5.1.2 GPS Satellite Surveying: Some Considerations

FACTORS INFLUENCING THE ADOPTION OF GPS FOR LAND SURVEY APPLICATIONS



GPS does not need to be only competitive against conventional terrestrial techniques of surveying, but also other extraterrestrial techniques and new technologies. GPS relative positioning technology can, in principle, be employed for a wide range of activities. As a starting point in discussions concerning the special advantages (and disadvantages) of GPS for land surveying applications, applications are classified into three general categories (section 2.3.2):

Class A (Scientific): better than 1 ppm
Class B (Geodetic): 1 to 10 ppm
Class C (General Surveying): lower than 10 ppm.

Several criteria for judging the utility of GPS over other competing technologies can be identified for each of these categories:


GPS vs Conventional Terrestrial Surveying -- ADVANTAGES:

Intervisibility not required

Operations are weather independent

Network independent site selection, hence sites placed where needed

Around-the-clock operation

Economic advantages from greater efficiency and speed of survey

Geodetic accuracies easily achieved

3-D coordinates are obtained


GPS vs Conventional Terrestrial Surveying -- DISADVANTAGES:

High productivity places greater demand on survey planning and logistical considerations

No sky obstructions can be tolerated, therefore cannot be used underground, under foliage or structures

GPS surveying is generally "targeted" to satisfy a specific survey need

No azimuth control for subsequent non-GPS surveys

Horizontal and vertical coordinates from GPS must be transformed if they are to be useful for conventional survey applications

GPS accuracies are generally higher than the surrounding existing control

High capital cost of GPS instrumentation

New skills needed

	

Geophysical, Engineering, Large Scale Cadastral and Mapping Surveys


The overwhelming majority of civilian GPS surveys will be carried out in support of geophysical prospecting activities, engineering projects, large scale point coordination for Geographic Information System (GIS) data collection, land parcel surveys, and for map control. Indeed, GPS is ideally suited for surveys associated with the establishment of a coordinated cadastre (apart from the constraints of sky visibility in highly built-up areas). The relative accuracy requirements are likely to be of the order of 1 part in 104 to perhaps better than 1 part in 105.

The majority of these prospective users will have had little or no previous experience with satellite surveying, therefore the introduction of GPS into surveying practices will be mainly influenced by cost-benefit considerations. Reservations concerning GPS technology will only be overcome if the surveyor is convinced that GPS could perform the positioning function, to the required accuracy, in a shorter time and with greater efficiency (and hence less cost) than any other technique. This means that the surveyor would have to satisfy himself/herself that the GPS positioning technology is superior to the conventional EDM and theodolite procedures with which he/she is already very familiar.

Although a definitive statement on the relative competitiveness of GPS and EDM-theodolite technologies is not possible, some criteria for evaluating competitiveness can be identified:


When GPS is forced to operate in a pseudo-traversing mode, EDM-theodolite techniques maintain an edge in competitiveness over the standard static (or even "rapid static") GPS baseline survey mode. However, there is probably a critical station-separation above which GPS would be the more efficient technique. Initially, interstation distances of ten or more kilometres (less in rugged terrain) are likely to be most efficiently bridged using GPS. Shorter baselines can be economically surveyed using modern GPS surveying techniques in which station occupancy times have fallen to just a few minutes or less. An additional factor to be considered is that GPS gives height information as well, although this height is not orthometric elevation and must be "corrected" by subtracting the geoid height quantity.


Receiver Cost

The average cost of an electronic tacheometer (or "total station") is between $10000 and $20000. As a result of small production runs, and the fact that such instruments are a combination of mechanical, optical and electronic components, the costs of these surveying instruments have not undergone the dramatic price reductions experienced in the handheld calculator or personal computer market. Aside from inflation, the prices of traditional surveying instruments are not expected to vary significantly through the 1990's. Prices of over one hundred thousand dollars were the norm for first generation GPS receivers. However, instrument prices have been falling and third generation phase measuring instrumentation is already of the order of US$10000 to $15000 for the basic models and perhaps double this for top-of-the-line "geodetic" receivers. There is a greater interest in "enhanced" GPS navigation receivers. These, when operated in differential mode, can deliver accuracies from a few metres to sub-metre over baselines up to hundreds of kilometres, and generally cost below US$10000. (A pair of receivers would be needed for relative positioning, hence doubling the cost.)


