GPS for Large Scale Surveying

Lt. Col(Dr.) B. Nagarajan
Survey Training Institute, Uppal, Hyderabad

GPS has proven to be a very accurate and cost effective tool for the surveys. B. Nagarajan discusses the basic concepts involved and various techniques adopted for precise GPS measurements. He also provides the details of two case studies undertaken at Survey Training Institute, Hyderabad to understand the capabilities of GPS techniques in large scale survey.

Regardless of the type of survey being conducted, most surveys share the need to establish, maintain, and record vertical and horizontal control in the region of the survey. 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 Global Positioning System (GPS) is one such technique, which meets the requirements of the surveying fraternity all over the globe.

Use of GPS (which was primarily developed as a navigation system by the U.S. Department of Defense) in surveying, can provide an accuracy of baseline length measurements in the order of one part per million on a regular basis, thus improving the measurement accuracy by one order more than what was achievable by the best measuring technique a decade ago. This high order achievability in measuring techniques on a routine basis has opened up various avenues in research and application to the whole scientific community in general and Surveyors in particular. The use of GPS technique for scientific applications warranting long range precise measurements viz., earthquake prediction and monitoring, interplate movement studies, crustal movement studies, geophysical positioning, etc., has already got the acceptance of scientific community for the past few years in our country itself and also in other parts of the world. However its introduction in large scale surveying, involving quick and precise short base measurements, is yet to get the approval from concerned quarters. The main reasons for the same may be:

1. insufficient information available to the user community regarding GPS technique’s capability in large scale surveying and
2. non availability of sufficient case study results carried out in Indian condition only to prove its worth compared to conventional surveying techniques normally done with theodolites and levels and lately with Electronic Distance Measurement equipments.

This paper attempts to explain certain basic concepts involved when surveying with GPS. the various techniques adopted for precise GPS, measurements and also its anticipated measurement accuracy and cost aspects. GPS applications in the field of large scale surveying especially in Cadastral, Town planning and Engineering surveying are discussed in detail with results obtained from case studies conducted at the Survey Training Institute, Survey of India, Hyderabad.

GPS for Large Scale Surveying
Can GPS Replace Conventional Surveying Technique?
Large scale surveys generally need short bases to be measured with more emphasis on the out turn achieved rather than on accuracy of measurements done. There are several survey equipments available in the market today viz. digital theodolites, Auto levels, Electronic Tacheometers, Short and Long range EDM instruments, Total stations etc., which can provide faster ‘field-to-finish’ solution, meeting the needs of large scale surveying. In this existing scenario, the main question that intrigues the minds of engineers and planners regarding the use of GPS in large scale surveying are the following:

• Sufficient expertise is already available for operating and making measurements using conventional survey equipments. Is it worth going in for an entirely new technique in place of existing techniques of surveying?
• The cost of GPS equipment is also feared to be exhorbitant; then why not stick on with the existing procedure itself?
• What will be the cost benefit ratio if one plans to switch over to the new technique?

Before venturing into answering these questions one should clearly understand that the satellite based GPS technique cannot replace the existing conventional ground survey techniques in all its entirety but can only augment its capability to provide a more accurate and in some cases faster survey solutions. The advantages of using GPS techniques are as follows:

• GPS measurements do not require intervisibility between the points. This allows ground monuments to be placed where they are needed and not on the top of mountains as is now necessary for want of line of sight.
• GPS technique provides a three dimensional position for the point. That is, in one go, we get the horizontal and vertical position of the point, unlike in conventional surveying where we need two operations : horizontal traverse for planimetric control and a level loop for height control.
• A very high accuracy measurement can be made in a relatively short time for baseline lengths of few hundred meters to few hundred kilometers and can provide the same accuracy anywhere on the earth, in almost any weather condition and at any time of the day.

Looking at the advantages claimed, it is not difficult to understand that GPS is a powerful surveying tool which can provide either on its own or in combination with other ground survey technique the required speed, accuracy and economy in large scale surveying.

