7.
Recommendations for the best use of key GIS UI
functions
7.1
Objectives
The aim
of this section is to provide a set of
recommendations to guide the end user in
exploiting the more common functions of a GIS.
Thus it integrates both the GIS operation
checklist of section 6, and the checklist for
defining user requirements of section 5. This
section explicitly considers the following
issues:
á the level of skill of the end user
á the type/class of application
á the structure of the data
á the UI technology currently available on the
market
á the generic UI practice available
Most of
the recommendations in this section are aimed at
the final user with low or intermediate
experience in GIS technology, but with a sound
background in his/her own discipline or
professional sector (i.e. topography and
surveying, civil engineering, hydrology, geology,
urban planning, etc.). The key functions
discussed here have been selected from among
hundreds of possibilities because they are
frequently applied or critical in the domains of
environmental control and urban
planning.
Both
vector and raster operations are considered,
highlighting the difference in working with these
two data structures.
Today a
major breakthrough is taking place in the GIS
industry. New systems are under development that
will have greater capabilities and, more
importantly, flexible and easily-customisable
user interfaces.
However,
since most of the GIS available on the market are
still bound by current technology,
recommendations will mainly refer to the
state-of-the-art of the commercial products. That
is considered a meaningful contribution to
address the current practical needs of
users.
Since
GIS functions, operations and capabilities are
illustrated or discussed in a number of good
books, reports and manuals (see the selected list
of references), attention will concentrate on
suggestions and advice that are complementary to
what the interested reader can already find in
the literature.
7.2
Recommendations
The
recommendations cover the activities that are
most common in a project for GIS development and
implementation, namely:
project
set-up
data entry
data conversion
data validation
data
visualisation/rendering
map database
management
attribute data
management
map
processing/analysis
map/report
production
For some
selected key functions, a more detailed
description is provided in Tab. 1 to
10.
7.3
Recommendations for the use of some selected key
GIS functions.
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Data Conversion
RASTER TO VECTOR |
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The
conversion of spatial data from raster
(cellular) to vector structures
(models). |
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Before
importing and converting a
satellite/aerial image to the vector map
database, the user should carefully
classify the image into a few meaningful
classes. Likewise, the smoothing/
filtering techniques used to eliminate
superfluous or meaningless polygons in
the vector map, should be selected and
used with great care.
Before
attempting any conversion, the user
should evaluate the use of the
imagery as read-only background in visual
display.
To
exploit scanner/vectorising technology on
complex paper maps, the user should
follow the strategy outlined in section
7.2.2.
In
some cases the correct conversion of
complex raster databases to the vector
representation may require the advice of
an expert. |
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Data validation
IDENTIFICATION/CORRECTION OF TOPOLOGICAL
ERRORS |
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Locating
and correcting the digitised data to
reduce or eliminate spatial and
topological errors. Making spatial data
usable for certain types of analyses
(i.e. network tracers) requires
topological construction, which makes
explicit the relationships between
features (line connectivity, area
definition and contiguity,
etc.). |
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In
order to reduce the number of digitising
errors, the user should invest more time
both in properly designing the workflow
for data entry, and in accurately
executing the digitising
operation.
When
input map exhibits very many digitising
errors, the user should carefully
evaluate two following alternatives:
correcting such errors or restarting from
scratch the digitisation process. Under
many circumstances, the second
alternative may prove to be the most
feasible.
Since
error propagation through overlaying
operations is a frequent problem, the
user should carefully check the quality
of each base map before starting any
analytical operation. |
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Data visualisation/rendering
ZOOM and PAN |
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Zooming
is the process of magnifying or reducing
the scale of a map or image displayed on
the monitor. Panning is the process of
changing the position at which the view
is displayed, without modifying the
scale. |
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When
the user is mainly concerned with the
exploration of relatively small data
volumes (say < 2 Mb), the recommended
zoom and pan functions should fulfil the
following requirements: a) they should be
versatile, that is, the operation may be
accomplished using different approaches,
b) each approach should be intuitive; and
c) each approach should allow for
repeating the operation many times
without excessive stress on the user. At
present, several small and large GIS
tools meet the above
requirements.
When
the user has to deal with large data
volumes, data structure and software
optimisation become of ultimate
importance on the GUI.
Under
such circumstances, the user must select
a GIS that fulfils the requirements of
both an efficient GUI, and optimised
software architecture. At present, few
GIS are available on the market that
meet the above
requirements.
In
order to evaluate software efficiency
(algorithm speed), the potential
customer should test zoom pan functions
on a large (say over 20 Mb) data set
(map) obtained from the GIS vendor or
elsewhere. Always consider that
(re)drawing speed is affected by the
system memory more is better) and by the
frequency of disk reads. |
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Data visualisation/rendering
MANAGEMENT OF BACKGROUND IMAGES |
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The process of
displaying and managing on the same view
vector objects (point, lines and
polygons) in the foreground and
georeferenced raster imagery (i.e.,
satellite imagery) in the background. |
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In
scanning the documents that are the
source for the background image, the user
should carefully select the most
appropriate resolution, balancing raster
file size and "readability" of
the image.
If
the available GIS system lacks image
analysis capabilities, the scanned file
could be first pre-processed using one of
the shareware image modules that can be
found on the Net; then transferred to the
GIS for visual exploration and
analysis.
In
many circumstances, the use of a
grayscale palette (colourmap) for the
background image yields the best results
in terms of clarity and interpretability
(i.e. a shade relief image on top of
which the vector stream network is
placed).
If
the raster file is large, plot it at the
lowest resolution (i.e. 150
dpi).
