Recent advances in mobile GPS/GIS mapping technology

Recent advances in mobile GPS/GIS mapping technology

Ashok Wadhwani
Applied Field Data Systems, Inc.
16300 Katy Freeway, Ste. 250 Houston, Texas 77094
Tel: 281-5790492, Fax: 281-5790412
afdsaw@aol.com

Introduction
GPS/GIS Field data collection is a perennial problem for cartographers, surveyors, engineers and researchers. Until recently, the tools available for mapping applications have been bulky in size and weight, expensive, and difficult to learn. During the past year, tremendous advances have taken place in GPS technology (receivers), data collection hardware, and field data collection software. Not only has the autonomous GPS accuracy improved, but the data collectors have become smaller, lighter, and less expensive. The software has become cheaper and easier to learn. In addition, for applications involving offsets, lower priced laser range finders have been become available. All of these advances have made the GPS/GIS data collection tasks easier, more economical and faster to complete. (See Fig. 1)

Figure 1: Mobile Mapping

Advancements

GPS Receivers
With the elimination of Selective Availability (SA), autonomous accuracy is much better. Today, a single stand alone receiver can provide an accuracy of between 10-15 meters. These receivers are so small that they can easily fit in your shirt pocket and one can get these receivers for approximately $100. It is also now possible with the use of DGPS service to obtain sub meter accuracy in real time. (This eliminates the need for post processing). This DGPS service is available through the use of coast guard beacon receivers and satellite based DGPS service. The coast guard service is limited to the coastal areas of the country or around 50-200 miles radius of the station itself. For areas where the coast guard beacon service is not available, real time satellite based DGPS service is available almost throughout the world at reasonable cost.

Data Collectors
Traditional Units for data collection
The traditional units for GPS data collection use either an onboard storage memory or an external data logger. In most cases, the software on these units is extremely complicated and difficult to learn. The data collectors are vendor specific to GPS engines only and cannot be used for any other applications. The units come with a proprietary operating system which makes modifications or difficult to incorporate. These units have slower processors thus making the manipulation and processing of data and maps impossible or very slow. Also, the small screen size limits the display of data without scrolling down or panning often. The units have very low battery life and are expensive to replace.

New Generation Data Collectors
With the introduction of Palm Pilots followed by Microsoft's launch of a pocket PC operating system, a new generation of handheld Personal Digital Assistants (PDA's) have flooded the market. It is now possible to use these lightweight handheld PDA's , with GPS/GIS data collection software, for field applications.

In addition, regular windows based laptop PC's are now available in ruggedized waterproof versions. One can use these PC's for mapping applications in tough outdoor environments.

Some of the new units available are manufactured by Casio, Hewlett Packard, Sharp, Compaq, Fujitsu, Panasonic, and Walkabout Computers. (See Fig. 2)

Advantages of New Generation Data Collectors
The new data collectors are economically priced between $300 - $6000. Widespread familiarity with the Palm, Pocket PC, and Windows operating systems has resulted in a quick and easy GPS/GIS integration with these devices. As these units can be used with or without GPS, other field data collection applications can be handled. They have a longer battery life, 14-16 hours, and are lightweight, thus making it easier for the user to carry the units all day in the field. In addition, most data collectors have touch screens and come with a color display. Almost all have voice activated systems which comes in handy if one wants to dictate notes. Most devices have 2 serial ports, USB ports, and infrared ports. Some of them have built-in modems, and offer extended memory and Compactflash card options.

Figure 2

Software

Traditional Field Data Collection Software
The traditional Data Collection software is difficult to learn and is vendor specific to their GPS engine only. Also the software available operates only on a Windows operating system and is expensive.

New Generation Software
Some of the new software introduced during the past year or so are: Fieldworker, Solo, ArcPad, Sitemate, Patchworks, and Composer.

Advantages of New Generation Software
Today, the new generation of software offers the user various options that can be used for his or her applications. The software is very economically priced, between $50-$3000, and has the capability to add background maps or digital orthophotos. Most of the new generation software allows the user flexibility to use any type of GPS engine beginning from a low priced recreational type unit to a high accuracy survey grade unit. The software has the capability of reading from 2 serial ports allowing the user to use a GPS receiver as well as an additional sensor, such as a laser range finder. The software can also accept digital camera input allowing the user to capture not only the location data but also the actual picture of the feature. Once the location, features and attribute data have been collected, all of the data can be exported in different GIS formats, such as ArcView shape files. Most of the new generation of software has a version available for Windows as well as a Pocket PC operating system. This results in a much shorter learning curve. (see Fig. 3)

Figure 3

Laser Range Finders
While GPS technology has made rapid advances, there are still inherent problems with data collection when utilizing GPS. First, there is the need for occupation of the point where GPS readings are required and sometimes its just not possible to reach the point of interest. Second, the point may be reachable, however, due to other factors, such as disturbance of wild life, areas of high traffic volume, or even evidence in crime mapping, you may not want to get there. Third, in some areas, GPS receivers cannot receive the signals, such as, in heavy tree canopy areas and near high rise buildings. Fourth, GPS mapping is slow if you have to map several features such as trees in a forest or electric poles on a road because you have to occupy each individual feature.

