Conventional topographic maps
possess wealth of information which
represents the spatial variation of
the terrain. Despite being the models
of spatial environment, the maps
have limited utility in the context
of manipulation of the data because
of its non-digital form. On the other hand, digital cartographic databases
(DCDBs) provide flexibility to
relate and integrate data from varied
data sources which has multifold
advantages. This has set in new
trends and has lead to the concept of
integration of varied information under
the umbrella of 'Geomatics' mainly
to support integrated land resources
management which is the prime
requirement for sustainable growth
and infrastructural development.
Geomatics integrates various scientific
disciplines which pertain to acquisition,
organization, management, archival,
infrastructure, standardization, and
dissemination of the data. Apparently
traditional map making and geomatics
have some similarities but practically
many differences. In the geomatic
environment, the complex integration
of spatial and non-spatial data can be
accomplished comparatively with ease
and efficiency provided special care is
taken while planning and generation of
DCDBs, which amongst other things
include, adherence to data accuracy
specifications, an adequate choice of
data structure, an optimum data model,
a workable data exchange format, an
efficient data storage system and a
workable dissemination policy keeping
in view the futuristic requirements.
Integration and interaction of a
variety of databases is a crucial aspect
and is complex in nature. It is best
supported by Geographic Information
System (GIS) which deals with
the integration of spatial and nonspatial
data from varied data source.
GIS facilitates capturing, storing,
checking, integrating, manipulating,
analyzing, and displaying of data
which is spatially referenced to the
earth. It enables provisions of needbased
information to the users for an
efficient decision making process.
The advent of Global
Positioning System (GPS)
technology has added another
dimension to the concept of geomatics.
It enables near real time response
in navigation and provision of
ground control points by using the
techniques of satellite tracking.
India should be prepared to have
alternatives, should the GPS signals
be denied to us in any future crisis.
Data updation
The mapping agencies at present are
not only confronted with the problem
of generating the cost effective digital
databases, but the basic need is to
maintain and update the databases to
make them more meaningful from
the users' point of view. The present
workshop may, therefore, be a step
forward in the right direction.
The necessity of 3D topographic
databases had already been projected
by a number of users and this
requirement may necessitate the
generation of 3D databases on
Analytical Plotters or/and Soft copy
Photogrammeric Systems using aerial
photography as the data source in
combination with stereo imageries.
To achieve this task, which obviously
is not simple, our data procedures
have to take up stereo digitization
on a mass scale using the latest high
and low end digital photogrammetric
systems in combination. The linear
details like roads, water-bodies,
embankments, canals, forest limits
etc. can be interpreted with ease
and stereo digitized as per the predetermined
data structure. The data
updation can be accomplished to
an appreciable extent using stereo
imageries in combination with
the large scale photographs.
Some first hand experiences
on data updation
Spatial data updation
using stereo spot
imageries (1993-1994)
a. Keeping the trends in
photogrammetry in view, we, in
Digital Mapping Centre, Dehradun,
had taken up Experimental Test
Programme in the year 1993-94
and carried out the feasibility study
with the clear aim of evolving a
technique for cost effective speedy
updation of DCDBs. The technique,
analysis, results, and methodology
for the updation of DCDBs using
SPOT stereo imageries (10m
resolution) in combination with
aerial photography were worked
out. In this test programme, in
depth studies up to the stage of
checking of the ground truth was
carried out for the test area.
b. The broader aim of the
test programme was:
(i) To identify the limits up to which
stereo imageries can be used
to detect the change in 1:50Kmaps if used in isolation.
(ii) To interpret the contrast in the
imagery in places where it is
not possible to interpret details
due to limited resolution.
(iii) To analyze if reference to
large scale photographs can
help in furthering the data
interpretation/updation.