Precise Positioning Measurement Techniques with GPS
There are two types of GPS observables that are of interest to the user. One of them is the Pseudorange, which is the distance synchronization error between them, and has a noise level of a few metres. The accuraccy of a single point positioning with pseudoranges is 10-15 m with a P code receiver and 15-50 m with C/A code receiver. With selective availability (SA) and Antispoofing (AS), the two intensional accuracy degrading techniques enforced by U.S. DoD, “ON” in the system, the single positioning accuracy has been reduced to 100 m, but better accuracy is achievable using postmission epimerides. If GPS data are collected by two receivers simultaneously differential positioning can be implemented. The differential positioning can be implemented. The differential accuracy using pseudoranges is 3-8 m and is mainly used for navigation.

Another type of observable that can be obtained from GPS is called ‘carrier phase’. It is basically a between-epoch range difference and has a noise level of a few millimeters. In a single point mode, carrier phase measurements do not significantly improve accuracy, as the orbital and atmospheric effects dominate the error budget. However, in differential mode base line accuracies of 1-3 ppm can be obtained using broadcast ephemerides in static mode. When post-mission precise ephmerides are used in the data processing, the base line accuracy is increased to 0.1 to 0.3 ppm in static mode. For precise positioning in surveying applications, carrier phase measurements are generally used in spite of the fact that it involves complex algorithms in its solution. Pseudo range measurements are restricted mostly for navigation solution. However techniques are being developed to utilise both code and phase measurements for successful GPS applications.

Since the differential positioning gets the common errors cancelled and thus improving the relative accuracy, we generally use this technique for precise positioning with GPS. Generally three differential positioning techniques are used when observing with GPS. They are:Static, Kinemetic and Rapid Static differential positioning techniques .

Static Differential Positioning In this technique at least two receivers collect carrier-phase data in stationary (static) mode for an extended period of time. Post processing software analyses all data simultaneously to obtain the differential position between the two receivers. Because the long observation sessions allow a careful treatment of systematic errors, static differential positioning yields more accurate results than any other technique. Therefore this procedure is used extensively for a variety of high-precision applications such as establishing national mapping control networks and monitoring of earth’s crustal deformations. Typical distances between receivers vary from several tens of kilometers to thousands of kilometers. Observation sessions of up to several hours may be required to achieve high accuracy over such long distances.

Kinematic Differential Positioning If all survey points are in a local area and all baseline lengths are within several kilometers, then some of the systematic errors in carrier phase measurements will be negligible and will have no effect on the differential positioning result. In that case one resort to kinematic differential positioning technique in place of static positioning. A very reduced length of station occupancy is the chief advantage to this technique. Kinematic differential positioning can generally be carried out as two different techniques.

1. Pseudo Kinematic Surveying: This method calls for one receiver to remain static at the reference site while another receiver occupies all remote sites in sequence. At each site the roving receiver collects measurements for a few minutes. After atleast one hour, the whole procedure is repeated and all remote sites are reoccupied. The procedure works best with a large number of sites to avoid waiting periods between stations reoccupations. The data collected in the first and second run are combined in a processing scheme similar to the one used in static surveying. User need not keep the remote receiver operating and locked on to the satellite signals while they move between sites, an advantage in areas with signal shading problem.
2. Stop-and-go surveyin: In this method, some times referred as semi kinematic surveying, the carrier-phase ambiguities are resolved, before the actual survey starts. Once the ambiguities are resolved, surveyors move one of the receivers through all the remote sites in sequence. In this mode, surveyors can accurately determine the differential position of the remote sites with observation periods as brief as few seconds. The restriction with this technique is that when the roving receiver moves between the remote sites, it must maintain phase lock to at least four GPS satellites for a successful survey.

If surveyors must traverse areas that create severe signal shading, such as the space underneath a bridge, the method is useless.

Rapid Static Surveying This is the latest addition to the catalog of GPS differential surveying procedures. It is essentially similar to conventional static surveying but features a vastly shortened site occupation time. The reduction in observation time primarily results from faster ambiguity resolution which is achieved either by combining pseudo range measurement technology with carrier – phase measurements or by making use of redundant carrier phase measurements.

