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The Right GPS for Professional GIS Data Collection



Katherine Sandford
Regional Sales
Manager for Mapping and GIS, Trimble, Germany



Abstract
An accurate and data rich GIS drives enormous improvements in land mapping, infrastructure planning and utilities maintenance, but the information available from such a system is, naturally, totally dependant on the data that has been input. Key to the provision of high quality data to a GIS is the use of an effective field data collection system based around positioning with GPS. The market for such systems is awash with choice and it is often difficult for the uninitiated to make a decision as to the correct system for their needs.

This paper will look at the key differentiators that should influence the buying decision for a GIS field data collection system.

Introduction
With increasing pressure to do more for less, GIS users may be tempted to build their field data collection systems around inexpensive recreational GPS products. Aimed at outdoor enthusiasts such as hikers, mountain bikers and hunters, these products provide features that seem comparative and have price tags far below GPS systems specifically built for the GIS professional, thereby presenting a strong incentive to purchase. However, thorough investigation into the aspects of GPS performance, the logistics of gathering data in the field, post-processing requirements, peripheral integration and the ease of data flow are all necessary to make an informed purchasing decision. GPS performance

Recreational and professional GPS units are designed for different purposes. The recreational unit is designed to acquire a location fix quickly without the need for pinpoint accuracy or continuous uninterrupted use over a full work day. Hikers, for example, can generally find what they are looking for once they get within ten metres. Professional GIS users typically require very accurate placement of features, often within a metre or less, so that data layers can be overlaid and intricate spatial relationships determined. They also require continuous uninterrupted operation through the full length of a standard work day.

Accuracy
Recreational grade GPS may be suitable in an application where accuracy and reliability of data is not critical. However, to reach the sub-metre level of accuracy that is often the prerequisite of modern GIS systems a far higher degree of accurate and reliable GPS positioning is required. Additional to real time differential correction a higher specification in both hardware and software on the GPS receiver is required to obtain this higher level of accuracy. Many recreational GPS receivers are capable of real-time differential correction, but do not have the ability to reach sub metre accuracy even with the availability of the strongest, most accurate differential correction signal.

Engineering, design, and construction characteristics account for the variation in capabilities among GPS receivers. Professional units have been engineered and built to acquire more accurate location coordinates. Although many design features contribute to this higher level of performance, three factors—quality control, electromagnetic shielding, and antenna technology— set professional grade products apart from recreational receivers.
  • Quality control — Professional GPS units give users control over the quality of the position points that are collected. Through a simple interface, the user can establish specific thresholds for acceptable data quality. For example, selecting the number of satellites and elevation above the horizon needed to achieve suitable accuracy, or, programming the receiver to disregard any satellite signals that suffer from excessive noise interference. These quality control settings essentially allow the user to filter out any poor data that may degrade the overall quality of the location coordinates, resulting in greater accuracy in the final dataset.
  • Electromagnetic Shielding— by their very nature GPS signals are extremely weak and are easily degraded by interference from nearby electronic devices such as laptop computers or personal digital assistants (PDAs). Given the fact that many GPS receivers and GPS cards are linked to computers and PDAs, this can pose a serious problem. High-end GPS products have built-in shielding technology that minimizes the effects of stray electromagnetic signals from other equipment.
  • Antenna Technology— acquisition of weak GPS signals requires a sensitive antenna, especially when receiving transmissions in urban canyons and under tree canopies. The antennas provided with professional grade GPS units are designed to pick up signals in almost any environment. A further issue is the degradation of accuracy due to multipath signals. These GPS signals have been degraded through reflection from buildings and other overhead features on route to the receiver. The antennas on professional receivers recognise and filter out multipath signals.

    Figure 1 below illustrates the importance of GPS receiver design, by comparing the performance of a handheld GPS receiver which does not benefit from advanced antenna design and shielding with a modern professional GPS receiver (Trimble GeoXT™) built using the latest technology advancements. While 80% of positions from the older receiver were within four metres of truth, 80% of positions from the professional receiver were within 0.80 metres of truth.

Figure 1. Comparison of positional accuracy between older and modern professional handheld GPS receivers.

Attribute collection
For GIS users, accurate attribute collection is just as crucial as location acquisition. However recreational units will not generally allow you to collect attribute information about a feature or export data to a geodatabase. Only professional GPS products offer customisable interfaces and routines for detailed attribute collection.

It is important to consider the software that comes with your GPS receiver and what it will allow you to do. There are a number of equally important tasks around the central activity of field data collection — these include creating a data dictionary and integrating it with your GIS database, checking satellite availability, configuring the receivers, downloading data, exporting it to the GIS, revising procedures based on actual field conditions and experience, and maintaining equipment.

A GPS with dedicated field software will make data collection much easier. If you buy an inexpensive recreational unit you may find it very difficult to do anything other than collect waypoints. In reality, you need to do be able to create a data dictionary and collect attributes about particular features, and then transfer this information to the office computer. Good field software will include data transfer functionality.

The more sophisticated GPS units on the market today have features and functions that can make the collection of data much easier. For example, the ‘repeat’ function lets you copy any previously entered attribute information into the next feature. For example, if you were recording manhole information where changes only occurred in the minority of attributes from one manhole to the next the repeat function would significantly streamline the collection process.

In the Province of Rome (Italy), professional mobile GPS devices are used to map manholes as illustrated below.


