GPS Q&A: Industry experts answer reader's GPS questions

Q. What applications are best suited to GPS data collection and why?

A. John Bohlke, Sokkia Corp.: GPS serves as an ideal tool for GIS, navigation and surveying applications. The automotive, surveying, marine, military and aviation industries appreciate the benefits of GPS. GPS enhances many GIS applications such as utility inventory, resource mapping, environmental management and infrastructure management. Other GPS applications include precision agriculture, property management and public health and safety mapping.

Wendy Corcoran, NovAtel Communications: GPS applications that would be best suited for data collection are those that do not require immediate guidance or results such as in real-time, but even some real-time will have to have raw data backed up for future reference. These applications are typically in the Survey and GIS fields. Although there are exceptions, the majority of Survey and GIS projects will collect data in the field and then process the information within a few days to assure the quality of the survey. Even when these applications are used in real-time though, data collection should still be done as insurance in cases where problems arise with the project - specifically legal ones. If position only is stored for a Survey or GIS project, there is no way to verify an incorrect position or possibly correct it if the raw data is not available.

Craig Hudson, II Morrow: GPS technology can benefit any application that needs geographical input. Some of the applications that have successfully integrated GPS positioning are asset mapping, resource management and geographical planning. If your application is unique, you will need a greater understanding of GPS technology to insure success. Understanding the environment of your application and the limitations of GPS is critical. Applications in high density urban areas can be frustrated by satellite "blockage" and multipath errors. This is a common problem and GPS users need to remember that the GPS signal is "line of sight" from the satellites.

Arthur Lange, Trimble Navigation: GPS/GIS data collection systems are capable of recording features and their attributes in addition to GPS position. The GPS position is one of the many details that can be stored with a well designed GPS/GIS data capture system.
      GIS managers who have a need to collect many types of field information can increase their productivity by utilizing an up to date GPS/GIS data capture tool. The list of specific applications that are well suited to GPS is as long as the list of disciplines that use field data. In fact, it seems that for every application that the GPS manufacturers can collectively think of, users come up with two or three more!
      Here are a few generalities. A manager who has field assets and inaccurate maps could profit from using GPS to improve their map accuracy. Use the GPS/GIS data collector to directly digitize the asset's location, as built. Managers with paper maps and a need to move to digital data will benefit from using GPS to quickly and accurately collect point, line, and polygon features. A GIS manager with an extensive GIS database will use GPS to validate the accuracy of their existing features. Note that GPS/GIS data capture systems can be used to validate the full set of attribute data in addition to location. Remote imagery can be geo-rectified very quickly and accurately with GPS.
      Presently, uses of GPS for GIS data collection are using GPS in applications as diverse as mapping the plots in cemeteries (another simple example of asset management) to mapping animal waste in the Australian outback for the purposes of overgrazing control. The possibilities are limitless.

John E. Stenmark, Leica Surveying Group: GPS is working its way into some unexpected areas. For this discussion we will concentrate on four 'traditional' applications.
      Surveying - GPS provides very fast and highly accurate acquisition of survey-grade (1cm) measurements. It has revolutionized many aspects of surveying. Differential GPS is required for the surveying operation. Typical survey applications include cadastral, seismic/geophysical, topographical, mining and hydrographic surveys.
      Mapping - GPS is fast and flexible. It can be used by a single operator operating independently. Mapping accuracies of 1cm to 1-5m are easy to achieve. Like the surveying operation, differential GPS must be used to produce this accuracy.
      Navigation - Navigation with GPS is fast and portable. It can be used anywhere and does not require any differential processing. Typical accuracy is 25-100m. Precise navigation (1-5m) is performed using real-time differential techniques. This is used for vehicle control and other applications.
      GIS Data Collection - GPS is excellent for collecting features and attributes. When used in a differential process you will enjoy mapping accuracy. Onboard software guides the user through the acquisition of the location of a feature as well as additional attribute information about it.

Q. What's the difference between "survey grade" GPS receivers and "mapping grade" GPS receivers?

A. Bohlke: The survey and mapping grade classification of GPS receivers refers to general expectations of accuracy, functionality and cost. The manufacturing of survey grade receivers (also called geodetic receivers) focuses primarily on accuracy. Survey grade receivers can collect data with centimeter accuracy or better. For that reason, these receivers are suitable for many applications that normally require traditional survey equipment.
      In contrast, functionality is the primary focus when manufacturing a mapping grade GPS receiver. Mapping grade receivers usually include data collection capabilities that allow the user to identify features and record their attributes. This classification of receivers generally provides an accuracy over a range of one to several meters and, in some cases, sub meter accuracy. Overall, mapping grade receivers cost less than survey grade receivers because they offer lower accuracy.

