The Nature of Geographic Information

25. Summary


Positions are a fundamental element of geographic data. Sets of positions form features, as the letters on this page form words. Positions are produced by acts of measurement, which are susceptible to human, environmental, and instrument errors. Measurement errors cannot be eliminated, but systematic errors can be estimated and compensated for.

Land surveyors use specialized instruments to measure angles and distances, from which they calculate horizontal and vertical positions. The Global Positioning System (and, to a potentially greater extent, the emerging Global Navigation Satellite System) enables both surveyors and ordinary citizens to determine positions by measuring distances to three or more Earth-orbiting satellites. As you've read in this chapter (and may know from personal experience), GPS technology now rivals electro-optical positioning devices (i.e., "total stations" that combine optical angle measurement and electronic distance measurement instruments) in both cost and performance. This raises the question, "If survey-grade GPS receivers can produce point data with sub-centimeter accuracy, why are electro-optical positioning devices still so widely used?" In November 2005, I posed this question to two experts--Jan Van Sickle and Bill Toothill--whose work I had used as references while preparing this chapter. I also enjoyed a fruitful discussion with an experienced student named Sean Haile (Fall 2005). Here's what they had to say:

Jan Van Sickle, author of GPS for Land Surveyors and Basic GIS Coordinates, wrote:

In general it may be said that the cost of a good total station (EDM and theodolite combination) is similar to the cost of a good 'survey grade' GPS receiver. While a new GPS receiver may cost a bit more, there are certainly deals to be had for good used receivers. However, in many cases a RTK system that could offer production similar to an EDM requires two GPS receivers and there, obviously, the cost equation does not stand up. In such a case the EDM is less expensive.

Still, that is not the whole story. In some circumstances, such as large topographic surveys, the production of RTK GPS beats the EDM regardless of the cost differential of the equipment. Remember, you need line of sight with the EDM. Of course, if a topo survey gets too large, it is more cost effective to do the work with photogrammetry. And if it gets really large, it is most cost effective to use satellite imagery and remote sensing technology.

Now, lets talk about accuracy. It is important to keep in mind that GPS is not able to provide orthometric heights (elevations) without a geoid model. Geoid models are improving all the time, but are far from perfect. The EDM on the other hand has no such difficulty. With proper procedures it should be able to provide orthometric heights with very good relative accuracy over a local area. But, it is important to remember that relative accuracy over a local area with line of sight being necessary for good production (EDM) is applicable to some circumstances, but not others. As the area grows larger, as line of sight is at a premium, and a more absolute accuracy is required the advantage of GPS increases.

It must also be mentioned that the idea that GPS can provide cm level accuracy must always be discussed in the context of the question, 'relative to what control and on what datum?'

In relative terms, over a local area, using good procedures, it is certainly possible to say that an EDM can produce results superior to GPS in orthometric heights (levels) with some consistency. It is my opinion that this idea is the reason that it is rare for a surveyor to do detailed construction staking with GPS, i.e. curb and gutter, sewer, water, etc. On the other hand, it is common for surveyors to stake out property corners with GPS on a development site, and other features where the vertical aspect is not critical. It is not that GPS cannot provide very accurate heights, it is just that it takes more time and effort to do so with that technology when compared with EDM in this particular area (vertical component).

It is certainly true that GPS is not well suited for all surveying applications. However, there is no surveying technology that is well suited for all surveying applications. On the other hand, it is my opinion that one would be hard pressed to make the case that any surveying technology is obsolete. In other words, each system has strengths and weaknesses and that applies to GPS as well.

Bill Toothill, professor in the Department of GeoEnvironmental Sciences and Engineering at Wilkes University, wrote:

GPS is just as accurate at short range and more accurate at longer distances than electro-optical equipment. The cost of GPS is dropping and may not be much more than a high end electro-optical instrument. GPS is well suited for all surveying applications, even though for a small parcel (less than an acre) traditional instruments like a total station may prove faster. This depends on the availability of local reference sites (control) and the coordinate system reference requirements of the survey.

Most survey grade GPS units (dual frequency) can achieve centimeter level accuracies with fairly short occupation times. In the case of RTK this can be as little as five seconds with proper communication to a broadcasting 'base'. Sub-centimeter accuracies is another story. To achieve sub-centimeter, which most surveyors don't need, requires much longer occupation times which is not conducive for 'production' work in a business environment. Most sub-centimeter applications are used for research, most of which are in the geologic deformation category. I have been using dual frequency GPS for the last eight years in Yellowstone National Park studying the deformation of the Yellowstone Caldera. To achieve sub-centimeter results we need at least 4-6 hours of occupation time at each point along a transect.

Sean Haile, a U.S. Park Service employee at Zion National Park whose responsibilities include GIS and GPS work, takes issue with some of these statements, as well as with some of the chapter material. While a student in Fall 2005, Sean wrote:

A comparison of available products from [one manufacturer] shows that traditional technologies can achieve accuracy of 3mm. Under ideal conditions, the most advanced GPS equipment can only get down to 5mm accuracy, with real world results probably being closer to 10mm. It is true that GPS is often the faster and easier to use technology in the field when compared to electro-optical solutions, and with comparable accuracy levels has displaced traditional methods. If the surveyor needs to be accurate to the mm, however, electro-optical tools are more accurate than GPS.

There is no way, none, that you can buy a sub-centimeter unit anywhere for $1000-2000. Yes, the prices are falling, but it has only been recently (last three years) that you could even buy a single channel sub-meter accuracy GPS unit for under $10,000. The units you mention in the chapter for $1000-2000, they would be 'sell your next of kin' expensive during that same time period. I am not in the business of measuring tectonic plates, but I deal with survey and mapping grade differential correction GPS units daily, so I can speak from experience on that one.

And Bill's response that GPS is well suited for all survey applications? Well I sincerely beg to differ. GPS is poorly suited for surveying where there is limited view of the horizon. You could wait forever and never get the required number of SVs. Even with mission planning. Obstructions such as high canopy cover, tall buildings, big rock walls... all these things can result in high multi-path errors, which can ruin data from the best GPS units. None of these things affect EDM. Yes, you can overcome poor GPS collection conditions (to an extent) by offsetting your point from a location where signal is good, but when you do that, you are taking the exact measurements (distance, angle) that you would be doing with an EDM except with an instrument that is not suited to that application!

The Global Navigation Satellite System (GNSS) may eventually overcome some of the limitations of GPS positioning. Still, these experts seem to agree that both GPS and electro-optical surveying methods are here to stay.

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