Numerical Investigations
Comparing the various techniques available for precise measurements with GPS in the foregoing paragraphs, we see that the static positioning techniques demand more observation time resulting in fewer base line measurements, of course with greater accuracy. The truly Kinematic positioning outputs the results in a preset time interval resulting in greater out-turn and accurate positioning but not at the required ground points. For large scale surveying we need a technique that is in-between the static and true kinematic, which should give a greater out – turn and at the same time at the required ground points.Psuedo-kinematic and ‘stop and go’ techniques can be considered as the ideal GPS measurement techniques for large scale surveying purposes. Pseudo-kinematic technique can be used advantageously in areas where there is a fear of signal shading due to vegetation, built up areas, tall buildings and obstructions, as there is no requirement for the receiver to maintain its lock to the satellite during movement of the rover receiver. But in open areas ‘stop and go’ technique may prove useful.
 

At SurveyTraining Institute, Hyderabad we undertook two case studies for understanding the capabilities of GPS technique in large scale surveying as part of our training and research programs. Since we felt that signal shading is going to be a problem in most of the areas in Indian context it was decided to use Psuedo – kinematic technique for both the case studies.

Case Study 1
AimThe Aim of this study was to demonstrate the possible applications of GPS technique in large scale mapping and compare its planimetric positioning accuracy with that of an electronic total station as it being the latest ground based technique for providing accurate ground control. Efforts were also made to compare the heights derived from GPS observations with that of Double Tertiary Leveling network.

Procedure As explained earlier, one master receiver was left at the base station, (i.e. a known Survey mark), making continuous observations. The rover receiver was moved to successive sites having observation at each site for 5 to 10 minutes. The cycle was repeated after an hour with same receiver occupying the sites successively for the second time. The radial network pattern of observation sites is shown in Figure 1.

As explained in the literature, the two 5 to 10 minutes observations separated by an hour are functionally equivalent to a static survey hour long base line observation. Therefore the same software used for static differential positioning can be used for computing the base line measurements in this case also. The processed results give the differential positioning of the various sites with respect to the base station i.e. x, y, z in meters. As the coordinates of the base station are known on Everest ellipsoid it can be converted into corresponding cartesian coordinates. The differential corrections to various sites can then be added to get the coordinates of the various sites in cartesian frame corresponding to Everest Spheroid. We can convert the entire sets of co-ordinates in terms of Everest Spheroidal co-ordinates. Depending on the choice of suitable map projection, the spheroidal coordinates can be converted to grid co-ordinates.

Results The planimetric control in terms of Easting and Northing (x,y) coordinates were then compared with the coordinates obtained using ‘SOKKIA SET IIC Intelligent Total Station’. The compared results are shown in Table-2. Similarly the Dh or the height differences between the sites obtained from GPS technique were compared with the height differences obtained using Double Tertiary Leveling as well as with that derived from Total Station. The results are shown in Table 3.

Discussion of Results From Table 2 we see that the standard deviations of differences in easting and northing are in the order of sE = 0.260 m and sN = 0.065 m respectively, suggesting an inaccuracy of 27 cm in planimetric positioning by GPS technique when compared to Total station traverse measurements.

In large scale surveying, say for example 1:1000 mapping the pIotable error is about 25 cm. In such an event, the inaccuracy of 27cm, in positioning is quite acceptable. As regards the height also, Table 3 shows that the GPS derived heights show a maximum deviation of 21 cm when compared to DT leveling heights. For 50 cm or 1 m contouring the accepted accuracy limit is upto 25 cm or 50 cm respectively, being half the value of contour interval. Therefore maximum error of 21 cm can be easily tolerated when GPS relative heights are used for contouring purposes also.

Case Study 2
Aim The aim of the study was to investigate the capability of Kinematic Differential GPS technique in large scale mapping when combined with conventional ground survey techniques for detail surveying. The problem chosen was to compare the Cost Benefit ratio when one carryout 1:5000 large scale original surveying with conventional plane tabling and then with GPS technique combined with Total station detail survey.

Procedure An area of 1 km x 1 km was chosen in Ibrahimpatan village in RR District of Andhra Pradesh. The test area was chosen in such a way it covers built up area, industrial area and also agricultural area. Pseudo-kinematic technique with GPS was used for the first two days to provide diversified control network in the test area. With this precise frame work Total station equipment was used to pick up the coordinates of all the identifiable features on the ground that may be required to be plotted on scale 1:5000. The surveyor used three prisms (one provided by the vendor and other two fabricated in No.l6 Party (STI) itself) for data collection with the Total Station. In four days with 4 to 6 hours observations, about 500 points were picked up within that selected area of 1 km x 1 km. A planetabler accompanying the surveyor with total station not only plotted the surveyed points in the field itself, but also completed the field drawing of topographical features. The time taken for various tasks are listed in Table 3.

