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Design of Large Area Spatial Framework for National (Natural) Resources Information System
Lambert Conformal Conic Projection :
This conic projection uses two
standard parallels to reduce some of the distortion produced when only one
standard parallels is used. The distortion related to shape and scale are
minimum in region between two standard parallels. The spacing between the lines
of latitude increases beyond the standard parallel .Suitable for middle latitude
region Local angles are accurate throughout because of conformity. scale is
correct along the standard parallels and is reduced between parallels and
increases beyond the parallels.
Methodology
Four regions
pertaining to topomaps 41E , 53E , 58F and 73M along east, west , north and
south extremes of INDIA were selected. Fictitious line and polygon features were
created in geographic projection system using ARCEDIT functionality of ARC/INFO.
For each map twelve projection files were created and using these projection
files the four basic coverages were projected considering district ,state and
center meridian for the selected Polyconic ,Lambert conformal conic and
Transverse Mercator projection system. For analytic comparison, Polygon
Attribute Table (PAT) files containing area and perimeter of polygon features
and Arc Attribute Table (AAT) files containing length of line features were
used. Details of distortions in area and length of features at state node and at
centre node are given in table.
Results and Conclusion
An
attempt is made to estimate errors introduced in geometric properties of the
features like area, perimeter and length due to shift in center of projection
from topomap to district, district to state and district to center level for
Polyconic , Lambert conformal conic and Transverse Mercator projections.
Tolerable limit of errors specified in NRIS Standards Document [1] is 0.3 % of
features area ,perimeter or length. Projection parameters and Results of study
are given in Table 1 and Tables
2A , 2B , 2C.
It is observed that while shifting projection center from topomap level
to district level , Polyconic and Transverse Mercator projections do not show
any error where as Lambert projection shows very negligible error which is less
then 0.003 % in area , perimeter and length of the features. Hence , all of
above projections are suitable at district level.
For district and state
level, errors are within tolerable limits for all of above projections but
Lambert Conformal Conic projections should be preferred due to its conformal
characteristic. For the centre level no projection is found suitable to create
seamless database .
Future Work
This study has evaluated the
three projections viz. Polyconic, Transverse Mercator and Lambert conformal
conic and found that none of these projections are suitable for creating
seamless database at center level. Therefore Geographic projection or grid based
projection /coordinate system should be adopted for creating seamless database
at centre level.
Acknowledgements The authors are thankful to
Shri A.R. Dasgupta, Project Director, NRIS-SIP and Dy. Director, SIIPA, for the
constant guidance and support provided by him for carrying out this study.
Referencers
- Node Design and Standards for NRIS ,July 1998. NO.
SAC/SIIPA/NRIS-SIP/SD-02/98
- Map Projections manual by ESRI.
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