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Control point positioning using GPS

Dr. Jayanta Kumar Ghosh
Dr Jayanta Kumar Ghosh & Ch. Venkata Appa Rao
Civil Engineering Department
Remote Sensing & Photogrammetric
Engineering Section Indian Institute ofTechnology, Roorkee
Uttaranchal 247 667

GPS technique has been well established as a means for establishment of control points in surveying and mapping. In carrying out this, the absolute coordinates of at least one site have to be known accurately in WGS84 coordinate system. As WGS84 coordinates for a particular site was not known neither it could be derived, in order to establish a reference station, Single Point Positioning technique of GPS has been applied for several hours of observation spanning in three consecutive days and night. The observed value shows precise planimetric position but grossly imprecise in height. However, the study provides some insight on the influence of some variables like a-priori value of parameters, time of observation etc. on result. The paper also discusses the probable sources of errors, precautions to be taken and direction of further study.

Control establishment is an important exercise in mapping process. The mapping accuracy is directly based on the accuracy of control network. The control network is formed by a group of points whose position (x,y,z) are known to a high degree of accuracy. Accuracy defines the quality of a control network. The positions of other features of interest are determined with respect to these control points for mapping. To achieve such a high degree of accuracy a reliable and accurate method of surveying is required. Though several techniques are used to properly establish and provide accurate horizontal and vertical control networks, meeting the requirements of the user community, the best technique will be one that provides the control networks with required mapping accuracy at the least cost in time and money.

The GPS (Global Positioning System) is one of the viable systems which meets the requirements of the surveying fraternity all over the globe. It is a space-based all weather radio navigation system. It broadcasts precise, synchronized timing signals to provide precise estimates of position, velocity and time of the antenna location of the system receiver. There are several methods of measurement based on GPS that make possible to take survey measurements from space. The numerous limitations of the terrestrial surveying like requirement of inter-visibility of survey stations, dependability on weather, difficulties in night observations, 3D position parameters etc could be overcome using GPS techniques. These advantages over the conventional techniques coupled with economy in time and cost, accuracy, speed and versatility in operation make GPS the most promising surveying tool of the future. Thus, a great technological revolution is taking place at the development of GPS as it can be used in any conceivable problem under the sky, where the exact position of any object or phenomena involved.

GPS surveying is a relative technique with baseline being “observed” and computed from the reference to rover. As many baseline will often be measured from the same reference station, the choice and reliability of reference station are of particular importance. Thus, the absolute WGS84 coordinates of at least one site have to be known accurately as all measurements in GPS system are depicted in WGS84 coordinate system.

For any precise GPS survey the absolute coordinates of ONE site in a network have to be known in WGS84 to about 10 meters. There are three possibilities for obtaining reliable WGS84 coordinates for one site and these are:
  • WGS84 coordinates may be available.
  • WGS84 coordinates can be derived from local coordinates using appropriate transform function.
  • WGS84 coordinates can be computed by GPS point positioning.
Since, WGS84 coordinates for a particular site is not known neither it could be derived, Single Point Positioning technique of GPS has been applied for several hours of observation spanning in three consecutive days and night.

Methodology – Point positioning
In point positioning, coordinates of the antenna position at an unknown point are sought with respect to the WGS84 reference frame. In this method, the known positions of the tracked GPS satellites (the position of a satellite can be computed from ephemerides) are being used to determine the position of unknown point using single GPS receiver by a method similar to the method of resection used in plane table surveying (Fig. 1).

Fig. 1: Point Positioning of GPS receiver

In this figure, s1, s2, s3 and s4 represent four different satellites (least required) being tracked. The positions of these satellites are referenced to the centre of the earth in the X, Y, Z coordinate frame. The coordinates for s1 are shown as (xs1, ys1, zs1). The coordinates of r, the unknown point, as referenced to the centre of the earth, are assumed to be (xr, yr, zr). The observed code, Prs1, relates the known coordinates of satellite 1 with the unknown coordinates of the receiver using the equation for a line in three-dimensional space. That is,

Prs1 = Ö [(Xs1 - Xr)2 + (Ys1 - Yr)2 + (Zs1 - Zr)2] + error

Thus, from four satellites, four distance equation can be formed leading to computation of the four unknowns (xr, yr, zr and clock bias) can be computed.

Study area
The area of study is the campus of IIT Roorkee. The astronomical coordinates of Roorkee are known to be: 29° 52Ë 002 North, 77° 53Ë 522 East and its height above datum (geoid) is 268 meter.

Location of Control point
Since control point has to meet some specific requirements with special emphasis of its suitability for GPS observation, the choice and reliability of reference station is of particular importance. In taking care of a good site characteristics for GPS observation such as
  • A clear view of the sky;
  • No obstructions above the cut-off angle (say 15°);
  • No reflecting surfaces that could cause multi-path;
  • Safe, away from traffic and passers-by;
  • Possibility to leave the receiver unattended;
  • No powerful transmitters (radio, TV antennas etc.) in the vicinity, a point on the top of a building (the Remote Sensing and Photogrammetric Engineering Section) of Civil Engineering department at IIT Roorkee is found suitable and thus considered as reference station for GPS receiver and subsequently, serves as the control point for this study.
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