Discussion of Results A copy of the map indicating the area of survey using combination of GPS and Total Station techniques is shown in figure 2. The task which generally required 30 to 35 mandays when using conventional method of planetabling has been completed in just 15 mandays. If a surveyor and planetabler are both employed together, the entire survey can be completed in just 8 working days. The advantages claimed are:

It s a quick and time saving method cutting down the requirement of field work to minimum

• This method provides a more accurate mapping of the area than the conventional method as the accuracy of distance and height measurements made are better
• We get both digital and analog map of the area at a time
• Ideal for large scale project surveys, Cadastral and Engineering surveys; also suitable for developing and updating digital databases for GIS and LIS applications.

Suggested GPS Applications in Large Scale Surveying

Cadastral Surveying
In developing countries like India where there is greater pressure on land and land use, Cadastral records in which individual property lines are accurately demarcated and the area of different land uses are clearly identified are quite essential. As our case study 2 shows, Pseudo-kinematic techniques with GPS in combination with Total Station equipment can provide faster, economical and accurate solution for quick updation and maintenance of cadastral records. While the village boundary pillars and permanent features within the village can be accurately controlled using GPS technique, Total station can be used for providing control points at the individual land holding boundaries. As the two techniques GPS & Total station traverse have already proven to provide great precision in linear measurements, the cadastral authorities can hope to have a reliable set of coordinates avoiding unnecessary litigations and court cases. It can also help in rational computations of land tax and property evaluation.

Town Planning and Engineering Surveys
From the list of advantages using GPS technique, we see that no intervisibility between points is required as well as GPS survey can be carried out at any time whether day or night to get the same greater accuracy. These qualities make GPS a good contender for Town Planning and Urban surveys where the surveying work may have to be done during the night only, due to traffic and other disturbances during day time. Pilot studies have also proved that when new precise geoid models are available, relative GPS positioning techniques can provide orthometric heights in the order of ± 5 cm. accuracy. This capability can be advantageously used to build up digital terrain models (DTM’s) of dense grid. The use of different density gridded DTM’s in engineering and scientific applications are well known. The main applications being rail route alignment and monumenting the chosen rail-road corridor, earth cut/fill computations in tunneling and other engineering projects etc.

Meeting special requirements of Mine surveying
Though mine surveying employs the same principles and instruments used in other type of surveying, certain requirements make mine surveying uniquely different from other field surveys. For example underground surveys

1. require establishing control on the roof above the surveyors to avoid loss of station markers resulting from haulage traffic
2. require multilevel referencing and control to maintain planning coordination between underground working with surface
3. require a consistent control network that will enable the surveying team to reestablish stations after they are lost as frequently occurs from roof falls or contact with haulage equipment and
4. require to work in dusty and unstable surroundings

Since the establishment of survey control stations using GPS techniques does not warrant intervisibility between the survey points, the ground monuments required to be established for further detailed survey can be located at places where they are needed. Also since GPS observations can be carried out in almost any weather condition at anytime of the day, the potentiality of GPS technique to meet Mine Surveying requirements is worth testing.

Conclusions
GPS has proven to be a very accurate and cost effective tool for the surveys of today. Though GPS will not completely replace conventional equipment, it will definitely revolutionise the way, land, engineering and mine surveys are now performed. Precise positioning receivers and versatile GPS softwares are available in our country itself at affordable prices. Awareness about the utility of this space geodetic technique has also increased many fold amongst various scientific organisations in India today including Survey of India. With the proven capability of this technique to provide precise control network with least cost of time and money and the cost of equipment decreasing dramatically with increased users, the days are not far when the utility of GPS is going to be felt in every sphere of an activity including Large Scale Surveying.

References Cannon, M.E., The Contribution of GPS to the Information Society, SICM Journal ACSGC, Vol.44, No.3, Autumn, 1992