Ease of Operation

GPS will be more readily adopted if it can be shown to be at least as easy to use as medium or long-range EDM. That is, GPS observation and reduction procedures should not require a high degree of operator training. To allow in-the-field positioning (or "real-time" processing), the processing software should be resident in the receiver, or on a portable field computer, and include communication links between receivers and easy-to-use software that requires a minimum of operator intervention. Differential GPS operation using pseudo-range data can deliver low accuracy results in a largely "automatic" mode and is hence ideally suited for GIS mapping applications.


Productivity

As surveyors, it is usual to compare GPS efficiency with that of EDM-theodolite procedures in terms of time spent in the field acquiring data. The fallacies in such comparisons are illustrated by the following three survey scenarios:

 

Geodetic Surveys


GPS has already replaced TRANSIT for establishing, maintaining and densifying geodetic networks. Over distances of a few hundred kilometres, multi-station TRANSIT gave relative positioning accuracies of a few decimetres (this figure is largely independent of station spacing). Unlike TRANSIT, GPS can give accuracies of 1ppm even over distances as short as a few kilometres. Furthermore, GPS is much faster, with measurement times as short as half an hour (in standard mode), or just minutes (for "stop & go" techniques), being required to measure a baseline. GPS can also compete with conventional methods which are slow, labour intensive, and suffer from unfavourable error propagation that degrades accuracies over long distances.

Unlike triangulation, traversing and trilateration, control networks established with GPS are not bound by constraints such as station intervisibility and network shape. With GPS the spacing between control stations in unsurveyed regions can be increased over traditional techniques. The increased flexibility of GPS also permits control stations to be established at easily accessible sites rather than being confined to hilltops as has hitherto been the case.

According to the "rule-of-thumb" in section 6.2.3, the orbital information necessary to support 1ppm surveying should be accurate to 20 metres or better. At present the Broadcast Ephemerides appears to be adequate for surveying applications at the part per million level. Since immediate positioning results are not needed for the applications considered here, post-processed ephemerides could also be used. These could be obtained from an "ephemeris service" such as the International GPS Service (IGS), as discussed in section 6.2.3 and section 12.2.1.

Frequently, the relative positioning results obtained with GPS have a higher internal precision than that of the geodetic network to which they must be tied. Discrepancies between GPS and published coordinates will cause headaches for surveyors carrying out high precision GPS surveys for many years to come. Should the GPS-derived values be distorted by performing an adjustment in which the local control is held fixed? Alternatively, should only the minimally constrained solution (holding only one station fixed) be insisted upon? These are issues that will be raised in later chapters. The entire geodetic network may ultimately need to be strengthened and redefined using GPS as is happening in many countries, including Australia.

Scientific Surveys


The measurement of crustal deformation is central to our understanding of earthquake processes, plate motion, rifting, mountain building mechanisms and the near-surface behaviour of volcanoes. The scale of deformation associated with these processes varies from tens to thousands of kilometres, but much of the deformation of interest is found near plate boundaries that are typically 30 to 300km wide. The requirement for crustal motion studies is distinctly different from routine surveying and mapping requirements in that the objective is not one of establishing absolute coordinates, but of measuring changes in position, displacement or strain with time. Hence one seeks to repeat the measurements under as nearly an identical set of circumstances and to as high an accuracy as possible. The relative positioning accuracies that are obtained for GPS crustal movement surveys are typically between 107 and 108 (that is, a few centimetres in 1000km!).

Until the 1970's, terrestrial techniques provided the only geodetic observations of presentday crustal movements. Over the last two decades measurements using Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) technologies have resulted in accuracies of several centimetres over distances of many thousands of kilometres. Unfortunately, the equipment required and the personnel costs have discouraged the use of such systems in the density necessary to study crustal deformation mechanisms at the shorter baseline lengths that are of interest in most seismic zones. Therefore, although VLBI and SLR are very valuable methods for the determination of global plate movements and large scale deformation within the plates, their future general use for baselines less than roughly 300km in length can no longer be justified from an economic point of view.

