With the placement of 3 Brass Geographic Markers and the installation of a CORS Base Station at SUNY New Paltz, the SUNY New Paltz Department of Geography moves to the forefront in employing modern geographic tools in the Mid-Hudson region.

Location and Place.  Early geographers were often surveyors or navigators who combined their measurements of location with descriptions of the topography and natural environment – the place.  During the colonial period and the westward expansion of the United States, surveyors and geographers did not have the benefit of pre-existing maps or aerial photographs of the regions.  They needed to plot their positions by relying on astronomic observation and terrestrial measurements in order to accurately fix their location and thereby describe new places.

In the last hundred years, however, geographers have seldom made initial determinations of position, instead relying upon existing maps as the basis for most subsequent maps and studies.  Most, if not all, determination of position is being done by geodesists, surveyors, navigators and astronomers.

During the last ten years, geographic information system (GIS) computer technology has revolutionized our ability to map and manipulate large amounts of spatial data. Yet only a tiny percentage of this data is actually being located “on the ground” by geographers.  It seems that most locations used in geographic studies and GIS maps are still derived mainly from existing maps or remote-sensing resources.

Modern geographers now have a new tool – GPS.  Just as the early geographers looked to the stars and chronographs to fix their latitude and longitude (known as “geographic position”), modern geographers are now able to use satellite technology known as the global positioning system (GPS) to accurately determine their location on the earth to within a few meters – or less!  Moreover, handheld computer notebooks attached to the GPS receivers allow modern geographers to simultaneously input their data, virtually linking their description of place to their location on earth. Location and place – the two essential themes of geography.

GPS.  GPS is an acronym for global positioning system. The most widely-used system is the United States’ NAVSTAR GPS system which has at least 24 broadcasting satellites and offers continuous worldwide coverage. A comparable Russian system is known as GLONASS with only 7 satellites currently active.  The European Union has proposed their own system, known as GALILEO, which would consist of 30 satellites.

The NAVSTAR GPS system was originally designed by the U.S. Department of Defense in the mid-1970’s for military point positioning.  Once operational, however, it was quickly adopted by civilian users in ways never anticipated by the designers. The first two major civilian applications to emerge were marine navigation and surveying.

The space segment of   NAVSTAR GPS consists of   at least 24 broadcasting satellites orbiting approximately 20,200 km (12,500 miles) above the earth in six different orbital planes.  Each satellite orbits the earth in about 12 hours.  The system is designed so that at least four (4) satellites will be “visible” to a GPS receiver at any point on the earth’s surface at any one time.  Four satellites are the minimum that must be “visible” for most positional applications. At certain times of each day as many as 9 satellites are “visible” in the New Paltz area.

Each satellite is equipped with several very accurate atomic clocks. These clocks generate unique time-based codes which are constantly being broadcast by each satellite in the 1.5 GHz frequency range.  Here on Earth, your GPS radio receivers are each equipped with their own clock and software to decode the satellite signals.   The distance from each satellite to your receiver is measured by the basic formula Distance = Velocity x Time, with the velocity of the radio wave being approximately equal to the speed of light and the elapsed time of each signal being determined by your GPS receiver software.

This distance calculation is performed simultaneously on all visible satellites.  Simultaneous distance solutions using a minimum of three radii from known points in space (satellites) can intersect at only one point on the Earth’s sphere.  A solution with the fourth satellite is required to correct for timing errors.  This amazing calculation is done in a matter of  seconds and can be repeated at one second intervals as the GPS receiver constantly updates your position on the Earth’s surface.

GPS Users. The GPS technology, originally intended for military purposes, now enjoys hundreds of applications.  GPS users include a wide-range of various academic, governmental, professional, commercial, recreational, industrial, scientific and engineering disciplines.

These include marine navigation, precise agricultural mapping and positioning, aeronautical navigation, utility mapping, railroad mapping and navigation, truck fleet management, off-shore oil drilling control, geodetic and cadastral surveying and mapping,  photogrammetric mapping,  bridge and dam monitoring, GIS mapping, precise timing applications, communication network synchronization, power grid control, ocean buoy monitoring, weather forecasting, water vapor measurements, space weather forecasts, seismic exploration and mapping,  crustal motion monitoring, construction machine control, missile guidance and search-and-rescue coordination. The fastest growing segments of the GPS user community in the next few years will be personal vehicle navigation, outdoor recreation users and automatic E-911 location reporting for cellular telephones.