Corcoran: Typically, when a GPS receiver is referred to as "survey grade," it means the accuracies attainable (whether post processing or real-time) are <10cm. In order to achieve this accuracy the GPS receiver measures the carrier phase data from the satellite. This can be either a single (L1) or dual frequency (L1/L2) receiver. There are many receivers that measure the L1 carrier phase, but not all receivers can measure it accurately enough or only use the L1 carrier phase to smooth the less accurate C/A code measurements. A "mapping grade" GPS receiver is in the accuracy range of 10cm-3m typically (post processing or real-time). Depending on the scale of mapping required, the accuracy requirements change. For instance, if the base map being generated is 1:10,000 then you could scale a position from the map to better than 10m whereas if your map is 1:1,000 a scaled point would have to be within 1 meter.

Hudson: The difference between mapping and survey grade receivers is the desired accuracy in your application. Applying a survey grade receiver for mapping is possible but perhaps not practical. Conversely, a mapping grade receiver with meter level accuracy may not suffice for survey applications.
      If your requirements are for GIS data capture and asset management then the lower cost and utility found in a "mapping grade" receiver is an ideal choice. Higher precision receivers typically require a stronger understanding of GPS principles and come with a higher price tag. Requirements for topographic or hydrographic surveys and stake out require the performance of centimeter level GPS equipment. However, some survey grade receivers require satellite initialization, and cycle lock must be maintained throughout the job. GIS data collection equipment do not have these advanced requirements and hence are easier to use and understand.

Lange: A Trimble "survey grade" GPS receiver is designed for receiving carrier phase measurements and is capable of differential baseline accuracy on the order of 1 cm. A mapping grade receiver may be capable of carrier phase measurement, but its primary design criteria has been ease of use for GIS data collection. The quoted accuracy of "mapping grade" GPS receivers is in the sub-meter to 5 meter differential accuracy range, depending on the model.

Stenmark: A 'survey grade' receiver is designed to collect sufficient data to produce high accuracy (1cm) measurements between two receivers. Survey receivers typically record both code and phase information, and many make these measurements on both the L1 and L2 frequencies. Mapping receivers are available for a much lower cost, and are not intended for high accuracy work. Most mapping receivers collect only L1 code, although they may be optionally equipped to measure the L1 carrier. In many cases a mapping receiver is easier to use since the operator is not con-cerned with achieving the highest accuracy. A survey grade receiver can be used to perform all mapping operations and provides the most power and flexibility. When there is no need for high accuracy, it makes little sense to invest in survey receivers. Some organizations own both types.

Q. How can GPS be used to implement a GIS?

A. Bohlke: Implementing a GIS with the use of GPS begins with the creation of a customized attribute library. When using a handheld GPS data collection device, the customized library enables the user to collect attribute data about each feature in addition to its position. This procedure establishes a direct link in the field between the spatial data and the textual information. The data can be downloaded to a PC and exported to the GIS software. Depending on the extent of the GIS, the data may be used as the base map or as unique features located on an existing basemap.

Corcoran: For a GIS, data needs to be collected or gathered and in most cases the information needs to be geographically refer-enced. GPS will give a position for the data sample of interest in the GIS. Therefore, GPS is only a tool among many that is used for information gathering in GIS.
      The GPS receivers designed for GIS offer data collection software that allows the user to design an attribute dictionary, tag attributes with a GPS position in the field and usually has software to process the GPS data and/or output the GPS data in various GIS formats. Before purchasing equipment for GIS data collection, you should ensure you research and fully understand the accuracy required from the GPS, the packaging and accessories for the GPS field setup and the type of coordinates (UTM, State Plane) that the GIS works in.

Hudson: Think of GPS positional information as just another field in your GIS database. Gathering attributes in the field, for the creation or update of your GIS database, is your primary job. Invisibly, the GPS sensor adds positional data, time and date information to the "point" you are defining. GPS just makes your job easier.

Lange: GPS is one of the many tools that a GIS manager will use to collect data for his GIS. In addition to using a GPS/GIS data collection tool to record the location of "as built" features and their attributes, GPS is also used to register and rectify other layers in the GIS database. For example satellite imagery, aerial photographs, and scanned maps are often geo-referenced and rectified with GPS. This ensures that these data layers line up with all the other layers in the GIS database, including new layers created by directly digitizing the locations of field assets with a GPS/GIS data collector.

