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PRESENT-DAY TECHNIQUE IN DETERMINATION OF GEOSPATIAL-DATA POSITION FOR GIS APPLICATION. Geo-spatial data has got both physical dimension and geographic location on the earth. They represent the real world and can be represented in a map using colored symbols.
Geo-spatial data has got both physical dimension and geographic location on the earth. They represent the real world and can be represented in a map using colored symbols.
by three concepts:
i) Entity :- Distinct spatial feature or object.
Example: Building, church,
temple, road, lake etc.
ii) Attribute :- Description or characteristic of the entity in qualitative or
quantitative or in both format.
iii) Relationship :- Link between different entities.
Again each spatial data can be represented by three geometric entities such as point (● ), line ▬, and polygon (or polyline) and a suitable label.
A point in 3D-surface is specified by either Cartesian coordinates in x,y,z or by geographical coordinates in Φ, λ , h i.e., Latitude, Longitude and height.
A Line is a sequence of connected points.
A polygon is a closed area represented by lines.
GIS relies heavily on SPATIAL and NON-SPATIAL data for its wide application in various fields.
SPATIAL DATA SOURCES
1. Conventional topographical maps, Aerial Photographs, Cadastral Maps and
many other sources.
2. From Ground Surveying.
3. Digital Information’s obtained through Global Positioning System (GPS), Total Station etc.
4. Many other departmental sources.
G P S (Global Positioning System) is the satellite based new technique in the field of data collection and for Mapping purposes
Though it started primarily as a navigation system but within a short period, it has revolutionized the positioning concepts.
It provides highly accurate position of the observer in x,y,z coordinates on any point of the globe at any instant of time.
DIFFERENT TYPES OF G N S S (Global Navigation Satellite System)1.The Global Positioning Satellite System is the only fully operational GNSS system controlled by USA with about 28 earth orbiting satellites that transmits continuously precise signals to the earth via electromagnetic waves. The system enables a G P S receiver to determine its location, speed, direction and time.
2. RUSSIAN GLONASS System:- With about 10 to 12 satellites are operational at present,but when it will be fully operational, there will be 24 satellites in the system. All these satellites are to be placed at an altitude of 19,100 km above the earth in 3 orbital planes.
3. The European unions GALILEO positioning system is the next generation GNSS and is expected to fully operational by 2010.
4. India’s IRNSS (Indian Regional Navigational Satellite System) is the regional GNSS System meant for Indian region and is likely to be fully operational around 2012.
24 operational satellites (+ 3 or 4 spare)
6 orbital planes
4 equally spaced per orbit
20,200 kIn (10,900 n. mi.) altitude
12 hour orbital period
Solar powered, backup batteries, rocket boosters.
1. Control segment consists of 05 (Five) trekking /Monitor stations spread over the globe at
Hawaii, Kwajalein, Asumsion Island, Diego Garcia, Colorado and the Master Station at Colorado Springs.
2. The Master Station collects the data about the Satellite health, positions etc through its five Monitor Stations.
3. Satellite Positions are corrected by uploading the data.
4. The data so uploaded in each Satellite System is termed as Broadcast Ephemeredes.
User Segment: In User Segment, we have to identify the various components of an integrated G P S instrument.
* A G P S Receiver with Micro Processor, Control and Display Unit with Power Supply, Recording device using a highly stable atomic clock and a R F (Radio Frequency) section. Some times a Receiver is designated by number of Satellites it can monitor simultaneously, for example 12 Channel or 06 Channel and some times by the number of frequency the data is received from the Satellites for example dual frequency (L1 & L2) and single frequency (L1).
* Antenna: There are various types of G P S Antenna now in use namely Choke antenna, helix antenna, zypher geodetic antenna etc.,.
. There are 4 variables unknown:
. Longitude (X)
. Latitude (Y)
. Elevation (Z)
. Time (T)
. Therefore, you need info from four satellites.
. If only 3 satellites are present, the receiver will guess at elevation and compute latitude and longitude.