The cost of making differential measurements using GPS receivers is low enough to consider their use for large numbers of crustal movement measurements with baseline lengths ranging from a few kilometres to thousands of kilometres. The GPS receivers are inexpensive compared to SLR and VLBI equipment and the personnel requirements can be reduced to one person per receiver (or none at all when operated in the automatic base station mode). In fact, GPS provides a much needed technique to bridge the "spatial gap" between conventional trilateration-triangulation for short baselines and VLBI-SLR for long baselines. GPS will therefore be applied to many of the geophysical problems presently studied using ground techniques over 5 to 30km baselines. There is also a trend to using GPS for baselines up to several thousand kilometres long, in order to densify the global network of fundamental stations beyond those established by SLR or VLBI techniques.

This high accuracy mode is achieved using "top-of-the-line" receivers and very sophisticated "scientific" software. In addition to being used by specialists, the trend is now to use these techniques to strengthen the basic datum network so that "standard" GPS (geodetic) surveys at the ppm level can be easily incorporated within such a "zero-order" network without the distortion that presently has to be experienced when GPS results are combined with earlier conventional survey results.

The vertical accuracy, while it is less than that obtained for the horizontal components, will have a major impact on geodynamic studies. The cost is much less than for precise levelling for distances greater than a few kilometres. Elevation differences are obtainable with much greater ease and may be undertaken by a small group of investigators. This requires relatively little expenditure of time, compared to the effort and large team required for first-order levelling. Furthermore, ellipsoidal height differences are obtained directly, not orthometric height differences, which must be corrected for any geoid height changes to give the true geometrical (ellipsoidal) height change.

Ionospheric refraction effects on the GPS measurements can be quite large over the distances considered here. The effects at either end of the baseline would not necessarily be the same and thus would not be expected to cancel in long baseline measurements. Dual-frequency receivers capable of measuring and recording phase on the L1 and L2 frequencies are therefore essential for high precision geodetic applications. Another significant error source is in modelling the tropospheric refraction error. Although the dry part of the atmosphere can be adequately modelled, the wet part, due to the tropospheric water vapour, is more difficult to determine.

A major limitation in GPS baseline measurements is the accuracy of the satellite ephemerides used in the data reductions. For survey accuracies of 1 part in 107 or better, ephemerides accurate to the sub-metre level are required. Only carefully computed post-processed ephemerides would satisfy these accuracy requirements. Post-processed ephemerides also contain reference system information which must be reconciled with survey datums. The IGS was established with this dual role very much in mind: a source of high precision satellite orbit information, and a means of defining and maintaining a global terrestrial reference datum.

Summary Remarks


Just as GPS complements the other space techniques for high precision applications, it will also complement the traditional EDM-theodolite techniques for routine surveying activities. Indeed the traditional techniques are likely to continue playing the dominant role for some time to come -- for traverses less than a few kilometres in rugged terrain or less than a ten or so kilometres in flat terrain. For longer distances GPS is a potential competitor. However this is more likely to occur if the cost of modern GPS receivers capable of high speed (even real-time) surveying falls to a level comparable with "total station" instrumentation.

Figure below illustrates the accuracy and spatial ranges of the various geodetic methods of positioning, and reinforces the remarks made above concerning the "niche" that GPS occupies between EDM-theodolite traversing at the short range end of the survey spectrum and SLR/VLBI techniques at the long range end. (Note that the x-axis of the plot is logarithmic!)


Accuracy and ranges of geodetic methods of positioning.


As already mentioned, GPS surveying has several unique characteristics related to:


Elements of the GPS survey task:


Definition of the task: how many points? accuracy required? horizontal & vertical? connect to datum? distribution of points? resources available? etc.

Planning: logistical and structural considerations, connection to control, standards & specs for GPS surveys, number of receivers/parties, site selection, observation schedule, etc.

Reconnaissance: satellite visibility & availability, site conditions & access, station marking, etc.

Field procedures: equipment checklist, on-site procedures.

Post-processing & result presentation: baseline processing, minimally constrained solutions, fitting GPS network results to geodetic control, QC, heights, etc.

 


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© Chris Rizos, SNAP-UNSW, 1999