error are
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displayed in red on the graphics monitor (represented here
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The location of the point marked on the
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<A HREF="i.group.html">imagery group</A> file is given
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under the heading "image" and the subheadings "east" and
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"north". The location of the point in the target data base
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is given under the heading "target" and the subheadings
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"east" and "north". If the user would like to exclude or
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include a point, this can be accomplished by placing the
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mouse cross hairs on the point number to be included (if
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the point is absent) or excluded (if the point is
364
displayed) and pressing the left button on the mouse
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twice. When a point is excluded, it is not afterwards
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included in the calculation of the RMS error, or included
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in the final transformation matrix. However, it can be
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retrieved within <EM>i.points</EM> at any time by double
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clicking with the mouse as described above.
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To end the <EM>i.points</EM> program place the mouse cross
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hairs on the word QUIT; the marked points (including
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coordinates) will be saved.
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A good rule of thumb is to mark at least 12 to 15 points
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which are evenly distributed over the entire
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<A HREF="i.group.html">imagery group</A> file in order to
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obtain an accurate transformation equation for the
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rectification process. The RMS error may increase with
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more points added, but the transformation equation will be
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An RMS error of less than or equal to approximately one
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resolution unit (pixel or cell) for the image being
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rectified is generally considered acceptable.
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In order to use a digitizer with <EM>i.points</EM>, at
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least one digitizer driver besides "none" (the on-screen
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digitizer) must be available in the digitcap file.
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This program is interactive.
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<A HREF="http://grass.itc.it/gdp/imagery/grass4_image_processing.pdf">Image
406
Processing manual</A></EM>
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<A HREF="g.mapsets.html">g.mapsets</A><br>
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<A HREF="i.group.html">i.group</A><br>
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<A HREF="i.rectify.html">i.rectify</A><br>
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<A HREF="i.target.html">i.target</A><br>
414
<A HREF="i.vpoints.html">i.vpoints</A>
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<em><a href="gm_georect.html">gis.m: GEORECTIFY TOOL</a></em>
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U.S.Army Construction Engineering Research Laboratory
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<i>Last changed: $Date: 2007-10-29 12:12:37 +0100 (Mon, 29 Oct 2007) $</i></p>