To overcome some of the limitations of the GPS technology, laser range finders have now become available at a reasonable price and performance. These units can be used with or without GPS. The key to laser mapping is: you do not have to get there from here and occupy the feature! Instead, just shoot it with the laser. (See Fig. 4)

Figure 4

What makes a laser mapping system work
The eye-safe diode pulse laser measures distance without reflectors and has a tilt sensor built in to provide vertical angles. In addition, it has an option for a digital flux gate compass or angle encoder (not affected by magnetic fields) to provide an azimuth reading. The mapping grade range finders provide an accuracy of about 5 cm to 1.5 meters. Maximum range varies from about 500 ft to 2000 ft. The Laser Range Finder can be used in different ways depending on the application, for example, direct GPS integration, indirect GPS integration, or Independent Laser Mapping

Direct GPS integration
In cases where you have a clear view of the sky and are getting GPS signals, you can place the laser range finder at the same spot where the GPS antenna is placed - in fact on the same range pole if possible. The laser unit sends the distance and azimuth readings to the GPS data collector and the software converts the laser readings to LAT/LONG based on the GPS antenna position as the reference. (See Fig. 5)

Figure 5

Indirect Laser/GPS integration
In cases where GPS signals are not being received, the GPS antenna is placed in a clear area where signals are available and the Laser is placed in any suitable area where maximum number of features are visible. The laser sends the distance and azimuth readings to the data collector and the software calculates the LAT/LONG of the features based on the antenna position. (See Fig.5)

Figure 6

Independent Laser Mapping
Laser Range finders can also be used without GPS. Here you place the laser at any reference point (base map coordinates, survey points etc) and call it "ORIGIN". From this origin, you can shoot the laser to various points of interest and the distance and azimuth readings are sent to the data collector and stored. In this method the position of features are accurate to base maps and to each other. The data collected by this method will be relative data only. However, in the future when LAT/LONG positions are available for any reference or points, one can insert that value in the software and all the relative data will be converted to absolute data in LAT/LONG.

Applications
Even though the new products described have only been introduced in the past year, their use has already become very widespread in various applications, such as, environmental, geology, infrastructure, forestry, etc. This is mainly due to the portability and ease of use of the system. (See Fig 6)

Selection Considerations

GPS Receiver Selection Consideration
The most important criteria in selecting a GPS receiver is identifying the accuracy that the application requires. The receivers generally fall into 4 categories. Autonomous - 10-15 meters, 1-3 meters, submeter, and survey grade. For all but the autonomous category, you will need some way to correct the readings. The corrections can be - Post processing, Coast Guard Beacon, and Satellite based.

In the post processing option, you need to ensure that base station data is available from a nearby base station. If the Coast Guard Beacon method is chosen, one needs to identify if a beacon station is available near the site. In the satellite based option, one needs to make sure that the service is available in the area and the subscription paid for.

Data Collector Selection Consideration
The weight and size of the unit is an important factor when using these units in the field for a long time. The type of operating system (Palm O/S, Pocket PC, WINDOWS) to be used needs to be considered if integrating with other application platforms. If you require a display of a background map, it will be preferable to have a unit with a larger screen and color display. However, the color display may reduce your battery life. If the application involves use in a rough environment, the data collector selected must be ruggedized for outdoor use. Battery life is also an important factor in the selection of the data collector. The unit selected has to be compatible with the type of GPS receiver to be used.

Laser Range Finder Selection Consideration
If you need to collect large amount of data and in areas which are difficult to reach, a laser range finder will be a very helpful tool. If the work involved is in an area where magnetic fields are present, an angle encoder will be a better choice than the digital compass.

Software Selection Consideration
As certain versions of field data collection software have limited features, several factors need to be considered in selecting the right software.

The selected software should be compatible with the GPS receiver and the operating system chosen for the data collector. Some software packages do not allow you to create and edit features in the field. This restriction implies that for any changes and or additions to the data dictionary, you will have to stop the project in the field and access a PC to edit the dictionary, reload the new version of the dictionary, and restart the field project. This is a very time consuming process which can be avoided by simply selecting the software with field editing of the data dictionary capability.

Often field data projects are handled in different datums and projections, and as a result, one should choose software with capability to select various datums/projections. If laser range finders or any other external devices besides GPS are to be used, make sure that the software is compatible and raw data is converted into the proper units. It is good practice to view the quality of GPS data collected by viewing the DOP number. Hence, one should pick a software package which displays DOP values. If you need to go back to a previously located or known point, you will need the navigation capability in the software. Most often the collected data needs to be imported into a GIS. The software chosen should be able to provide a conversion routine to allow an import of GPS and Feature data into various types of GIS software, such as, ARCVIEW, MAPINFO, and AUTOCAD.

Conclusion
The availability of new technology in the GPS mapping arena has made the data collection tasks much easier and cost effective. I see this trend continuing towards lower prices and faster speeds. This will allow new and widespread use of mapping technology. However, with more options, different price ranges and easier availability of this technology, the users have to be extremely careful when planning and implementing their Mobile Mapping project.