Heads-Up Digitizing - 3

The following is a listing of how to do this using ARC/INFO. All commands for ARC/INFO will be printed in bold here. In this example, an image will be registered using information found in a coverage or the same extent. This image will be rectified using the information from the registration process.

  1. To register the image, use the command register with the coverage containing suitable Ground Control Points (GCP).

    2. Arc: register

    Usage: REGISTER <image> {cover} {cover_color} {band | COMPOSITE} {red_band}

    {green_band} {blue_band} {ARC | POINT | TIC | ALL}

    Arc: register campus.tif campus_cov 4

  2. Align a similar area in both the Coverage and Image windows. Select "View:Redraw Overlay Campus" to draw both maps in the Overlay window.
  3. Using the mouse as input, select a GCP on the image that may be found in the coverage. Click on the location in the image first, then the corresponding location on the coverage in the Overlay Window. ARC/INFO refers to this as a link.
  4. After establishing several links, the output may look something like this:

    5.  

  5. After creating 4 or 5 links, you should have enough information to lock the image together. To do this, Select "View:Link Actions".
  6. Select the Register button, and then the Redraw Overlay button with the Lock Image box checked. The window that pops up looks like this:
  7. In this Registration window, you can see that there are 10 links created. The pixels will need to be scaled at roughly 0.5 in both the X and Y directions, and the overall error is 5.1 pixels, or 2.58 feet.
  8. When the Overlay is redrawn, the lines of the coverage might line up better with the image better. At this stage, you must created several more links across the image.
  9. The image will be accurately registered if you choose GCP all across the image, and not just in a particular area. Sometimes, due to GCP restrictions with the coverage, you may not be able to select in more than a few locations on the image. Be warned: the overall RMS error is valid only in the areas that you have created links with, the error will be higher in areas without GCP links.
  10. At any time, you may choose to delete a link that may be unusually high, but be careful. If care is taken during this phase, then all links are probably accurately placed. You may decrease the reported RMS error by deleting links but the RMS error may then not be entirely accurate. Discretion must be used when deciding which links were misplaced and which are in the proper location.
  11. There should be at least 25 links for an image. Any less may yield an inaccurate RMS error value. Having more than 40 or 50 links may provide redundant data and not provide new information.
  12. This process is repeated until you have enough links to create an acceptable RMS error. Be sure to select the "Save Transformation" button. This creates a world file for the image. To look at this file:

    13. Arc: type campus.tfw

    Submitting command to Operating System ...

    0.50053464870430

    -0.01152274964816

    -0.01100095996915

    -0.50081913471566

    392148.90499453183000

    525587.55640567141000

     

  13. This 6 line file describes how the image should be scaled, rotated and translated through rectification.
  14. To rectify this image, use the RECTIFY command:

    15. Arc: rectify

    Usage: RECTIFY <in_image> <out_image> {NEAREST | BILINEAR | CUBIC}

    {DEFAULT | COLORMAP | NONE} {clip_cover | BOX} {xmin} {ymin}

    {xmax} {ymax}

    Arc: rectify campus.tif rect-campus.tif bilinear

    Rectifying...

    Arc:

  15. The new "rect-campus.tif" image is ready for on-screen digitizing.

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