MAP DIGITIZATION PROCESS IN GIS

MAP DIGITIZATION PROCESS

Digitizing is the process of converting features on a paper map into digital format. To digitize a map, you use a digitizing tablet (also known as a digitizer) connected to your computer to trace over the features that interest you. The x,y coordinates of these features are automatically recorded and stored as spatial data. (Davis, 2001)

Digitizing with a digitizing tablet offers another way, besides screen digitizing freehand, to create and edit spatial data. You can convert features from almost any paper map into digital features. You can use a digitizer in conjunction with the editing tools in ArcMap to create new features or edit existing features on a digital map (Murayama, 2012).

You might want to digitize features into a new layer and add the layer to an existing map document or create a completely new set of layers for an area for which no digital data is available. You can also use a digitizer to update an existing layer on your digital map.

 Sample table digitizer

THE FOLLOWING ARE THE PROCESSES/STEPS FOR MAP DIGITIZATION

Step 1: Set up your digitizing tablet and install the driver software

To use a digitizing tablet with ArcMap, it must have WinTab-compliant digitizer driver software. To find out if a WinTab-compliant driver is available for your digitizer, see the documentation that came with the tablet or contact the manufacturer.

If you installed ArcGIS before installing your digitizer, the Digitizer tab may not appear in the Editing Options dialog box. To add the tab, you must register the digitizer.dll file using the ArcGIS ESRIRegAsm.exe utility. You need to have administrator privileges to perform these steps.

Tip:

If you have installed the ArcGIS ArcObjects Software Development Kit, you can simply browse to the directory containing the digitizer.dll, right-click it, then register it from the shortcut menu.

1.     Close any open ArcGIS applications.

2.     Start the DOS Command Prompt, which is usually accessed by clicking Start, pointing to Programs (or All Programs), then clicking Accessories.

3.     In the Command Prompt window, type cd and a space, followed by the path to the directory containing the ESRIRegAsm.exe utility: C:\Program Files\Common Files\ArcGIS\bin. This changes the Command Prompt's active directory to the folder where the ESRIRegAsm.exe utility is installed.

4.     Press the ENTER key.

5.     Type ESRIRegAsm.exe, a space, a quotation mark, the full path to your ArcGIS installation location with the name of the DLL, and a closing quotation mark. The default path is "C:\Program Files\ArcGIS\Desktop10.1\bin\digitizer.dll". If you installed ArcGIS in another directory, substitute that path.

6.     Press the ENTER key.

7.     If the registration was successful, close the Command Prompt window. The Editing Options dialog box will have the Digitizer tab when you restart ArcMap.

 A specialist processing map data for digitizing

Step 2: Configure the digitizer puck buttons

After installing the driver software, use the WinTab Manager setup program to configure the buttons on your digitizer puck (you might have to turn on your digitizer and reboot your machine before you can use the setup program). One puck button should be configured to perform a single click to digitize point features and vertices; another button should be configured to perform a double-click to finish digitizing line or polygon features. You might also want to configure a button to perform a right-click so you can access shortcut menus.

With any development programming language, you can configure additional buttons to run specific ArcMap commands.

Step 3: Ensure the quality of your paper map

Your map should ideally be reliable, up-to-date, flat, and not torn or folded. Paper expands or shrinks according to the weather. To minimize distortion in digitizing, experienced digitizers often copy paper maps to a more stable material, such as Mylar.

Step 4: Establish control points on the paper map

Before you can begin digitizing from your paper map, you must first establish control points that you will later use to register the map to the geographic space in ArcMap. If your map has a grid or a set of known ground points, you can use these as your control points. If not, you should choose between four and 10 distinctive locations, such as road intersections, and mark them on your map with a pencil. Give each location a unique number, and write down its actual ground coordinates.

Once you have identified at least four well-placed control points, place your map on the tablet and attach it with special residue-free putty; masking tape; or drafting tape, which looks like masking tape but leaves less residue when it's removed. You do not have to align the map precisely on your tablet; ArcMap corrects any alignment problems when you register the map and displays such adjustments in the error report.

Step 5: Register the paper map

After identifying your control points, you must register your paper map in real-world coordinates. This allows you to digitize features directly in geographic space.

Registering your map involves recording the ground coordinates for the control points you identified. These are recorded using the Digitizer tab of the Editing Options dialog box.

After you have entered the ground coordinates, ArcMap displays an error report. The error report includes two error calculations: a point-by-point error and a root mean square (RMS) error. The point-by-point error represents the distance deviation between the transformation of each input control point and the corresponding point in map coordinates. The RMS error is an average of those deviations.

ArcMap reports the point-by-point error in current map units. The RMS error is reported in both current map units and digitizer inches. If the RMS error is too high, you can reregister the appropriate control points. To maintain highly accurate data, your RMS error should be less than 0.004 digitizer units (often inches) or the equivalent scaled distance in map units—the ground units in which the coordinates are stored. For less accurate data, the value can be as high as 0.008 digitizer units.

Step 6: Set the correct projection

If you know what coordinate system (projection) your paper map is in, you should set the same projection for the layer into which you're digitizing. If you are digitizing features into an existing feature layer, you must ensure that your paper map and digital layer share the same coordinate system.

Step 7: Enable digitizing mode and begin digitizing

To digitize features, you need to enable digitizing mode to create features.

Digitizing tablets generally operate in two modes: digitizing (absolute) mode and mouse (relative) mode. When you are in digitizing mode, you can only digitize features; you cannot choose buttons, menu commands, or tools from the ArcMap user interface because the screen pointer is locked to the drawing area. In mouse mode, however, there is no correlation between the position of the screen pointer and the digitizing tablet. When digitizing, you can switch between digitizing mode and mouse mode using the Editing Options dialog box. This allows you to use the digitizer puck to digitize features as well as access user interface choices (as a substitute for the mouse). Also, you can use your mouse to choose interface elements at any time, whether your digitizer is in mouse mode or digitizing mode (Wenzhong, 2010).

You can digitize features on a paper map in two ways: point mode digitizing or stream mode digitizing (streaming). You can switch back and forth between the two modes as you digitize by pressing F8.

When you start a digitizing session, the default is point mode. With point mode digitizing you convert a feature on a paper map by digitizing a series of precise points, or vertices. ArcMap connects the vertices to create a digital feature. Point mode digitizing works the same way with a digitizer as with screen digitizing with the construction tools; the only difference is that with the digitizer you are converting a feature from a paper map using a digitizer puck instead of a mouse.

When streaming, ArcMap automatically adds vertices at an interval as you move around the map. You might want to stream when creating a curved line, such as a river. Streaming, or stream mode digitizing, is commonly used with a digitizing tablet but can be used simply with a mouse.

To begin digitizing in stream mode, right-click and click Streaming when creating features. You can also press the F8 key to enter stream mode. If you click the map, streaming is suspended. This allows you to click buttons, menus, and other user interface elements. This means you can right-click to access the shortcut menu, enabling you to place a vertex using Absolute X,Y, Delta X,Y, or any of the other commands on that menu. Click the map again to start streaming. To exit from stream mode entirely, right-click and click Streaming or press F8.

References

Davis, B. (2001). GIS: A visual Approach. Canada: OnWord Press.

Murayama, Y. (2012). Progress in Geospatial Analysis. Tokyo: Springer Publishers.

Wenzhong, S. (2010). Principles of Modeling Uncertainties in Spatial Data and Spatial Analyses. USA: CRC Press.

A Supportive material prepared by

Ngogo MN.

Geography Dept.

St. Augustine University of Tanzania