Stenmark: In two ways. First, GPS is used to establish a system of control stations to which the GIS measurements are tied. Accurate reference stations are located for use in the differential processing of the GIS data. This ensures that the GIS data fits well with existing information. Second, GPS is used to collect the location and attributes of the features for the GIS. Small GPS units are used to collect raw data, and base stations are established on known reference points. This information is then downloaded and processed to determine accurate locations. Once processed, locations and attributes are passed on to GIS via various data exchange formats. In many applications, the navigation accuracy (25-100m) GPS may be sufficient.

Q. What new GPS technology is your company developing?

A. Bohlke: Sokkia is currently developing new GPS products for use in various surveying and mapping applications.

Corcoran: Some of NovAtel's innovative family of core GPS technology includes:
      MET/MEDLL - These two GPS receivers are designed to model and eliminate 30-90 percent of the signal distortion known as multipath. Multipath is caused by the reflection or bouncing of the GPS signal, off surfaces such as a building or car, while on route to the GPS antenna.
      RT-20 - This technology is a real-time DGPS receiver that delivers 20 cm or better accuracy and high update rates coupled with low data latency. It can be used for static or kinematic applications.
      MiLLenium - NovAtel is developing a dual frequency OEM board, MiLLenium, for integration into embedded and stand-along applications. Even in the presence of encryption, the MiLLenium guarantees superior performance from the P-Code Correlation Technology.
      End User Products - NovAtel is making a strategic move into end-user products. Recently, two high performance marine products were introduced into the market - Hydrographic Surveyor and the GPS Dredger. This fall NovAtel will introduce a GIS product called the GISMO.

Hudson: II Morrow has introduced a new handheld GPS navigator for the aviation market, the Apollo Precedus. Our pilot customers have asked for a smaller unit, larger display with more resolution and easily changeable and rechargeable batteries. II Morrow responded. This product is the first in a new class of GPS handhelds for the aviation market.

Lange: Trimble spends a great deal of effort on developing GPS receivers with the newest technology available. Three new developments incorporated into products are: 1) the Real-Time Surveyor with on-the-fly integer ambiguity resolution and centimeter accuracy in real-time, 2) high performance (C/A code) receivers with sub-meter point-by-point differential accuracy, and 3) the PC Card GPS receivers with 2-5 meter differential accuracy.
      Trimble has recently introduced the ASPEN series of GPS/GIS data collection products which are based on some of these GPS developments and also utilize pen computers. The use of pen computers allows a significant improvement in data collection efficiency. The ASPEN Pro model uses Trimble's high performance GPS engine which is capable of sub-meter differential accuracy while the ASPEN Field model uses Trimble's PC Card GPS receiver capable of 2-5 meter differential accuracy.
      Another new GPS/GIS product is Direct GPS for ArcView which allows GPS data to be collected directly into ArcView shapefiles. With Direct GPS software and the PC Card GPS receiver the user has the full capability of ESRI's ArcView Version 2.0 continuously available and the full capability of a GPS receiver, with full control over data collection and display parameters.

Stenmark: Leica is advancing on several fronts. We are introducing new survey and mapping grade receivers that provide higher accuracy with simpler operation. New base station receivers are also available. Processing and data management software is moving towards full integration of survey and mapping information, and the ability to combine information taken with different techniques (GPS, conventional surveying, etc.) A premium is placed on the ability to handle information from third party systems and receivers, making the Leica system a convenient place to combine and analyze data prior to sending it on to GIS, CAD, or other system.

About the Participants:
John Bohlke is a systems engineer with Sokkia Corp. in Overland Park, Kan. He can be reached at 913-492-4900 or 800-4-SOKKIA in the U.S.
Wendy Corcoran is a product manager, Survey and GIS, at NovAtel Communications, Ltd. in Calgary, Alberta, Canada. She can be reached at 403-295-4789.
Craig Hudson joined II Morrow of Salem, Ore., in 1988 and currently serves as GIS product manager. He can be reached at 503-391-3411 or 800-742-0011 in the U.S., or 800-654-3415 in Canada.
Arthur Lange is the GIS product manager for Trimble Navigation in Sunnyvale, Calif. He can be reached at 408-481-2994.
John E. Stenmark is a product support manager for Leica Surveying Group in Englewood, Colo. He can be reached at 303-799-9453.

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