Keep
in mind that the monitor (screen)
resolution is about 70-90 dpi. |
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Attribute database management
ESTABLISHING MORE COMPLEX RELATIONS |
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Implementation
of graphic/alphanumerical connections
between graphical and alphanumerical
databases. This connection is based in
the use of a GIS internal table as
linkage table to other tables in external
databases.
The
set or collection of data that describe
the characteristics of real world
entities or conditions is frequently too
large to be stored in a single table
associated to the graphic elements. This
data are usually managed by a relational
database management system (RDBMS). |
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The
graphical alphanumerical connection with
external databases must be implemented
when graphical and alphanumerical
elements are perfectly defined
considering the possibility of
establishing relational
links.
Control
of the topological definition of systemic
elements and systematic verification on
the consistency. |
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Data processing/analysis
CREATE, SAVE DATABASE VIEWS |
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Database
views may be created through SQL queries
based on logical operators or similar
select options. Logical operations deal
directly with the database (alphanumeric
information) and allow the user to
identify and select features by a
specific set of criteria. Generally,
features are identified and selected
according to a combination of several
conditions. In a typical application, a
specific item in the database is employed
to differentiate features that satisfy
different sets of selection criteria.
Graphical selections, using the mouse to
define an area of interest, may
also form part of a view. |
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Many
beginning GIS users see the tool as
purely a cartographic one, whereas
seasoned users quickly find that, once
the cartography has been automated, many
of the difficult issues in GIS are
database-related. Therefore, training in
the optimal use of the RDBMS in question
is advised, beyond the few pointers given
in the typical GIS training
course.
Database
queries, or views, may be constructed
previous to analysis, to be ready for
their eventual use. A geographic database
with (perhaps) millions of elements may
require minutes or even hours to generate
certain complex views. Therefore, in many
cases these may be created ahead of time
and stored (cached). This capability
differs according to RDBMS manufacturer. |
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Data processing/analysis
BUFFERING (dilation) |
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The
process of generating a polygon that
encloses an area within a specified
distance from one or more point, line or
polygon features. |
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To
test a model or simulate a process, very
many buffering operations could be
necessary, varying both corridor width
and constraining conditions. Under such
circumstances, dilation can be carried
out much more efficiently in a raster
environment. Hence, the user could
consider following the strategy outlined
in Tab. 7.8.
In
order to obtain meaningful maps
displaying proximity of given features,
it is of paramount importance to
carefully evaluate the physical meaning
of the operation. Too frequently, users
exploit this powerful function without
understanding that the surrounding or the
buffered feature is neither homogeneous
nor isotropic. |
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Data processing/ analysis
MAP OVERLAY (Spatial Joins) |
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The
process of superimposing two maps such
that the resulting map contains spatial
and attribute information from both input
maps. |
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In
order to obtain a meaningful overlay of
two maps, it is of paramount importance
to carefully classify or reclassify the
value of the entity displayed in each
input map.
If
very many overlay operations are needed
on a large set of vector geographical
data, it is worth to evaluate the
following strategy:
a)
reclassify all maps into limited number
of meaningful classes,
b)
convert each map into a grid structure
with a grid-cell size close to the
resolution of the input data;
c)
perform all spatial analyses in such new
format;
d)
reconvert the results of the analysis to
the original vector
structure.
As
already discussed, the last step may
prove to be rather cumbersome. |
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Data processing/analysis Data
analysis
DIGITAL TERRAIN MODELLING |
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The
process of generating and handling a
digital terrain model (DTM), a numerical
representation of the earth surface based
on a set of x, y and z coordinates. The z
value may also represent any other
spatially continuous attribute such as
sea depth, air pollution or population
density. |
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Depending
on the type of investigation and
landscape characteristics, either
grid-based or TIN DTMs provide the best
information on the relief and its
derivatives of the region under
investigation. In general, DTM structure
and data resolution should allow for
preserving in the 3D model the
morphological information pertaining to
the original input data (contour lines,
spot height, etc.). If a grid DTM has to
be generated from digitised contour
lines, the following strategy is highly
recommended:
a)
rasterise contours with a grid-cell size
less than one third the input contour
interval;
b)
resample (by pixel thinning) the
resulting, usually too dense, raster file
to the desired grid
spacing.
When
a TIN DTM has to be produced, the user
should find out if the algorithm is able
to generate automatically ancillary
points for creating meaningful triangles
in ridge or valley areas. Alternatively,
such points have to be added
manually.
In
some cases, the best way to produce a
faithful DTM consists in calculating
elevations directly from the aerial
photographs through digital
photogrammetric techniques. |
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Map/report production
GENERATE COMPLEX GRAPHICAL PRODUCTS (Map
Composition) |
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The
combination of map layers to compile a
graphical representation of data, both
geographic features (points, lines,
polygons) and cartographic annotation
(titles, text, legend, scale bars, etc.).
The resulting product can also
incorporate other graphical elements such
as histograms, scatterplots, variograms,
and the like. |
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It
is important to always think about the
map reader needs and focus to production
of outputs that meet reader requirements,
namely to have a final product that is
easy to read and interpret.
Before
producing a complex, high quality map,
the user should:
a)
determine the purpose of the map;
b)
identify the map reader;
c)
design the components of the map;
d)
determine the map scale; and
e)
select the most appropriate symbols,
colours and annotations.
The
production of an aesthetically valuable
map composition will be greatly
facilitated by properly organised map and
attribute databases.
Before
plotting a map, the user should view and
inspect relevant details of the maps
(mainly annotations) on the monitor
screen using a zoom scale equal to that
of the final plot.
To
avoid printing problems with large size
(A0) inkjet plotters, the user should add
maximum (say more than 40 Mb) RAM to the
output device. |
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