 GPS Accuracy.  The type of GPS receiver and the techniques employed will determine the relative accuracy of the position.  An autonomous position using a single, consumer-grade handheld GPS receiver will typically have an accuracy of 10-100 meters.  A better quality receiver with a differential correction applied can yield an accuracy of between 0.5 meter - 5 meters. The accuracy of the GPS solution typically increases with the quality of the receiver, the number of satellites being received and the use of differential correction.

Currently, the most accurate GPS receivers use so-called “carrier-phase” technology.  In addition to the time-based codes, these receivers also determine the phase differences in the satellites’ carrier frequency radio signals to compute precise distances. Surveyors and scientists using these dual-frequency, carrier-phase GPS receivers with applied differential phase corrections can expect baseline accuracies of between 0.5mm - 20mm (millimeters!) between two GPS receivers.

Differential Correction.  The term “differential correction” means that simultaneous measurements are being collected at a second GPS reference receiver located at an already known point on the Earth’s surface.  Based on a geometric comparison between observed positions versus the known position at the reference site, a three-dimensional correction can be applied to the autonomous receiver’s position. This correction (DGPS) can be either transmitted by a radio beacon in real-time or it can be post-processed.

While reference receivers are sometimes operated by individual users, the wider GPS community often relies on reference stations operated by commercial services or public agencies such as the Coast Guard, the Federal Aviation Administration, state Departments of Transportation or educational institutions.

CORS.  The reference GPS receivers which provide corrections to the public are generally known as “continuously operating reference stations” (CORS).  A network comprised of more than 275 CORS sites currently participates in the national CORS system with data files posted hourly on the National Geodetic Survey (NGS) website http://www.ngs.noaa.gov/CORS.  These files are posted in the industry standard RINEX format and can be freely downloaded for use in post-processing software.
 A number of CORS sites operated by the Coast Guard also broadcast their corrections in real time via low-frequency radio beacons as aids to marine navigation. The FAA also operates two geostationary satellites, known as WAAS, which broadcast regional real-time GPS corrections for aeronautical navigation.  Other users can also make use of these broadcast differential corrections to improve the accuracy of their GPS positions.

GPS Data Collection.  Many of the commercial-grade GPS receivers used for GIS-mapping applications incorporate data collection capability or can interface with handheld computers.  This mobile keyboard option allows the geographer to input attributes of the place while simultaneously recording the geographic position. Extensive database information can be attached to each feature for later import into a GIS.

An example would be a recent GPS survey of handicap parking spaces and building entrances on campus.  In addition to the geographic position of each space and entranceway, the geographers collected supplemental information about the place including dimensions, street address, notes on needed maintenance, etc. while standing at the location. Many of the GPS receivers used for GIS mapping can collect positions as either discrete points (nodes), continuous lines or closed polygons (areas).

GPS at SUNY New Paltz.  I have participated in two GPS-related programs sponsored by the Department of Geography.  In association with Leica Geosystems, Inc., the Department of Geography has hosted 3-day seminars entitled “Basic GPS for Land Surveyors”.  These seminars were intended to provide professional development credit for licensed land surveyors who are increasingly using GPS for survey measurements. Course material included a review of basic geodesy, GPS theory and precise GPS measurement techniques for land surveying and engineering applications.  The classes made use of the GIS computer lab for instructional purposes and for post-processing GPS data collected during outdoor fieldwork.

Surveyors use carrier-phase differential GPS to compute vectors in networks of so-called “baselines” tied to fixed reference marks.  The GPS methods used in surveying vary somewhat from the methods used for code-based point positioning.  For the most precise measurements, surveyors use “static” observation sessions lasting from ten minutes to three hours collecting thousands of redundant measurements which are post-processed to compute relative distances with millimeter accuracy between points.  Another technique, known as real time kinematic (RTK), combines a tripod-mounted GPS receiver with a low-power UHF radio transmitter broadcasting differential correction data to two or more similarly-equipped rover GPS survey units.

For example, during the class one team of surveyors hid dimes around the New Paltz campus and located their precise position with RTK GPS.  Other teams used similar equipment to find the dimes – hidden in the grass, beneath manhole covers, in sidewalk cracks – in a treasure hunt designed to demonstrate the potential accuracy of the carrier-phase equipment.
The Department also offers a 1-credit GPS (Global Positioning System) short course during the Fall semester as part of the college’s GIS program.  Here students have the opportunity to use a variety of mapping-grade GPS receivers to collect data, post-process the data in the GIS/Computer lab and import it into a GIS map.

