ArcScan provides tools that allow you to convert scanned images into vector-based feature layers. 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 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.
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 be derived from engineering, survey, and cartographic professionals. ArcScan provides an efficient way to streamline this integration when compared to traditional techniques, such as digitizing.
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.
Getting started with ArcScan
ArcScan works within the ArcMap environment and relies on its own user interface, which supports the tools and commands used for the vectorization process. As with other ArcGIS extensions, you must enable the ArcScan extension in ArcMap before you can use it. You must also add the ArcScan toolbar to your map to access the tools and commands that support the vectorization workflow.
Since ArcScan is designed to work with editing, you must start an edit session to activate the toolbar. This means that all the editing tools and commands can be used in conjunction with the ArcScan tools and commands. ArcScan uses editing settings, such as the snapping environment and target templates and layers.
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 either the ArcMap Unique Values or Classified rendering options 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.
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.
Interactive vectorization (raster tracing)
Raster tracing is useful in cases in which you need to have more control over the vectorization process or need to vectorize a small portion of an image. This process is known as interactive vectorization and is similar to the existing techniques used to create features while editing. Interactive vectorization consists of several components: raster snapping, raster tracing, and shape recognition.
Raster snapping
The ArcScan extension supports the ability to snap to raster cells. Although not required for raster tracing, raster snapping can help ensure that you create features accurately. You can snap to raster centerlines, intersections, corners, ends, and solids.
Raster tracing
The Vectorization Trace tool allows you to manually trace raster cells and generate either line or polygon features. Used in conjunction with raster snapping, raster tracing can be an effective and accurate way to convert raster data to vector 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.
The image below shows centerline tracing used to create line features.
With the Vectorization Trace tool, you simply point the cursor in the direction you wish to vectorize and click. With each click, features will be generated at the centerline of the raster cells. The current vectorization settings will influence the output vector geometry. You have the option to generate line or polygon features.
The image below shows raster tracing and snapping used to create polygon features.
Shape recognition
You can also capture vector features of a certain shape, such as a building or storage tank, via the shape recognition tools. This allows you to generate features simply by clicking once on the raster feature you wish to capture.
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.
Automatic vectorization requires settings that influence how the output vector features are generated. These settings, also known as styles, can be saved and reused with raster images that possess similar characteristics.
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 editing tools, such as topology, advanced editing, and spatial adjustment, to further refine the data, if necessary.
Selecting raster cells
ArcScan supports the ability to select raster 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. You can create raster selections interactively by clicking a series of connected cells or by executing an expression-based query. These selection tools can help you define the scope of your vectorization.
The Select connected cells dialog box allows you to perform complex cell selection based on pixel area and envelope extents.
Cleaning up rasters before vectorization
ArcScan also supports tools for editing raster images. You can draw, fill, and erase raster cells all within an ArcMap edit session. These steps, known as raster cleanup, allow you to eliminate raster cells that are not in the scope of the vectorization. Additionally, you can export the modified raster to a new file in case you need to preserve the original copy.
The graphics below show an image prior to and after cleanup.
