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
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