Signals from GPS satellites are continuously transmitted on two Carrier frequencies viz.
L1 and L2.
L1 = 1575.42 MHz i.e., 1575.42 x 106 million chips per second.(wave length λ = 19 cm).
L2= 1227.60 MHz i.e., 1227.60 x 106 million chips per second.(wave length λ = 24 cm).
Through L1 carrier, code like C/A code i.e.Coarse Acquisition, Precise code (P-Code) and the Satellite messages are sent to the earth continuously. Where as through L2 Carrier only P-Code and Satellite messages are sent.
Through Code measurements also known as Pseudorange measurements instantaneous position on the earth surface can be determined using G P S Satellites.
P (Pseudo range) = c x ε where ε = time delay
= c (tR – tS) + clock error = time difference between the generation
of code by satellite clock and its
receipt by the receiver clock.
The C/A code–frequency = 1.023 M Hz i.e., 1.023 million chips per second (λ = 293m)
and P-Code-frequency = 10.23 M Hz i.e., 10.23 million chips per second (λ = 29.3m)
GPS receivers which record carrier phase, measures the fraction of one wave length (i.e. fraction of 19cm for L1 and 24cm for L2 carrier) when the receiver first locks on to a satellite and continuously measure the carrier phase from that time.
The number of full cycles between satellite and the receiver from the initial start up which is termed as Ambiguity together with the measured carrier phase gives the satellite – receiver range (i.e. the distance between satellites and a receiver).
Hence, we can say
Range = Measured fraction of carrier phase
+ Ambiguity x wave length + clock error
Higher positional sub meter accuracy is obtained by carrier phase measurements where as code measurements give lesser positional accuracy from 10m to 100m in GPS data.
For both code and carrier phase measurements, a variety of positioning methods exists.
Code Positioning Technique:
1. Single Point Positioning: The coordinates of the antenna position on the surface of the earth can be achieved by intersecting the measurements from four (4) or more satellites. The accuracy achieved in single point positioning may be within +/- 100mtr. Single point Positioning is also termed as Absolute Positioning or Point positioning. In this method only one receiver is needed.
2. Relative Positioning or Differential Positioning: In this method of observation two receivers are needed. One of them is kept on a known station and the other is kept on unknown station. In Relative Positioning higher accuracies are achieved. Errors involved in the G P S observations are common in the known and unknown sites. During post processing of the field data the errors are compensated. Accuracy achieved in this type of observation may be within 2 to 10mtr horizontally.
Carrier Positioning Technique In Carrier Positioning, the observations taken may be Static, Kinematic, Semi-kinematic, Pseudo-kinematics and Rapid Static.
Static Carrier Positioning Technique: In this method two or more G P S receivers kept at the stations and longtime observation are taken. On post-processing the data with respect to the positional data of known station, a very good accuracy within millimeter level i.e., 1 cm + 1 ppm to 10 ppm can be achieved.
Rapid Static Carrier Positioning Technique: In this method one G P S is kept on a known station and observation is taken continuously on it, and the other G P S is kept on unknown station i.e, Rover Station and the observation taken on this points for about 10 to 15 minutes. Such type of observation is known as Radial Rapid Static Method. Some times G P S positions are alternately changed by taking observations on stations for 10 to 15 minutes such type of observation in known as Traverse Rapid Static Method. By post processing the G P S field data, the positional accuracy of the unknown points can be achieved within centimeter.
Kinematic Carrier positioning Technique: In Kinematic Carrier mode one G P S is kept at Static on a Known Station, and the other is used as a moving one.
Semi-kinematic: In Semi-kinematic one G P S called the Master GPS is kept on a known station, and the observation is taken on it continuously to resolve ambiguity.
Other GPS called rover is moved between each point by holding it stationery on the points for 10 to 15 seconds, and then moved to the next point without closing the instrument. This method for providing control is suitable for very small area.
(Ionosphere & Troposphere)
The Technique of Global Positioning System fall into following kinds of applications: