How To Make An ArcInfo Vector Coverage Using a Scanned Image and ArcScan

The following represents the core ArcScan procedures for making a vector coverage from a scanned image such as a TIF file. Because of the complexity of this operation, you may want to refer to the help documentation while using ArcScan. A functional list of ArcScan commands can be found by typing "arcdoc" at the Unix prompt. Once the ArcInfo Help document arrives, sequentially select:

         Contents
            Data automation
               ArcScan
                  Command references
                     Functional list of ArcScan commands

Within the final selection choose GridEdit commands for raster editing
to learn how to set up the background grid. Choose Vtrace commands for raster-to-vector conversion
to learn how to control the tracing environment.

Basically, the ArcScan line-vectorization procedure consists of:

displaying a grid that represents your scanned image,
defining the editing environment parameters that will best facilitate vectorization,
creating an ArcInfo line coverage that will receive your traced lines,
manually and semi-automatically tracing the grid lines that appear within your gridded image, then
saving your traced lines to the ArcInfo line coverage.

You do this tracing operation by moving back-and-forth, as necessary, between an interactive tracing mode and a manual tracing mode. In the interactive mode, ArcScan traces out a line along the designated line within your grid until it arrives at a point of ambiguity (usually where a decision has to be made by you as to which direction vectorization should continue). ArcScan then waits until you (sort of) nudge it along by indicating which direction ArcScan should proceed. In the manual mode you directly enter vertices along the designated line on the image. Use the manual mode in situations where there are many conditions that may cause ambiguity to the interactive mode.

Once a line is traced and stored in the line coverage, ArcScan smooths out the line to remove notches that develop during the tracing operation. (The degree of smoothing is controlled by several of the tracing parameters listed below.)

A third mode exists for creating traced lines: the automatic tracing mode. This mode is recommended for only the cleanest grids where there is little chance of ambiguity along a line. In this mode, ArcScan process all the lines within the gridded image without your intervention. Refer to the help documents, described above, for help on the automatic mode. The following discussion is limited to the procedures for using the interactive and the manual tracing modes.

Follow this general procedure for vectorizing lines on the grids.

Starting with a scanned image of lines (e.g., geologic contacts, faults, or topography) convert the image to a grid in ArcInfo:
Arc: imagegrid <in_image_name> <my_grid>
(In this tutorial replace <in_image_name>, <my_grid> and <my_cover> with the names of your image, grid and coverage, respectively.)
Once the grid is generated, activate ARCEDIT and set up the initial editing environment.
Arcedit: disp 9999 1 Arcedit: mapextent <my_grid> Arcedit: ge edit <my_grid> Arcedit: ge colors opaque grey white Arcedit: draw

(Note: If you are going to register your resultant vector data to real-world coordinates, e.g., UTM or State Plane projection, then the image needs to have at least four tics where the ground coordinates - such as longitude/latitude - can be chosen as the points to enter tics.)
Create a new coverage using the tics displayed on the grid.
Arcedit: create <my_cover>
You will be prompted to enter tics for my_cover:
Creating MY_COVER Digitize a minimum of 4 tics. Signal end of tic input with Tic-ID = 0 Tic-ID:
1. Zoom in to the location on the grid where the #1 tic resides,
2. Type "1" in the text window and press the Enter.
3. Position the cross hairs over the #1 tic and press the left mouse button.
4. Repeat this for operation each of the other 3 tics, typing in sequence 2, 3, 4 for tics 2 through 4.
5. After the fourth tic is entered, type 0 and press the Enter key.
The screen will look like this:
```
         Tic-ID: 1
         Tic-ID: 2
         Tic-ID: 3
         Tic-ID: 4
         Tic-ID: 0
      
```
Next you will be prompted with the following:
Enter initial boundary Define the box
Position the cross hairs just outside the lower-left corner of the grid and press the left mouse button. Then position the cross hairs just outside the upper-right corner of the grid and press the left mouse button. This operations defines the area of interest.
Set up the vectorizing environment within ARCEDIT. You can issue the following commands to get this process going. (To ease your typing pain, you can view the page source of this HTML file, select and copy these commands from the source, then paste the commands to an ArcInfo AML file which you can then run within ARCEDIT.)
Arcedit: vt raster <my_grid> Arcedit: vt arrowlength 32 Arcedit: vt arrowwidth 3 Arcedit: vt backtrack no Arcedit: vt branch straight Arcedit: vt dash 0.040 Arcedit: vt eol manual Arcedit: vt eos edit Arcedit: vt fan 10 30 Arcedit: vt fg 1 Arcedit: vt gap 0.03 Arcedit: vt gtol 0.004 Arcedit: vt hole 0.004 Arcedit: vt js medium Arcedit: vt ls 3 Arcedit: vt lv 1.1 Arcedit: vt lw 0.101 Arcedit: vt ms 0 Arcedit: vt mode center Arcedit: vt retrace yes Arcedit: vt sr 0.030 Arcedit: vt skip 0 Arcedit: vt sta 15.000 Arcedit: vt stc 0.000 Arcedit: vt std 0.020 Arcedit: vt str 0.060 Arcedit: ef arc Arcedit: de arc Arcedit: de node dangle Arcedit: sds 2 Arcedit: nodesnap closest,0.03 Arcedit: draw
Once you set up the tracing environment you will be ready to start tracing. As you trace lines you will need to pan around and zoom in and out of the areas of interest on the grid. You can use the Pan/Zoom buttons on the upper left corner of the ARCEDIT display window to zoom in close to a line that you will start tracing. As you are tracing you can move along the line using the Pan option (the CTRL-A key is a shortcut). Once you select the pan option move the mouse pointer into the display window. The mouse pointer changes to cross hairs. Position the cross hairs near the start of a line that you want to trace, and press the left mouse button. The screen will pan to the desired location.
To start tracing lines type the following:
Arcedit: vt add

Use the 1, 2, 3, 4 & 9 keys to do the following:
Key 1: Initiate manual tracing.
Key 2: Change the direction of automatic tracing.
Key 3: Initiate automatic tracing.
Key 4: Start a new tracing point.
Key 9: Terminate tracing along the designated line
For keys 1, 2, and 3 you can use the left, middle and right mouse buttons.
In auto-tracing mode ArcScan will seek out a path along the designated feature and continue tracing until an ambivalent condition arises. If you don't like the direction the autotracing is headed, you can force ArcScan to change directions using the #2 mouse button.
As you add vertices occasionally save your work.
```
         Arcedit: save
      
```
When finished adding arcs to the trace coverage, save your work and exit ARCEDIT.
Arcedit: save Arcedit: q

Your new coverage, my_cover, would then contain the vectorized arcs. You can proceed to project or transform the coverage into real-world coordinates using the tics established in step 3, above.

Potential problems with starting VT Add

If, when attempting to use "vt add", you get the following error:
Unable to locate the starting point
this error could indicate that the current value of the VTrace ForeGround value is not consistent with the value associated with the cells in your grid which represent lines. For instance the grid cells may actually have values of 255 for lines but the current VTrace Foreground value is 1. To correct this do the following:

Quit out of ARCEDIT, then call GRID from the Arc: prompt:

         Arc: grid

Within the GRID program do the following:
Grid: mape <my_grid> Grid: image <my_grid> Grid: cellvalue <my_grid> *

Point to and click on one of the grid cells representing a line. GRID will return the value of the cells that represent lines (such as 255).

Return to ARCEDIT, as above, and type the following (for a foreground value of 255):

         Arcedit: vt fg 255

11/08/99