Global Positioning System
A global positioning system (GPS) is an accurate satellite-based radio-navigation system. It provides three-dimensional positioning information (longitude, latitude and elevation) as well as time. Today, many growers use GPS-derived products to enhance operations in their vineyard operations. The accuracy of GPS allows growers to create vineyard maps with precise acreage for vineyard blocks, road locations and distances between points of interest. GPS allows growers to accurately navigate to specific locations in the vineyard, year after year, to collect soil samples or monitor crop conditions. Combining GPS and geographic information systems (GIS) has enabled the coupling of real-time data collection with accurate position information, leading to the efficient manipulation and analysis of large amounts of geospatial data.
GPS receivers come in different accuracy classes and utilize different correction sources. The accuracies range from recreational class non-corrected receivers at 15 meters to survey grade professional equipment that can attain millimeter accuracy. Of course, the cost of these receivers varies widely with performance and features.
The quality of GPS units and operational errors associated with the GPS system determine the accuracy of GPS-located positions. There are several sources of GPS errors:
Types of Correction
U.S. Coast Guard: The U.S. Coast Guard provides a free differential correction beacon signal. The Coast Guard signal is an AM radio signal that is broadcast from several locations, and travels as a “ground wave” over the earth’s terrain.
Commercial Geo-stationary Satellite Differential Correction Services: Several commercial geo-stationary satellite differential correction services are available for subscription charges.
Satellite-based Differential Correction Services Satellite-based differential correction services are also referred to as satellite-base augmentation systems (SBAS).
Real-time Kinematic Differential Correction: A fourth type of differential GPS correction, commonly called real-time kinematic GPS (RTK GPS), provides GPS position accuracy to within 1 centimeter. RTK GPS requires a separate base station located within approximately 5 miles of the mobile GPS units and is usually positioned at a known survey point and then calibrated. The RTK base station is a known location equipped with a GPS unit.
Differential global positioning systems (DGPS) reduce GPS errors and provide more accurate and reliable readings. In order to obtain sub-meter positional accuracy (i.e. accuracy to less than 1m) a differential correction is required.
Datums and Coordinate Systems
When comparing geographic data obtained from different sources, you must reference the data to the same datum and coordinate system, since the various datums and coordinate systems provide different coordinate values for each geographic location.
Not surprisingly, the Earth is not a perfect sphere. In fact, it is an oblate spheroid—flattened at the poles and bulging at the Equator. While a spheroid approximates the shape of the earth, a datum defines the position of the spheroid relative to the center of the earth. A datum provides a frame of reference for measuring locations on the surface of the earth. It translates the coordinates, latitude and longitude lines, in a flat map to the actual shape of the earth and vice-versa.
A coordinate system enables every location on the earth to be specified by a set of coordinates of known location on a grid. There are two primary coordinate systems that you will need to know, including the Geographic Coordinate and Universal Transverse Mercator (or UTM) coordinate systems.
The Geographic Coordinate System: The most well known coordinate system is the Geographic Coordinate System (GCS), which uses measures of latitude and longitude to determine your location.
The Universal Transverse Mercator: The Universal Transverse Mercator (UTM) coordinate system uses a different grid scheme. UTM zones run north and south and UTM zone designators run east and west.
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