Vectorization in ArcScan

Part 1: Raster Tracing

 

About ArcScan

ArcScan is an ArcGIS extension to convert raster images to vector-based GIS layers. This process can be performed interactively or in an automated fashion.

In this part we will learn how to set up the raster snapping options and environment, snap to raster cells, and trace raster cells to create line and polygon features.
In part two, we will edit the raster layer to remove unwanted cells, apply vectorization settings, preview the vectorization, and generate features using the batch vectorization mode.

Project Setup

In this exercise, we will generate features from a scanned parcel map by interactively tracing raster cells.

1. Download the ArcScan.zip to your folder in MyDocuments and unzip it.
2. Open the ArcScanTrace.mxd.
3. Open the ArcScan toolbar by checking View > Toolbars > ArcScan.

Changing the raster layer symbology

Raster layers must be symbolized as bi-level images to use the ArcScan tools and commands. You will change the raster symbology from stretched to unique values.

1. Open the layer Properties of the raster image.
2. Click the Symbology tab and select Unique Values. Click OK.

Setting a bookmark to locate the trace area

Spatial bookmarks are named extents that can be saved in map documents. Creating a bookmark is useful if you have to zoom in to areas of frequent usage. We will create a new bookmark to zoom into the area we are first editing.

1. Zoom in to show this area:


2. Click View > Bookmarks > Create.
3. Name the bookmark Trace lines.
4. Now click the Full extent button to zoom out. Go back to your bookmark by choosing View > Bookmarks > Trace line.

Tracing a line

The ArcScan extension is only active in an edit session. That is why we need to start editing.

1. Open the Editor toolbar and Start Editing.
2. Raster snapping requires settings that inflence the behavior of the tracing. These options are set in the Raster Snapping Options dialog box. Click the Raster Snapping Options button on the ArcScan toolbar to open the Raster Snapping dialog box.
3. Set the maximum line width value to 7. This setting will ensure that you are able to snap to raster cells that represent the lot boundaries. Click OK.
4. Now, we also need to set the snapping options in the Editor toolbox. Click the Editor menu and click Snapping to open the Snapping Environment dialog box.
5. Click the plus sign next to Raster to expand it.
6. Check the Centerlines and Intersection options for raster snapping.
7. Now that you have set up your raster snapping environment, you are ready to begin tracing the raster cells. You will use the Vectorization Trace tool for this step. Select the Vectorization Trace button on the ArcScan toolbar.
8. Move the pointer until it snaps to the intersection of the lot boundaries and click to start tracing. Point the Vectorization Trace tool downward and click to start creating the line feature. Continue to point and click around the lots until you get to your starting point.
9. Right-click to finish the polygon or click F2.

Creating polygon features by tracing raster cells

Now that you have successfully traced raster cells to create line features, you will create polygon features using the Vectorization Trace tool.

1. Go to the Trace polygons bookmark by clicking View > Bookmarks > Trace polygons.
2. Click on the Vectorization Trace tool.
3. Move the pointer until it snaps to the lower-left corner of lot 061 and click to start tracing.
4. Trace the lot boundary in a counterclockwise direction. Click F2 to complete the polygon.
5. Stop Editing and save your edits.

In this exercise you learned how to set the raster snapping options and environment, snap to raster cells, and trace raster cells to create new line and polygon features. These steps covered the main components of the raster tracing process. In part 2, we will show you how to edit a raster layer and automatically generate features for an entire layer using the batch vectorization tool.

Creating a Layout

Digitize some more lots with the tools introduced above and create a layout of the vectorized areas. Add a title, your name, and other common items you think are appropriate. Export it as Trace.pdf and submit it to your Webfolder.


Part 2: Batch Vectorization

 

In this part, we will edit a scanned parcel map to remove cells from the raster that are not in the scope of the vectorization. Once the raster has been cleaned up, you will generate features using the batch vectorization mode.

1. Open ArcScanBatch.mxd from your directory.
2. Change the symbology of the ParcelScan.img raster to bi-level (Properties > Symbology > Unique Values).

Cleaning up the raster for vectorization

1. Go to the Raster cleanup bookmark (View > Bookmarks > Raster cleanup).
2. Start Editing in order to make changes to the ParcelScan.img layer.