One of the initial exercises was to compare the geographic position of points on campus scaled from USGS topographic maps and other sources with the actual position observed using GPS.  Some of the field assignments using GPS included the mapping of handicap parking spaces in relation to building entrances, mapping of trash containers in relation to waste volumes, mapping of cigarette butts in relation to outdoor ashtrays, mapping of pedestrian patterns in relation to existing sidewalks, etc.  The ability to virtually connect geographic position with data and then later display it in a GIS was impressive.

Geographic Monuments.   For future comparisons of geographic position by various methods, three (3) permanent geographic markers have been installed on the SUNY New Paltz campus.  These markers are 3-inch diameter circular brass plaques inscribed “SUNY New Paltz, Department of Geography, Latitude and Longitude Marker” cemented into the ground at three locations on campus.  The markers were donated to the Department by Berntsen Survey Markers, Inc. (Madison, WI) and installed with the cooperation of the college maintenance department.  The precise position of each marker was determined in conjunction with the September, 1999, land surveyors’ seminar.

These brass plaques closely resemble the official survey monuments used by the National Geodetic Survey (NGS) and other public agencies.  The plaques can be found at three locations on campus, namely (a) on the patio of Terrace Restaurant, (b) in the center of Old Main Quad and (c) in the sidewalk near the easterly entrance to Parker Theater

The geographic position (NAD83/1996) of each marker was determined by GPS measurements made to three regional geodetic control points in the state’s High Accuracy Reference Network (HARN). These geodetic reference marks are located at Stewart International Airport, Dutchess County Airport and near Tillson, NY.  The horizontal positional accuracy of the newly-established marks is expected to be within 10 millimeters.

 For such precise measurements, land surveyors employed dual-frequency GPS receivers mounted on tripods. Observation times at each point ranged from 30 minutes to 2 hours using so-called “static” GPS techniques.  The data was post-processed to derive GPS baseline vectors between various points and then the network was further adjusted mathematically using a least squares adjustment.

The geographic positions (latitude/longitude) are as follows:

    SUNY 99-001 (Terrace)…. 41°44’29.954” N,   074°05’09.744” W(NAD83/1996)
    SUNY 99-002 (Old Main)   41°44’37.665   N,   074°04’56.516 ’W(NAD83/1996)
    SUNY 99-003 (Parker)…..  41°44’26.219’ N,   074°05’03.541’  W(NAD83/1996)

The datum for geographic position is North American Datum of 1983 (NAD83) revised 1996 (HARN).  For comparison purposes, one second of latitude or longitude at New Paltz, NY, is equivalent to approximately 30 meters (98.4 feet) in horizontal distance.

SUNY New Paltz CORS.   The college is also in the process of installing a permanent GPS receiver reference station at Resnick Engineering Hall. This is a Leica RS-500 GPS receiver with a permanently-mounted AT-503 GPS radome antenna.  It is designed to operate 24 hours a day, collecting measurements at the rate of every 5 seconds and downloading hourly observation files to a cooperative CORS internet site.

These files will be posted in industry-standard RINEX format for use in post-processing.

This CORS station will enable the college to serve an important public function and will benefit a large number of GPS users in the area, including academics, scientists, public agencies, surveyors and GIS professionals.  It is anticipated that the New Paltz CORS site will become a participant in the NOAA/NGS CORS Network.

At the present time, there are no Continuously Operating Reference Stations (CORS) in the mid-Hudson Valley participating in the NGS CORS program.   The nearest sites are US Coast Guard at Hudson Falls, NY (Washington County); NYS DEC at Delmar, NY; Lamont-Doherty Earth Observatory at Palisades, NY; New Jersey Institute of Technology at Newark, NJ; and Penn DOT at Wilkes-Barre, PA.

In addition to the surveying and mapping applications for CORS data, networks of  permanent, continuously-operating GPS receivers are also being used for various research applications including precise timing applications, weather forecasting, water vapor measurements, space weather forecasts and seismic and crustal motion monitoring.

Conclusion. The continued focus on GPS by the Department of Geography is an important complement to the GIS program. It will guarantee that the next generation of geographers will be able to precisely define their location while describing their place.