A Look at GPS
Continued ...

National Geodetic Survey CORS GPS Data

The National Geodetic Survey coordinates a network of Continuously Operating Reference Stations (CORS) across the United States and in some surrounding areas. This network is part of the National Spatial Reference System and is the closest thing we have to a national coordinate system base. If you want to tie all of your GIS data into a national coordinate system, then use the Base Station data provided from CORS on the Internet at NOAA. When you post-process differentially correct your GIS mapping grade GPS data with the data from the closest CORS station, you will ensure that your data fits within a national framework for spatial data. It has been recommended that even real-time corrected GPS data be differentially corrected using post-processing to obtain the highest accuracy levels. Survey grade or Geodetic grade observations for control densification, stakeout, or imagery ground control, should be network adjusted with one or more CORS stations to obtain a position that is tied in to the National Spatial Reference System. This will ensure consistent and repeatable observations in the future and help keep the GIS database as accurate as possible. CORS data is distributed in RINEX2 format and can be used with both resource and survey grade systems. Be sure to check the logging rate of the CORS base station you use so that you can divide evenly into your receiver's logging rate (e.g. CORS base is at 5 second logging rate so any multiple of 5 will work for your rover.). Also, try to avoid using any CORS station further than 200km from your GPS rover.



Upcoming solar maximum impacts on all GPS operations

The current sunspot cycle, known as number 23, will reach its maximum in late 2000. During the previous cycle which peaked in 1989, GPS surveyors noticed an unstable ionosphere with scintillations that made GPS signals untrackable for periods of time. Even surveyors using dual-frequency receivers, which should have been less susceptible, were affected in some parts of the country. With the increased research into DGPS and RTK GPS since 1989, scientists are now saying that we could expect problems with these types of receivers due to irregularities in the ionosphere that cause rapid variations or scintillations in the amplitude and phase of GPS signals. These scintillation effects can be a significant problem at low latitudes around +/- 20 degrees from the geomagnetic equator, particularly right after sunset. As always, begin and end every observing session by occupying a known point as a check station. Selecting the best receiver for your needs

Selecting the best receiver for your needs

Low End GPS Units

Navigation or recreational grade GPS receivers are powerful tools for the money and in terms of cost/benefit ratios, anyone who works with GIS should consider buying one. Essentially, all of the major brands( Magellan, Garmin, Eagle , Raytheon , Lowrance to name a few) have the same features and the quality generally appears to be good. I have used the Garmin 12XL in the worst possible conditions and found it to be the most reliable piece of gear we had on the climbing expedition. The newer models offer color LCD screens, the ability to upload background maps (although in proprietary formats only), connections to PC software, and DGPS capability with a third party beacon receiver system. Prices range from as little as $99.00 to over $500.00.

Since the Department of Defense has turned off SA, one can expect the accuracy of these units to be between 5 and 10 meters for an autonomous position. This is certainly good enough for some types of GIS work and fine for collecting ground control points for 30m LANDSAT data, but can you create new GIS layers easily using these types of systems? Probably not, unless you are very creative or can link up to a laptop or palmtop computer in the field to manage GIS feature creation. The main limitation is that these units are only designed to store waypoints for use in navigation. Logging of line or polygon features is impossible in most of these units and some units will not record the elevation of a waypoint. If you are collecting point features only and can live with the stated accuracy, then this is a great way to get started in GIS mapping. A notepad and pencil can replace the sophisticated data dictionary that is found in some of the more expensive resource grade units mentioned in the next section. Mapping lines and polygons could be accomplished by taking careful notes and connecting the dots back at the office. Be aware though that none of these GPS units will let you do post-processed differential corrections. It is essential that the unit be able to interface to a PC to download waypoint files to be truly functional for this type of application. Be sure to order any cables and software you may need to accomplish this and an external antennae if you are going to map roads from inside a vehicle. These units may be suitable for use in Automatic Vehicle Location(AVL) systems given their low cost and ability to send out data packets in NMEA format . A good review of these systems can be found at this personal website http://joe.mehaffey.com/#FAQs.

Mid-Range GPS Units

So called resource or mapping grade GPS receivers are the type most common in GIS operations and rightfully so. These are powerful instruments with the firmware and processing software that make them easy to use and allow even first time users to create GIS maps. Examples include Trimble PRO XRS , Ashtech ProMARK X , and Leica GS50. The primary difference between this category and the recreational grade receivers is not in the GPS hardware necessarily, but in the data collection firmware installed on a datalogger. The datalogger will simply take the GPS position at a given instant and will add vertices to the line or polygon that the user is creating. The datalogger allows for sophisticated attribute entry and storage of many features. In many respects, it is the datalogger, firmware, and post-processing software that one pays for. Before purchasing a unit, you will want to test the system for compatibility with your GIS, ruggedness of the hardware, ease of use and evaluate the upgrade policy. Also, look at the type of batteries that are used and if the hardware can be upgraded to a dual frequency geodetic grade receiver in the future.

Another issue to consider here is do you want real-time DGPS or can you live with post-processed differential corrections? I am a strong advocate of post-processing all data even if collected in realtime but there are times when realtime is essential, like when navigating to a parcel boundary corner or buried valve. The price jump for a realtime DGPS system is steep so you want to consider the advantages carefully and make certain that if you decide to post-process, that you have access to a reliable base station via the Internet.

