DIGITIZING:
Digitizing in GIS is the process of converting geographic
data either from a hardcopy or a scanned image into vector data
by tracing the features. During the digitzing process, features
from the traced map or image are captured as coordinates in
either point, line, or polygon format.
Types of Digitizing in GIS:
The most common method of encoding spatial features
from paper maps is manual digitizing. It is an appropriate
technique when selected features are required from a paper map.
Manual digitizing requires a digitizing table that is linked to a
computer workstation. The digitizing table is essentially a large
flat tablet, the surface of which is underlain by a very fine mesh
of wires. Attached to the digitizer via a cable is a cursor (puck)
that can be moved freely over the surface of the table. Buttons
on the cursor allow the user to send instructions to the computer.
The position of the cursor on the table is registered by reference
to its position above the wire mesh.
Figure: Manual Digitizer
The procedure followed when digitizing a paper map using a manual digitizer has the following five stages:
• Registration: The map to be digitized is fixed firmly to the table top with sticky tape. Five or more
control points are identified (usually the four corners of the map sheet and one or more grid intersections in
the middle). The geographic co-ordinates of the control points are noted and their locations digitized by
positioning the cross-hairs on the cursor exactly over them and pressing the ‘digitize’ button on the cursor.
This sends the co-ordinates of a point on the table to the computer and stores them in a file as ‘digitizer co-
ordinates’.
• Digitizing point features: Point features, for example spot heights, hotel locations or meteorological
stations, are recorded as a single digitized point. A unique code number or identifier is added so that attribute
information may be attached later. For instance, the hotel with ID number ‘1’ would later be identified as
‘Mountain View’.
• Digitizing line features: Line features (such as roads or rivers) are digitized as a series of points that
the software will join with straight line segments. In some GIS packages lines are referred to as arcs, and their
start and end points as nodes. This gives rise to the term arc–node topology, used to describe a method of
structuring line features.
• Digitizing area (polygon) features: Area features or polygons, for example forested areas or
administrative boundaries, are digitized as a series of points linked together by line segments in the same way
as line features. Here it is important that the start and end points join to form a complete area. Polygons can be
digitized as a series of individual lines, which are later joined to form areas. In this case it is important that
each line segment is digitized only once.
• Adding attribute information: Attribute data may be added to digitized polygon features by linking
them to a centroid (or seed point) in each polygon. These are either digitized manually (after digitizing the
polygon boundaries) or created automatically once the polygons have been encoded. Using a unique identifier
or code number, attribute data can then be linked to the polygon centroids of appropriate polygons. In this
way, the forest stand may have data relating to tree species, tree ages, tree numbers and timber volume
attached to a point within the polygon.
Comparison of Scanner Types
Hand Scanner
Portability: Highly portable, can be easily carried around.
Speed: Fast for small areas or individual documents.
Resolution: Varies, but often sufficient for barcodes, QR codes, and small text.
Connectivity: Typically wireless or USB-connected.
Applications: Inventory management, logistics, point-of-sale systems, and mobile document scanning.
Flatbed Scanner
Size: Larger than handheld scanners, requiring a dedicated space.
Speed: Slower than handheld scanners, especially for multiple documents.
Resolution: Generally higher resolution for scanning larger documents or images.
Connectivity: USB-connected or network-enabled.
Applications: Scanning books, magazines, photographs, and large documents.
Drum Scanner
Size: Large and specialized equipment, often found in professional settings.
Speed: Very fast for high-resolution scanning.
Resolution: Extremely high resolution, suitable for large-format images and graphics.
Connectivity: Typically connected to a computer or server.
Applications: Professional photography, graphic design, and archiving high-quality images.
Sheet-fed Scanner
Size: Similar to flatbed scanners, but with a built-in document feeder.
Speed: Faster than flatbed scanners for multiple documents.
Resolution: Offers similar resolution to flatbed scanners.
Connectivity: USB-connected or network-enabled.
Applications: Scanning large volumes of documents, such as invoices, receipts, and reports.
Introduction:
Data encoding is the process of getting data into the computer. It is a process that is fundamental to almost
every GIS project. For example:
• An archaeologist may encode aerial photographs of ancient remains to integrate with newly collected
field data.
• A planner may digitize outlines of new buildings and plot these on existing topographical data.
• An ecologist may add new remotely sensed data to a GIS to examine changes in habitats.
• A historian may scan historical maps to create a virtual city from the past.
• A utility company may encode changes in pipeline data to record changes and upgrades to their pipe
network.
Once in a GIS, data almost always need to be corrected and manipulated to ensure that they can be structured
according to the required data model. Problems that may have to be addressed at this stage of a GIS project
include:
• the re-projection of data from different map sources to a common projection;
• the generalization of complex data to provide a simpler data set; or
• the matching and joining of adjacent map sheets once the data are in digital form.
This unit looks in detail at the range of methods available to get data into a GIS. These include keyboard
entry, digitizing, scanning and electronic data transfer. Then, methods of data editing and manipulation are
reviewed, including re-projection, transformation and edge matching. The whole process of data encoding and
editing is often called the ‘data stream’.
Analogue data are normally in paper form, and include paper maps, tables of statistics and hard-copy (printed)
aerial photographs. These data all need to be converted to digital
form before use in a GIS, thus the data encoding and correction procedures are longer than those for digital
data. Digital data are already in computer-readable formats and are supplied on CD-ROM or across a
computer network. Map data, aerial photographs, satellite imagery, data from databases and automatic data
collection devices (such as data loggers and GPS) are all available in digital form.