Post-processing
If GPS is being used for high-accuracy navigation purposes, real-time DGPS (differentially corrected GPS) is the only option as the need for accuracy is immediate. But for most GIS data collection work, where high accuracy in real time is not critical, post-processing after data collection produces much more accurate GPS data as well as lowering the logistic and operational overhead in the field. Post-processing techniques are often essential to ensure that a feature's position can be defined to the required accuracy level.

Post-processing achieves better accuracy than real-time differential correction because of the ability to use data collected both before and after the time the position was recorded. When working in real time mode the corrections are predictions based on broadcast corrections from a few seconds earlier. Post-processing uses multiple base observations from before and after the time the position was recorded. Due to the comparatively unconstrained processing environment available in the office, post-processing software is also more powerful and uses more sophisticated algorithms in calculating errors and corrections.

For post-processed differential correction to work effectively, the software requires raw, unmodified GPS positions or measurements. Some GPS receivers apply correction techniques to GPS positions to provide more accurate coordinates in real time. If a GPS receiver also outputs information about how these modified positions were calculated, the post-processing software can recalculate the raw positions and differentially correct them.

Some GPS message formats, such as NMEA, only output modified GPS positions and only provide basic information about them. This information is not complete enough for the post-processing software to recalculate the raw position, so it cannot differentially correct GPS positions provided. Although a number of recreational GPS receivers are capable of post-processing output, this is only the most basic data and often in a format that is not compatible with most office post-processing systems.

The foundation of a good GPS unit is its ability to correct it in real time as well as provide complete data sets for post-processing. If it can perform these tasks, the GPS receiver should remain valuable for many years.

System performance

Integration
Today’s field data collection systems go far beyond the simple isolated collection of a position with associated hand entered attributes. Modern field campaigns often require electronic input from a variety of peripherals, such as a barcode reader, digital camera, or cell phone for real time wireless communication with the central GIS database. Recreational GPS units are generally not designed for this level of integration and can be troublesome when it is required to integrate them into a broader system. Efficiency of field crews can be greatly reduced when it is necessary for them to contend with a mass of cables and multiple devices as they frequently climb in and out of vehicles and manoeuvre down manholes.

Professional GPS receivers are rugged, integrated units providing antenna, receiver and open software platform all within a handheld computer. In addition to software that enables you to collect and store attributes about a feature the open software platform will provide easy integration with a range of peripherals that may be required for a particular field campaign, for example bar code readers, GSM communications or digital cameras. With the proliferation of Bluetooth communications between such devices it is generally possible to assemble a field system with a minimum of cables, thereby greatly enhancing the efficiency of field crews.

A further important point with respect to integration is related to the transfer of field data to and from a GIS. Most recreational GPS receivers do not provide the advanced functionality that enables data conversion to other formats. It is important that the process of data conversion from field to office is efficient and consistent, and can be carried out without the need for manual intervention. Utilising a recreational GPS receiver may require significant programming and integration time to enable the input of data to a GIS with ongoing overhead of multiple steps to achieve the transfer.

Ruggedness
Ruggedness does add to the cost of a field computer. However, the additional outlay can be very quickly recovered through reduced downtime, fewer return visits due to field failures or bad weather, increased security of data and reduced on-going outlay for replacements. Also, in contrast to low-cost PDAs, ruggedised systems designed for outside data collection typically have sufficient battery-life for a full-day’s work in the field.

Until recently the only mobile computers that could withstand the rigours of field use were heavy, power-hungry and cumbersome. The offerings were typically customised for a specific application and often used a proprietary operating system. The convergence of PDA functionality and tough environmental specifications, coupled with improvements in battery performance, has allowed the development of field computers ideally suited for GIS data collection.

The picture below shows an archaeological site, on which rugged field devices are being used to facilitate mapping of the area.


Enterprise performance
The question to ask is: “Does my GPS receiver allow me to transfer data to a GIS?” If you are converting GPS points to a specific GIS format, such as shape file format, you will need to buy a professional unit. Most recreational receivers cannot convert data to other formats. You should also consider whether the GPS receiver you buy will be able to perform as part of a larger enterprise. Will it ‘scale up’ and work with the other equipment you have? Is technical training and support available?

Scaling up
If you intend to deploy a number of workers in data collection field work, it is important that the process of data conversion from field to office is efficient and consistent, and can be carried out without the need for manual intervention. A professional system will enable this to happen without the need for complex process intervention.

Training, Support and Service
Investing in GPS training can eliminate costly errors in data collection, ensure data integrity, raise quality-control standards and guarantee compliance with existing standards for digital data collection. GPS training benefits all users, from novice to expert, because technology is constantly evolving in the office and the field. A field worker is only one part of a network and must understand how the rest of the network functions. And people in the office need field time with the equipment as that will influence what they do in the office.

Field and office workers from the Polish Agency for Restructuring and Modernization of Agriculture were trained as one group as illustrated in the photograph below.


Further consideration should also be given to ongoing technical support, service and upgrades for both hardware and software. To ensure that equipment bought from hard-won budget will not become obsolete, it is well worthwhile investing in professional-grade GPS receivers from a reputable manufacturer that is focused on the GIS industry specifically and is in business for the long term.

Conclusion
Based on price alone the purchase of recreational GPS units to act as the core of a GIS data collection system can appear the obvious answer.

However careful consideration needs to be given to the logistics of gathering data in the field, the necessary overall system configuration and integration, real-time and post-processing accuracy requirements and integration with the GIS. Modern professional GPS receivers are designed and built with the express requirements of the GIS industry taken into account. Although the professional units are initially more expensive to purchase than their recreational counterparts, the return on investment achieved by utilising these units will very quickly surpass the initial additional outlay.

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