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An overview of ArcScan

Release 9.3
Last modified April 24, 2009
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Note: This topic was updated for 9.3.1.

The ArcScan extension provides tools and commands that allow you to take scanned images of maps and convert them into vector-based feature layers, such as shapefiles and geodatabase feature classes. The process of converting raster data to vector features is known as vectorization. Vectorization can be performed manually by interactively tracing raster cells or automatically using the automatic mode.
The process of converting raster data into vector features relies on user-defined settings. These settings allow you to influence the geometric composition of the output vector features. Once you have determined the optimal vectorization settings for your data, they can be readily saved and reused.
The ArcScan extension also provides tools that allow you to perform simple raster editing to prepare your raster layers for vectorization. This practice, known as raster preprocessing, can help you eliminate unwanted raster elements that are not in the scope of your vectorization projects.
The interactive vectorization experience, referred to as raster tracing, requires that you trace the raster cells in the map to create vector features. The automated vectorization experience, referred to as automatic vectorization, requires that you generate features for the entire raster based on settings that you specify.

Who should use ArcScan?

Organizations that need to convert raster images into vector-based feature layers are the primary candidates for using the ArcScan extension. Since a large amount of geographic information still exists in the form of hard copy maps, having a tool to integrate these documents into a GIS is crucial. These legacy documents can derive from engineering, survey, and cartographic professionals. ArcScan provides an efficient way to streamline this integration when compared to traditional techiques, such as digitizing.

How does ArcScan work in the ArcGIS environment?

The ArcScan toolset is an add-on component of the ArcGIS Desktop suite. Licensed as a separate extension, ArcScan works within the ArcMap environment and relies on its own user interface, which supports the tools and commands used for the vectorization process.

Getting started with ArcScan

As with other ArcGIS extensions, you must enable the ArcScan extension in ArcMap before you can use it. ArcGIS manages the licenses and notifies you if one is not available. You must also add the ArcScan toolbar to your map to access the tools and commands that support the vectorization work flow. Any map document that is saved with the ArcScan toolbar displayed automatically retains the toolbar the next time the document is opened. Since ArcScan is designed to work with the Editor, you must start an edit session to activate the toolbar. This means that all of the Editor tools and commands can be used in conjunction with the ArcScan tools and commands. ArcScan uses Editor settings, such as the snapping environment, current edit tasks, and target feature layers.
Learn more about the Editor toolbar
View an illustration of the ArcScan toolbar

Symbolizing raster layers for ArcScan

ArcScan can vectorize any raster format supported by ArcGIS so long as it is represented as a bi-level image. This requires that you symbolize raster layers with two unique colors. You can use the ArcMap Unique Value or Classified Rendering option to separate the raster into two colors. Most scanned documents tend to consist of two colors that delineate the foreground and background values. Typically, the foreground is represented as a dark color, such as black, and the background is represented as a light color, such as white. However, these colors can be reversed or represented by different values. As long as the two colors possess unique values, ArcScan supports vectorization for the current foreground raster cells.

Selecting connected cells

ArcScan supports the ability to select connected cells. Connected raster cells are raster pixels that share contiguous borders. This can be in a side-by-side or diagonal arrangement. This functionality allows you to select portions of the raster for various reasons, such as vectorization, export, or removal.

Raster selections can help you focus on the important parts of the raster data while allowing you to isolate parts of the data you are not interested in. Raster selections can be performed interactively by clicking a series of raster cells or by a Structured Query Language expression-based query using the Select Connected Cells dialog box. Both methods allow you to create a new selection set, add the current selection set, or remove something from the current selection set.

Cleaning up the raster data

ArcScan supports tools to perform simple edits on raster layers. One set of these tools is called Raster Cleanup, and it supports its own edit session. Raster cleanup is sometimes essential for raster layers that contain a lot of unwanted cells, commonly referred to as noise. You can also use the Raster Cleanup tools to add new raster cells to the image.

Once you have performed the edits on the raster, you can save them to the current file, export the edited raster to a new file, or perform the vectorization and discard all changes. Used in combination with the raster selection tools, Raster Cleanup is a powerful function that can make your vectorization experience more efficient and, therefore, increase productivity.

Using the raster snapping tools

The ability to snap to raster cells has been introduced with the ArcScan extension. This functionality allows you to accurately trace raster data to create vector features, which can enhance the interactive vectorization experience. The supported raster snap agents include centerlines, corners, intersections, ends, and solids.

ArcScan relies on the current Editor snap tolerance settings for raster snapping tolerance. Other settings include showing snap tips, which can help you distinguish which part of a raster’s connected cell you are snapped to.

Tracing raster cells

Interactive vectorization is characterized as the process of manually tracing raster cells. This can be accomplished using the standard Editor Sketch tools or the ArcScan Vectorization Trace tool. Used in conjunction with raster snapping, raster tracing can be an effective and accurate way to convert raster data to vector features.

Depending on the current edit target layer, you have the option to create line or polygon features. You can also control the geometric composition of the output vector features by adjusting the vectorization settings prior to tracing. Once you have added new features to your database by tracing, you can leverage other tools, such as topology, advanced editing, and spatial adjustment, to modify the data, if necessary.

Automatic vectorization

Automatic vectorization is defined as an automated technique for converting raster data into vector features. This process relies on user input to control how to perform the vectorization. Factors such as image resolution, amount of noise in the image, and the actual content of the scanned document all play a role in determining the outcome of the vectorization.
ArcScan supports two vectorization methods: centerline and outline. Centerline vectorization will generate vector features along the center of the raster linear elements. Outline vectorization will generate vector features at the border of the raster linear elements.
As previously mentioned, the success of a vectorization may be determined by the state of the scanned document at the time of the conversion. It is sometimes necessary to modify the image prior to the generation of features. This process, referred to as raster preprocessing, can help you clean up certain portions of the raster that will help define the scope of the vectorization. The Raster Cleanup tools provide the means to perform these steps. In addition, raster selections can be used in combination with or independently of raster cleanup to isolate the raster cells you want to vectorize.
Besides manipulation of the original raster, the most influential factor in automatic vectorization is the settings. The settings control which cells are vectorized as well as the amount of generalization and smoothing that is applied to the output vector data. You can modify the settings and preview them immediately in the map to see how they affect the vectorization. Once you have determined the appropriate settings, you can vectorize the entire raster layer or a defined area of it. After data creation, you can use other Editor tools, such as topology, advanced editing, and spatial adjustment, to further refine the data, if necessary.

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