Geoscience Reference
In-Depth Information
These three geophysical methods are being employed for soil mapping of salinity, water content,
and nutrient levels along with location of claypans and buried farm infrastructure. Other geophysi-
cal methods not currently used in agricultural geophysics but having a promising future are, for
example, seismic methods, which might be useful for determining shallow subsurface geology,
soil engineering properties, and the level of soil compaction; self-potential investigations employed
to find leaks in animal waste storage ponds and treatment lagoons; or geomagnetic surveying that
can support farm infrastructure mapping. A common denominator for all these geophysical meth-
ods and their applications is the need for proper geolocation (geospatial location) of the geophysi-
cal quantities being measured. This can be provided by GPS which, depending on the accuracy
required, can be used in one of the positioning modes discussed in this chapter. GPS can, in general,
provide 3D geolocation of the sensing device as well as a precise timing stamp (1 pps signal), if
timing information is required.
One way of defining or classifying geospatial applications that are relevant in the context of this
book is to adopt some accuracy requirements, such as follows (Rizos, 2002b):
•
Scientific Surveys (category A): better than 1 ppm
•
Geodetic Surveying (category B): 1 to 10 ppm
Precise engineering deformation analysis, geodynamics applications
•
•
General Surveying (category C): lower than 10 ppm
Geodetic surveys for establishing, maintenance, and densification of geodetic control
•
•
Engineering and cadastral applications
•
Mapping/Geolocation (category D): better than 2 m
Geophysical mapping, and so forth
•
•
General-purpose geolocation tasks primarily for GIS data capture, some geophysical
mapping
Regardless of the accuracy requirement and GPS technique used, each GPS mapping project
consists of five basic steps:
•
: All the initial preparations that take place before GPS and geophysical
sensor data are collected
Data Collection
Mission Planning
•
: Collecting GPS and geophysical sensor data in the field
•
Manipulation
: All the processing of GPS data that occurs between the collection period and
data analysis, such as the downloading, export, quality control, and processing of GPS data
Analysis
•
: Using GPS data as spatially referenced information in a research problem: here—
geolocation of geophysical quantities mapped
Application
•
: Applying the results of the analysis phase in the real world
Stanoikovich and Rizos (2002) and Stewart and Rizos (2002) provide excellent reviews of the
GPS survey planning process and carrying out of a mapping task.
GPS receivers most commonly used in agricultural geophysics are Trimble (such as Trimble
AgGPS series; www.trimble.com/agriculture.shtml), Topcon receivers (http://www.topcongps.
com), Magellan WAAS receivers (www.magellangps.com) and Garmin WAAS products (www.
garmin.com). When purchasing a GPS receiver, one of the most important requirements to care-
fully consider is the accuracy. The accuracy of the receiver has the greatest impact on its cost. Thus,
the geolocation accuracy requirement should be a defining factor for the GPS receiver selection.
Another important factor is the source of differential correction the receiver can accept (i.e., DGPS
radio beacons, commercial satellite differential service providers, or the WAAS correction). Many
receivers are designed to use only one correction option. Others, such as, for example, Trimble
AgGPS 132 or Topcon GMS-110, allow a choice of the source of differential corrections. Thus, it is
