For the management of National Resources, computerized digital databases are generated for different administrative units like village, district , state and for different themes like landuse, roads, canals, elevation points etc. At present, thematic maps generated by different agencies is the main source of inputs for the creation of National Resources Information System (NRIS) district databases. Dependency on GPS data and satellites data including stereo pair is much less. It is required to update, validate and enhance databases periodically for reliability of GIS outputs and integrity of data analysis. To meet such requirements GPS technology provides facility for data collection pertaining to new developments and also for checking the accuracy of existing databases.
After removal of Selective Availability (S/A), the applicability of GPS data in facility mapping, infrastructure planning and GIS database generation will increase by many folds. In context of NRIS, it is necessary to prepare fresh base maps for many districts using IRS data and control points provided by GPS observations due to restriction policy of digitization of SOI maps and non availability of current maps. Thus, GPS is an important source for NRIS database generation.
This paper discusses interface issues and data integration problems in context of authors experience regarding usage of GPS receivers for NRIS database validation and enhancement
The NAVSTAR GPS (Navigation Satellite Timing and Ranging Global Positioning System ) is a satellite based radio navigation system providing precise three dimensional position , navigation and timing information to suitably equipped users on a continuous basis. GPS receivers measures code or carrier phase or both to provide meter level accuracy in point positioning mode and up to few centimeters in differential mode. The brief description of GPS system components viz. space segment, control segment and user segment is given below.
The space segment consists of 24
satellites arranged in six different orbital planes of
inclination 55 Deg. w.r.t. equatorial plane. These satellites
are orbiting the earth at a height of 20200 km from the
surface of the earth and have periodic time of 12 hours.
Minimum 4 satellites are visible for positioning on
ground/sea/air at any time throughout the year. Each satellite
is transmitting coded signals known as pseudo random noise
(PRN) signals modulated on L1 (154 * 10.23=
1575.42 MHz (= 19.05 cm)) and L2 (120 * 10.23 = 1227.60 MHz (=24.45 cm)) carrier frequencies. Transmitted signals on both frequencies are modulated with navigation and system data including satellite ephemeris, atmospheric propagation correction data and satellite clock bias information. The L1 signal contains both, P- code and C/A code. The L2 signal contains P code only. The P code frequency is 10.23 MHz which corresponds to wavelength of 29.31 meter with period of 267 days and 7 days for a satellite. The C/A code frequency is 1.023 MHz which corresponds to wavelength of 293.1 meter with period of 1 millisecond. Data signal frequency is 50 bps and its cycle length is 30 second.
The Operational Control Segment for GPS consists of the Master Control Station near Colorado Spring (USA), three Monitor Stations and ground antennas in Kwajalein, Ascension and Diego Garcia, as well as two more monitor stations in Colorado Spring and Hawaii. The tasks of the control segment are to:
- monitor and control the satellite system continuously for uploading data into the satellites.
- predict the satellite ephemerides and the behavior of the satellite clocks.
- periodically update the navigation
message for each particular satellite.
This relates to various types of GPS receivers like navigation, survey, single frequency, dual frequency etc.
To study the role of GPS data in NRIS database content validation and enhancement with respect to following objectives.
- Determine location accuracy and alignment of NRIS database elements like roads, railway lines, canals and selected area features like water bodies ,forest boundaries etc.
- Determine height accuracy of elevation points and contours in databases
- Provide accurate control points for registration and rectification of satellite images
- Provide accurate control points for registration of cadastral map
- Provide control points for DEM generation and evaluation
- Error modeling of GPS observations.
- Provide control points and Spatial
Framework for base map generation
GPS satellites are available for all 24 hours and observation can be taken at any place at any time. For better accuracy planning is required with respect to identifying area of interest, distribution of points, data collection strategy, availability of required no of satellites with good GDOP value and above 15 degree of elevation angle. Detailed procedure followed during experiment is as below,
- Preparation of map for different coverages for which data are to be collected
- Selection and marking of features/control points on map with identification code
- Total time of data collection and assigning time slot for different area
- Preparation of satellite visibility chart
- Logistics planning
- Selection of reference points position and rover points positions for data collection
- Determining initial approximate
coordinates of observation points in WGS 84 system
GPS observations were taken at well distributed
points within map sheet no 46 F/9 using two LEICA SR 9400 GPS
receivers one working as reference receiver and other working
as rover receiver. During first schedule simultaneous
observations were taken at reference point (continuously for 6
hours) and at seven rover points(for 30-40 minutes). Similar
observation pattern was followed for second schedule.
Specific mission parameters like ambiguity resolution limit, models for atmospheric and ionospheric correction, baseline length limit etc. were defined before data processing.
The coordinate solutions were determined using phase and code measurement.
The computed WGS 84 coordinates were converted to Everest Datum using computed local seven parameters.
Heights above ellipsoid were converted to MSL value using external geoid model.
- For all observed points WGS 84 coordinates are given in table-1.
- Differences in NRIS database coordinates and coordinates computed using local 7 parameters are given in table-2 for selected 8 points with larger base-length. Differences are within 10 -15 meter limits.
- Differences in GPS coordinates and
coordinates estimated using 7 parameters (for whole Indian
region) and Geoid model are given in table-3. Comparatively
larger errors are found while using global parameters for