(iv) To carry out in depth evaluation
in respect of the test area including
the stage of verification of ground
truth and conclude results.
c. During the feasibility study, it was
observed that in one of the sheets
where large scale photography was
available, the change in contrast
could be fully exploited by referring
to the photographs. This was
found to be of immense help and
furthered the capture of additional
data to a great extent. This data
was also evaluated by subjecting
the products to 'Ground Truth'.
Experimental test programme
using new generation satellite
spot-v stereo imageries
a. We have also taken up an experimental test programme on a
high ended digital photogrammetric
system for mapping 'Virgin
Areas', using new generation
satellite SPOT-V Stereo imagery,
without ground control points.
b. The stereo imagery has a resolution
of 5m in panchromatic mode. An
effective resolution of 2.5 metres
is obtained by sampling with the
supermode process (THR on two
digital channels). The presence
of on board 'Star Sensor' which
works in tandem with DORIS
enables precise satellite attitude
and the star tracker data gives
the position. The experiment
has given encouraging results.
Issues related to data updation
Three distinct problems specific to map
updation using satellite stereo imagery
in photogrammetric environment are:
a) Need for thorough comparison
with existing map or
cartographic database.
b) Change detection.
c) Interpretation of new features for stereo digitization.
d) Deletion of old features to the extent necessary.
e) Accommodation of errors in old maps.
Even the simplest revision process is
sufficient to indicate the difficulties
not normally associated with new
map making. Discrepancies in
respect to some features may come
up between the existing map and
the data captured by stereo-imagery
or aerial photography which may
not be attributed to any change. If
confirmed that the new detail cannot
be made to fit the old details, it
becomes necessary to place the new
detail with best approximation and,
in most of the cases, the accuracy of
the end product may still be within
map specification accuracy limits.
A proper confidence limit has to
be developed by the operators
and propagated by way of need
based training so as to enable them
to capture the desired data after
deliberate comparison between the
existing map/digital files, the stereoimagery,
and the aerial photographs.
From our experience we have
seen that proper exposure and inhouse
job training supplemented
with product evaluation can be of
immense help in this regard.
Issues in creation of DCDBs
In the prevailing 'Data/Information
Explosion', various powerful
computer systems are available at the
doorstep of user agencies who may
be experiencing a blurred feeling
because of the recent and inevitable
transition to digital technology. The
feeling is rendered more uncomfortable
because of the availability of
variety of computer systems, arcane
terminology, dense theory, intricate
protocols, and impractical examples.
Under this blurred environment
there lies a guiding torch in the
form of GIS, which facilitates
organization and its application.
In GIS environment, a digital
cartographic database is the nucleus
surrounded by data processing and data
analysis techniques which facilitate
queries and analyses. Consequently,
the design and management of a
cartographic database is of utmost
importance. Both these aspects have
a major implication on DCDBs in
geomatics environment and have
compelled computer engineers and
cartographers to focus their attention
more on the database rather than
graphic form or the artistic perception
of graphics. Some of the major
issues in the digital cartography
in the present context are data
integration, data standardization,
quality control, and data storage or
archival and data dissemination.
Data integration
Various data producing agencies
produce data in their own formats.
Problems are bound to arise during the
course of integration of this variety of
data with the cartographic database
of Survey of India. The need of the
hour is to work out a systematic data
fusion methodology in the absence
of which many users may resort to
quick and non-standard solutions for
various localized utilities with narrow
aims. This may lead to practical
problems at the stage of transference
of plans to actions on ground. Such
problems have been envisaged in the
integration of cadastral information
with Survey of India topographic map
and also in the generation of image
maps from satellite data and digital
cartographic database (Dasgupta,
1994). The data fusion standard need
to be worked out so that different
data sets, based on their own datum,
structure, formats, and projections
can be efficiently matched.
