Beyond mapping

Over the years topographic mapping generally involved acquisition of data related to the features of the earth, its environs of user’s interest and its graphic depiction on a suitable media, generally paper, ensuring that the accuracy specifications, which depend upon the type of end product, are strictly adhered to.

Over the years topographic mapping generally involved acquisition of data related to the features of the earth, its environs of user’s interest and its graphic depiction on a suitable media, generally paper, ensuring that the accuracy specifications, which depend upon the type of end product, are strictly adhered to.

Beyond mappinggeneration of DCDBs

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).