The most important factor to consider in my mind is the PC software that will turn your raw GPS data into a GIS data layer. How detailed can you make the data dictionary? What types of attributes can you add in the field? What attributes will the software add automatically? You will certainly want to be able to filter out poor quality positions or at least flag them for more careful analysis before creating a new GIS layer. The software should support robust coordinate transformations and should handle Height Above Ellipsoid (HAE) to Mean Sea Level(MSL) conversion using the latest geoid models. I also like software that supports archiving and management of GPS data collection sessions and the ability to import in background map layers in the form of GIS or image data layers. Some of the newer systems support the revision of GIS data layers in the field by letting you import a GIS file into the data logger and make corrections or updates while you walk or drive around. The ability to do carrier phase or high accuracy measurements is a good feature to have as it puts you in the realm of 10-20 cm accuracy measurements at a fraction of the cost of a geodetic grade receiver.

Adding external sensors to the GPS allows for these units to do even more work. A laser rangefinder allows one to determine the coordinates of a location without actually having to occupy it. It simply takes the location of the GPS antennae and using a compass, attitude sensor, and laser rangefinder calculates the vectors to the point to be surveyed and adds them to the GPS coordinate. Other more sophisticated equipment like soil pH meters, Geiger counters, and depth sounders can be interfaced directly into the datalogger to store coordinate information with the respective value returned from the external sensor.

One can expect to spend between $3,000 to $20,000 for a mapping grade receiver depending on the quality, features, software, DGPS capability and upgrade policy that accompany the receiver and datalogger.

High End GPS Units

Th final category of GPS receivers that are useful to the GIS community are the so-called geodetic grade receivers, which include such examples as Trimble 4000 SSi, Ashtech, and Leica GPS System 500. These are precision scientific instruments and are priced accordingly, and one finds the greatest range in features and prices here, making purchasing a difficult task. The following discussion is divided up into hardware and software to simplify the issues at hand.

Geodetic grade hardware is not designed to be carried on the person except in the case of the RTK GPS total stations. Generally, one sets up the instrument on a point to be surveyed, with a great deal of fuss, and lets it collect data for however long is required as determined by the accuracy of the position required. Several modes of surveying are employed and can be characterized by whether or not the instrument is moving (kinematic) or stationary (static). Fast or rapid static observations are generally on the order of 7-15 minutes and result in a horizontal accuracy of around 2 cm if everything is working well. To achieve an accuracy greater than this requires longer occupations and it is very difficult to be more accurate than 0.5 cm. Kinematic observations are used to create topographic profiles and imply that the GPS is taking a measurement at a specified time or distance interval while moving. Airborne GPS systems for aerial photography missions use kinematic observations to determine the x,y,and z coordinate of the camera at the instant of exposure.

Real-time kinematic systems, or RTK GPS, are a very useful tool for navigating to a point with an accuracy of around 2 cm in real time and are used for construction stakeout, land surveying and other applications that require very accurate real time positioning. These RTK systems are very useful and allow for a one man survey crew to accomplish the work of a two- or three-man traditional survey crew. These RTK systems generally cost a minimum of $25,000, will be more susceptible to solar flare maximum events, and are currently limited to a range of around 15 km from the RTK base station. The concept behind their operation is similar to DGPS but much more complicated than just calculating the differential correction to be applied to an autonomous position.

When considering the purchase of a geodetic grade GPS system, you must carefully evaluate the cost and complexity of using such a system. In some cases it may be more cost-effective to subcontract out these services. Features to consider include the following: Does the system have the capability to be upgraded to RTK? Does the manufacturer provide on-site training and at what cost? How many hours of data can be stored in the data logger? Does the data logger have Total Station surveying software included? Will it be upgradeable to the new GPS frequencies to be implemented in the next few years? Does the software support network adjustments of GPS observations when two or more base stations are used? Can RINEX2 CORS data files be imported in as base station data? Does it give you an accuracy assessment of your surveyed points? What is the policy on software and firmware upgrades? And finally, will the data logger support the collection of attributes like some of the mapping grade systems? Because of the high cost of this type of system, you will want to look at protecting your system from theft when you have to leave a base station unattended.

In conclusion, there are a wide variety of systems available to GIS users and one must balance accuracy, ease of use, and attribute collection capability with the cost of the system. After deciding on the level of accuracy required, decide if real time DGPS is what you need , then look at the manufacturers compatibility with your GIS. From here it will only be a matter of evaluating the individual features of each manufacturer and selecting a dealer in your area that can offer the sales and services you need.

About the Author:

Kyle Bohnenstiehl has been involved in remote sensing, GPS and GIS for over 10 y ears and is currently Supervisory Geodesist for the Hopi Tribe Land Information System Office. He can be reached at Kyle@nagis.com

NAGIS Inc. a non profit mapping company that works with native tribes and third world NGOs to provide low cost mapping and entry to GIS.



Visit our Design Tools for a listing of GPS software.

Visit our Resources for more GPS Links of Interest