| GPS Consumer Series: You Get What You Pay For
The differences between GIS data capture tools and consumer-oriented GPS receivers
By Chuck Gilbert Today, GPS receivers range in price from as little as $250 USD to several tens of thousands of dollars. Very few seemingly similar products have such a wide price range. It is difficult for some people to imagine what characteristics can justify a 200-to-1 price difference. However, the justification is there for those who look closely at the product capabilities.
It is easy to recognize the value of a centimeter accurate real-time survey system with the ability to store thousands of points, compared to a 100 meter accurate toy that cannot even store the data it computes. However, the distinction is less obvious between a low-end GIS data capture tool and a relatively sophisticated consumer-oriented GPS receiver for campers, boaters and aviators. Looks like a rose, but...
In the realm of GIS data capture tools, the prices range from about $3,000 USD to $20,000 USD. Meanwhile, the consumer oriented products are typically between $250 USD and $1,000 USD. Often these two classes of products have a similar appearance and operating characteristics; even the accuracy specifications can be identical. At a glance, it may be difficult to understand what justifies such a dramatic price difference. The visual similarity is usually due to the GPS manufacturers saving a little on production costs by sharing similar components, such as the exterior casing. However, the internal components and the software can be vastly different.
What does one obtain for the huge difference in price? There is a lot of variation in the GPS industry, but the following examples elaborate on the most important differences between the GIS data capture tools and the hand-held GPS toys of wealthy yachtsmen. Feature Attributing
The point of using a GIS data capture tool is to obtain spatially accurate attribute information for a GIS. The GPS receiver does supply a position, however, the attribute data is really the focus of the data collection.
A consumer-based receiver will usually have little-to-no attribute collection capabilities. The design goal of these receivers is to assist in navigation. Most consumer GPS receivers only allow the user to store a small number of locations and names as waypoints for navigation, but little else. Often the waypoints and names cannot even be exported.
In contrast, a well designed GIS data capture tool will allow the user to collect a wide variety of point, line, and polygon features along with detailed attribute values for each feature. The features and attributes that can be collected should be customizable so that they are a direct reflection of the user's GIS database structure. Based upon whether any particular feature has been defined as a point, line, or polygon, the GPS data collection system should handle the associated data appropriately. For example, all of the data associated with a point feature should be combined or averaged to create a single position representative of the point feature. If the feature is linear, all of the associated data should be combined in the form of a single continuous line. When the user is collecting a polygon feature, the polygon should be closed automatically by the GPS system so that the users can complete the job by traversing, say, only three of four polygon sides. When appropriate, the GPS system should be able to automatically generate label points to which the attribute data will be attached. This saves the user the need to collect or create an additional point out in the middle of a lake or a forest fire.
The user should be able to store hundreds of attribute values along with every feature, and the GPS system should inspect the integrity and validity of all attributes as they are entered or stored. With such data validation the user will be warned if an inappropriate attribute value is entered; such as a pole height of "Green," or a Road Condition of "8.5 meters." In the more sophisticated GIS data collection systems, the user can return to any particular attribute value and edit the data that was entered in the field.
A GPS system that was designed specifically for the collection of GIS data will be able to support the special needs of the GIS user. There will be controls that allow the user to momentarily pause and resume data collection. With such a system, users should also be able to handle dynamic segmentation situations; such as line segments that have multiple, continuously or frequently changing attribute values. Additionally, a well designed system will handle the common scenario wherein the user wants to collect point features that are positioned along or near a line feature. The user will be allowed to collect those nearby point features without having to drive the road twice or to stop and start the line feature multiple times.
Finally, a system designed for the collection of GIS attribute data should have a powerful and flexible software package that can be used to export the attribute and position data into a variety of GIS interchange formats. The more powerful packages will automatically export different features to specific layers in the target GIS. Additionally, a well designed export program will also create macro files or batch files that will aid the user in importing the data to a particular GIS. Accuracy comes in many flavors...
Non-Differential Accuracy
Although the accuracy specifications might be identical, it is crucial that the user considers exactly how that accuracy is achieved. For example, some consumer products claiming accuracy in the 10 meter range can achieve this only by averaging multiple, non-differentially correctable positions together. This is a nice idea, however, the time required to reliably average down to less than 10 meters varies from 30 minutes to several hours, depending upon the severity of Selective Availability. (Editors Note: For more on Selective Availability, see this month's GPS Q&A on pages 42-43.) The result is that the user must wait, stationary, at every location for a long time; and even then may have questionable results. In contrast, the GIS data collection tools will use differential GPS (DGPS) in order to obtain high accuracy as quickly and reliably as possible. Virtually any GIS data collection system will generally obtain better than five meter differential accuracy after only one second, whether stationary or moving. The more sophisticated GIS data collection systems will continuously achieve differential sub-meter accuracy on a second-by-second basis.
Be wary also about how the accuracy has been specified. The specification of some products will stretch the truth by quoting the accuracy at only the 50th percentile, meaning that half the data falls within the specified accuracy and half does not. However, most professionally designed tools for GIS data capture will quote accuracies at the 68th or 95th percentile. Differential Accuracy
Most consumer products are capable of computing differentially corrected positions, but only in a very limited manner. Such DGPS capable consumer products boast an accuracy of two to five meters. However, the 2-5 meter accuracy can be obtained only when the receiver has a telemetry link that connects it, in real-time, to a GPS base station. Such a telemetry link can easily cost much more than the GPS receiver! Additionally the necessity of the real-time link often will severely limit where the differential GPS is possible. If the user ventures beyond the range of the real-time link, the accuracy immediately is degraded to the 100 meter range.
A common alternative to the user-supplied telemetry link is that the user subscribes to a DGPS broadcast service. However, such services are a recurring cost that ranges from about $50 USD per month to over $500 USD per month; depending on the quality and range of differential coverage.
GIS data capture tools are generally capable of utilizing such a link if the user should so desire, however, it is not a requirement. The typical GIS data capture tool can compute DGPS positions in either real-time or as the result of a very short and simple post-processing procedure. In fact, most users who use GPS for GIS data collection perform this post-processing at the time data is transferred to their computer for the GIS. Quality Control
Another advantage of the GIS data collection systems is the ability to obtain quality assurance information about the data that you collect. This capability varies widely by manufacturer across the various GIS data collection systems. In the realm of consumer products it is almost always totally absent. In the few consumer products that do provide a degree of quality assurance information it is generally so limited as to be useless. Note that quality assurance principles should be applied not only to the position data, but also to the attribute data that is collected. Summary
Only two major points have been addressed above; accuracy and attributing. There are many additional important factors that are also overlooked by the consumer receivers, such as the data capacity, the reliability in hostile environments, external sensor recording, or the user's ability to edit or manipulate the data.
Really, the differences are so wide-spread and profound that the two types of receivers, consumer and GIS data collection, shouldn't ever be directly compared. GIS data collection systems are tools, designed for professionals, to solve complex problems. The consumer oriented receivers are best categorized as toys. An extremely sophisticated and powerful toy to be sure! But still, designed with recreational navigation as the primary design goal. Although some users may be tempted to save a little money and buy a low-cost consumer receiver instead of the appropriate GIS data capture tool; I do not recommend this any more than I would suggest using a consumer receiver to navigate a commercial aircraft. The risks to data integrity and the ease of use penalties are so severe that any initial savings are lost almost immediately as a result of wasted time collecting data and low quality results. Good luck, and as always, test it yourself. About the Author:
Chuck Gilbert has over a decade of experience as a GPS user. He has been employed as an applications engineer for Trimble Navigation since 1989. Back |