When performing batch vectorization, it is sometimes necessary to edit the raster image prior to generating features. This process is referred to as raster cleanup and involves the removal of unwanted cells from the raster image that are not in the scope of the vectorization. ArcScan provides the tools to perform raster cleanup. You will now use these Raster Cleanup tools to remove unwanted text from the ParcelScan image.

3. Click the Raster Cleanup menu and click Start Cleanup to start the raster cleanup session.
4. Click the Raster Cleanup menu and click Raster Painting Toolbar to display the Raster Painting Toolbar.
5. Click the Erase tool located on the Raster Painting toolbar.
6. Erase the text located at the top of the parcel lot 001.
7. In addition to the Erase tool, the Raster Painting toolbar supports another tool designed to erase cells. This tool is called the Magic Erase tool, and it allows you to erase a series of connected cells by simply clicking or dragging a box around them.
8. Click the Magic Erase tool located on the Raster Painting toolbar.
9. Drag a box around the text located in the center of the parcel lot 001 to remove it.
10. Remove the rest of the text parcel lot 001 with either the Erase tool or the Magic Erase tool.

Using the cell selection tools to assist with raster cleanup

In the previous steps, you learned how to use the Erase and magic Erase tools to remove unwanted cells from the raster image. However, if the image you are working with requires much cleanup, these techniques could be time consuming. To help streamline this process, you can use the cell selection tools in conjunction with the raster cleanup tools.

1. Zoom to the Cell selection Bookmark (View > Bookmarks > Cell selection).
2. Click the Cell Selection menu and click Select Connected Cells.
3. In the Select connected cells dialog box, enter a value of 500 for the total area of raster pixels. This expression will select all the cells that represent text in the raster. Click OK.
4. Click the Raster Cleanup menu and click Erase Selected Cells to delete the selected cells.

Using the vectorization settings

Batch vectorization relies on user-defined settings. These settings influence the geometry of the generated features. These settings may vary depending on the type of raster data you are working with. Once you have determined the appropriate settings for your raster, you can save them within the map document or to a separate file. You will use the Vectorization Settings dialog box to apply the settings.

1. Click the Vectorization menu and click Vectorization Settings to open the Vectorization Settings dialog box.
2. We will now modify the vectorization settings to ensure optimal results when generating features.
3. Change the Maximum Line Width value to 10.
4. Change the Compression Tolerance value to 0.1.
5. Click Apply to update the settings. Click Close.

Previewing the vectorization

ArcScan provides a way to preview the batch vectorization prior to generating features. This can help you save time by allowing you to see how the settings will affect the vectorization. When the settings are changed, the preview can be updated by clicking the Apply button located on the Vectorization Settings dialog box. This design allows you to fine-tune the vectorization settings.

1. Click the Vectorization menu and click Show Preview to display the vectorization preview. The cells that represent the text are now selected.
2. Click the Raster Cleanup menu and click Erase Selected Cells to delete the selected cells.

Generating features

Batch vectorization relies on user-defined settings. These settings influence the geometry of the generated features. These settings may vary depending on the type of raster data you are working with. Once you have determined the appropriate settings for your raster, you can save them within the map document or to a separate file. You will use the Vectorization Settings dialog box to apply the settings.

1. Click the Vectorization menu and click Vectorization Settings to open the Vectorization Settings dialog box.
2. The final step in the batch vectorization process is to generate features. The Generate Features dialog box allows you to select the vector layers that will store the new features and execute the vectorization.
3. Click the Vectorization menu and click Generate Features.
4. Choose the ParcelLinesBatch layer. Click OK.
5. Right-click the ParcelScan.img raster layer in the table of contents and click Zoom To Layer to view all of the new features that were generated.
6. Stop editing and save your edits.

Creating a layout

Create a layout of the vectorized areas. Add a title, your name, and other common items you think are appropriate. Export it as Batch.pdf and submit it to your Webfolder.

 


Lab created by Eva Grund, March 2005, based on ESRI's ArcGIS 9 - Using ArcScan for ArcGIS manual, Chapter 2.