UNIT 12: PLANNING A DIGITIZING PROJECT
Written by Eileen McGowan, Holyoke Community College
Digitizing is the transformation of information from an analog format,
such as paper, to a digital format that can be stored and displayed with
a computer. Digitizing is performed on a digitizing table or tablet but
may also be done on a computer screen ( heads up digitizing). The digitizing
table has a fine grid of wires embedded in it that acts as a Cartesian
coordinate system. The coordinate may be in plane or geographic coordinates.
The procedure involves tracing map features in the form of points, lines
or polygons with a puck which relays the coordinate of each sample point
to be stored in the computer. The tablet and puck acting together with
the computer can locate the puck s position relative to reference information
provided by the operator (3- point orientation).
Example Application
A city wants its land use map digitized so it can be overlaid on other
city layers for planning purposes. In order to fulfill this request you
must know several things.
-
What coordinate system is used by the city? This is important so the
map layers will be compatible within the GIS environment. It is better
to use the same coordinates so they do not have to be changed later. Every
transformation adds error to your map.
-
What is the accuracy of the layers to be associated? If it is significantly
different, the layers may not match (who owns the wetland).
-
What is the accuracy of the land use map? Someone may have to appear
in court to defend those decisions made based on this map.
-
Will this map be used for policy setting? You may run the risk of zoning
inappropriately.
-
What GIS program does the city use? It is always easier to input the
data in to a useable form and not have to find a translation program.
-
Can the program be used to digitize? If possible, use the GIS program
that the map will be used in to digitize.
The map is needed next week so planning will be important.
Suggestions:
Each time you digitize, digitize as much as possible. This will make
your technique more consistent. For more consistency, only one person should
work on a given digitizing project.
If the source consists of multiple maps, select common reference
points that coincide on all connecting sheets. Failure to do this could
result in digitized data from different data sheets not matching.
If possible, include attributes while digitizing. Doing so will save
time later.
Learning Outcomes
The following list describes the expected skills which students should
master for each level of training, i.e. Awareness/Competency/Mastery.
Awareness:
To become aware of the value of planning a project before it is begun,
thus limiting the probability of failure.
Competency:
The student will be competent in the use of scale and coordinate
systems, aware of software constraints and ability to determined required
accuracy.
Mastery:
To learn the value of planning a digitizing project or any GIS project.
Preparatory Units
Recommended:
-
Unit 11 - Registration and
Conflation
-
Unit 13 - Projecting Data
Awareness
Vocabulary:
-
Accuracy- How close a measurement is to the true or real value, Because
hypothetically we can always measure to one more decimal place, we can
never obtain the true value.
-
Precision- the ability to reproduce the same result. A measurement can
be very precise without being accurate.
-
Orientation points -Three or more reference points on a map attached
to a digitizing table that are used by software to locate the pucks position
on the table. The coordinates of the referencing points are entered into
the computer. The locations of all subsequent points are assigned coordinates
based on their position relative to the reference points.
Planning is the most important part of any project, especially any data
input pr project like digitizing. Proper planning will save hours of editing.
The answers to a series of questions will lead you to successful completion
of your project.
Questions to ask:
-
For what purpose will the data be used?
-
Will it be merged with a larger database?
-
With what GIS software will the data be used?
Competency
The purpose of the digitized data will dictate what your source will
be. The accuracy of the source data will be degraded every time a transformation
takes place. The skill and precision with which the operator digitizes
will be a factor in the accuracy of the final product. The accuracy needed
for the final product will dictate what should be used for the source.
For example, a USGS topographic map with 10 meter contour intervals and
a possible vertical error of 5 meters would not be used to delineate wetlands.
A change of 5 meters in water level would result in major flooding.
The information may be intended for use in a larger project. Because
of this, compatibility with existing forms of data must be addressed. Many
times the spatial data from a digitizing project is merged with outside
data. The data could be a database such as Microsoft Access or another
set of spatial data. The first situation would require the existence of
a common field to link the spatial information you digitized to the database.
The second would require the two sets of data to be the same scale and
in the same coordinate system with several matching points for geo-referencing.
If possible, digitizing should be performed using the same GIS software
that will later be used to display and analyze the data. File formats created
by digitizing with different GIS programs may not compatible. If the GIS
software does not have digitizing capabilities, digitize with a compatible
program or in ASCII format, a generally readable format. Failure to digitize
in a compatible format will make your work unusable. At the least one format
will need to be translated to another. Remember, the translation process
may introduce its own errors.
Mastery
A comprehensive understanding of all aspects of the GIS project will
help with all steps of the planning process for your digitizing. The big
picture needs to be examined before completing the steps that lead to the
end product.
An understanding of how the digitized product will be used. Who are
all possible users of this information and will it be compatible with the
data they already use.
Creation of a digital product that easily merges with the intended
GIS software.
Follow-up Units
-
Unit 13 - Digitizing maps
-
Unit 14 - On-Screen Digitizing
Back To Core Curriculum for Technical Programs Welcome Page
Currently maintained by Steve Palladino
Created: May 14, 1997. Last updated: October 5, 1998.
Content comments to
Eileen McGowan
Formatting comments to Steve
Palladino