Data standardisation
The salient requirements of data
standardization of digital cartographic
databases may include:
a) Standardization of technical terms
related to topographic data.
b) Standardization of scales and
projections with respect to
their utilities and services.
c) Standardization of various data
sources for generation and updation
of digital cartographic databases
so that data source conforms to
the accuracy specifications.
d) Standardization of symbols
and patterns and their
cartographic attributes.
e) Standardization of text
and its attributes.
f) Standardization of data
structure, data encoding, and
data exchange formats in such
a way that it facilitates speedy
data retrieval/exchange.
g) The feasibility to follow a single
data exchange format with variety
of information generated by
various agencies. In such case,
the 'think tanks' may have to
decide whether standardization
of a family of data exchange
formats will be appropriate.
h) Standardization of relationship
of spatial variables in order to
enable efficient GIS analysis.
i) Standardization of data structure
(vector, raster) and feasibility
of considering dual structure in
the same cartographic database
by retaining/ maintaining
non-intelligent information
in raster mode such as, sand
feature, scattered trees etc.
j) Standardization of datum needs
serious viewing keeping the
international status in view. Pros
and cons have to be weighed
since the proposition is not simple
because of the efforts involved
in adopting it considering the
wide extent of our country.
Quality control
Under the current environment, many
Govt./Non-Govt. agencies may be
generating their own database from
various data sources. All of them may
not be following the basic standards
of digital cartography which is likely
to lead to wrong analysis in a GIS
environment. This may eventually
contribute to erroneous decision making and catastrophic results in
areas where accuracy specifications
are of prime importance, such
as defence, land use, road and
railway networks, measurement
analysis, planning and execution of
development tasks etc. Under the
circumstances standardization of
quality control needs no emphasis.
The best thing available to a user
could be a 'Quality-tag' for each
database, with respect to accuracy
specifications, after the databases go
through quality checks. The quality
control tests need to be realistic.
The tests have to be standardized
such that no non-standard database
finds its way in the pipe line else it
may integrate with other databases
and consequently bring down their
purity. This leads to the concept of
'Validation of Cartographic Databases'
which is of prime importance.
Generation and quality control of
Digital Elevation Models (DEM)
needs a special mention. DEM is
the mathematical representation of
earth's terrain in the form of X, Y,
Z triplets. An accurate DEM will
faithfully depict the elevation related
information of the earth. Foolproof
validation techniques need to be
worked out and standardized because
DEM is prone to propagation of errors.
If proper care is not taken prior to the
delivery of the data and it's archival,
the DEM data generated may not serve
the purpose for which it was generated
and the realization could be too late.
Data storage and archival
Data storage and its archival with
respect to digital cartographic
databases demands voluminous
space. Adequate media needs to be
identified as per the latest state of
art available in international market
for mass storage for large databases.
Standardization with respect to design
of inventory system, optimum storage
and its foolproof security, number
of copies, and storage in duplication
at different locatix
ons to safeguard
against natural calamities are some of the salient aspects which need
standardization and implementation.
Data dissemination
The policy of data dissemination has
to be standardized so that the data
generated is available to genuine
govt. and non-govt. users. This aspect
has a special impact on the security
of the data. This subject has been
actively addressed by the concerned
Govt agencies. As regards the nonrestricted
data, the procedures for
supply of data needs standardization
such that the data is available to the
users else the whole exercise of data
creation may not serve much purpose.
Conclusion
Integration of varied information
in GIS environment is not simple
but complex and challenging. Great
responsibilities lie on the shoulders
of cartographers, space scientists,
computer engineers and administrators
to ensure that they address the issues
like data fusion, standardization,
quality control of digital products
etc. This will facilitate creation of
meaningful cartographic database for
generation of quality information which
should be second to none in the world.
References
Peter Reinartz et el. - First result
on accuracy analysis for DGM and
ortho images derived from SPOT
HRS Stereo Data over Bavaria.
Major P N Koul - Spatial data
updation using stereo imageries,
ISPRS workshop, Working Group
IV/2, Institute of Remote Sensing,
Anna University (Nov-Dec, 1995).
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