1. What is a GIS? A Geographic Information System (GIS) is a system for creating, storing, analyzing and managing spatial data and associated attributes.
A GIS can produce information that answers specific questions and allows you to share that information with others. By visualizing relationships, connections, and patterns in data, you can make informed decisions and increase efficiency throughout your organization.
Source: ESRI: Redlands, CA
2. What is the GPS and what is a GPS receiver? The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. The satellites are constantly moving, making two complete orbits in less than 24 hours. These satellites are traveling at speeds of roughly 7,000 miles an hour. Navstar is the defense departments name for GPS.
Source: Garmin: USA
3. What is real-time differentially corrected GPS (dGPS)? Real-time DGPS occurs when the base station calculates and broadcasts corrections for each satellite as it receives the data. The correction is received by the roving receiver via a radio signal if the source is land based or via a satellite signal if it is satellite based and applied to the position it is calculating.
The Wide Area Augmentation System, or WAAS, is being developed by the Federal Aviation Administration (FAA) to provide precision guidance to aircraft at airports and airstrips that currently lack these capabilities, using a system of satellites and ground stations that provide GPS signal corrections. WAAS is broadcast from geostationary satellites so the signal is often available in areas where other DGPS sources are not available.
The Coast Guard signals are broadcast in the frequency range of 285 to 325 kHz (just below the usual AM-radio band) where radio waves travel as ground waves and are not limited to line-of-sight reception like FM-radio stations. The signals are series of pulses similar to those of the GPS satellites. Referred to as Minimum Shift Keying modulation, the signal is less sensitive to electrical interference and noise than AM-radios. The range of the Coast Guard beacons is approximately 150 miles in good weather (electrical storms cause interference). Accuracy decreases with distance from the transmitter. A disadvantage of the Coast Guard differential corrections signal is the rate at which the beacon transmits or repeats messages. Most Coast Guard sites broadcast at 200 bits per second. At this broadcast rate, the age of a satellite's differential correction can be as old as four seconds. For some applications, such as guidance, this update rate may be unacceptable. For guidance applications, update rates of two to ten times per second may be required. Typical Coast Guard beacon receivers have two channels. One channel receives the differential correction and the other is searching for the best incoming signal. This helps to ensure against loss of a DGPS signal if at least two beacons are within range.
Satellite-based correction signals
The simplest type of differential corrected signal is transmitted from a geostationary satellite. Companies such as Omnistar, Accqpoint and Racal provide this service. The correction signal is available throughout most of North America. The accuracy of high quality receivers is generally considered to range from one to three meters RMS. Interference from man-made sources is minimal. Satellite-based signals may have an advantage for operation around trees and buildings since the satellite is nearly overhead at most locations and within the DGPS receiver's line-of-sight.
Real Time Kinematic (RTK) RTK is a process where GPS signal corrections are transmitted in real time from a reference receiver at a known location to one or more remote rover receivers. The use of an RTK capable GPS system can compensate for atmospheric delay, orbital errors and other variables in GPS geometry, increasing positioning accuracy up to within a centimeter.
RTK is used, not only as a precision positioning instrument, but also as a core for navigation systems or automatic machine guidance it provides advantages over other traditional positioning and tracking methods, increasing productivity and accuracy. Using the code phase of GPS signals, as well as the carrier phase, which delivers the most accurate GPS information, RTK provides differential corrections to produce the most precise GPS positioning.
4. Which coordinate system should I use? Several coordinate systems are in use for mapping and may cause problems with compatibility between different data sets. Users frequently need to transform position data into a plane (flat) coordinate system, either to merge them with another data set, to plot a map of the GPS results, or to perform further calculations for such parameters as area, distance or direction (plane coordinate systems are usually easier to work with than geodetic coordinates). When using data and maps from several sources, coordinates must be based on the same datum. The coordinate system differences, which are caused by a different reference frame, ellipsoid and data adjustment, are significant (up to several hundred meters) and cannot be ignored.
GPS receivers can usually report position information in more than one format. The most common format is lat/long (latitude and longitude). Lat/long coordinates are recorded in angular units of degrees, minutes and seconds. One second of latitude is equal to about 30 meters. GPS receivers may display lat/long in degrees plus minutes to four decimal places (instead of minutes and seconds). Most geographic information system (GIS) software is capable of using more than one format and may automatically convert lat/long coordinates to a coordinate system such as Universal Transverse Mercator (UTM) or State Plane Coordinates (SPC) to calculate distances in meters or feet.
UTM and SPC systems project portions of the earth's curved surface onto a flat map and report locations as actual distances from a reference point in meters and feet, respectively. Hence, no conversions are necessary to calculate distance or area.
Universal Transverse Mercator Coordinates
The UTM coordinate system is a worldwide system originally adopted by the U.S. military in 1947, and since has been widely used by civilian mapping in many countries. The UTM system is consistent throughout the world and one set of equations will allow calculation of coordinates at any location. The world is divided into 60 zones each spanning 6 degrees in longitude and extending north and south from a latitude of south 84
State plane coordinate system
In the 1930s, the U.S. Coast and Geodetic Survey established a plane coordinate system for each of the 48 states. One to five zones were established in each state with a Lambert Conformal or a Traverse Mercator projection. The specific projection and the size of the zone were selected to fit the geometry of the state and to keep distortions at or below one part in 10,000.
5. What can GIS/GPS do for me? GIS/GPS can help with the collection of data. Examples of data that can be collected with GIS/GPS are yield points, leaf samples, trap locations, disease find locations and field boundaries.
· Field experiments can be set up using data points collected with GIS/GPS. Using GPS points one can come back to the same sampling points from year to year with out having to mark locations with flagging tape.
· Soil sample locations can be documented and mapped.
· Normalized difference vegetation Index (NDVI) can be used to look at live green vegetation from aerial or satellite images.
· Visualize patterns in data
These are only a few of the things that can be done with GIS/GPS.
6. What equipment will I need to get started?
· GIS software – ESRI – www.esri.com
ERDAS – www.gi.leica-geosystems.com
Intergraph - www.intergraph.com
· GPS – the different units are discussed in which GPS should I purchase?.
· Color Printer
· Computer (desktop, laptop, PDA)
7. Which GPS should I purchase? Things to consider when buying a GPS
Cost - How much do you want to spend on the GPS unit? RTK units are the most expensive; generally the cost increases with accuracy and data rates.
Data/power connection - At some point, you will probably want to be able to transfer data, such as data points, between your GPS and a PC/laptop/PDA. Some low-end GPS receivers do not have the ability to transfer data, so they may not be the best choice. Some GPS receivers can be powered from an external source, such as 12 volts in an automobile or tractor, which may be an issue, rather than relying on batteries in the GPS receiver.
Accuracy - If the accuracy of the position reported by your GPS receiver is an important consideration for your intended use, then you should take some time to understand how the Global Positioning System works, and the limitations of GPS receivers. Consumer-grade GPS receivers are used in a wide variety of situations, but are not suitable for things like determining grove boundaries, or variable rate fertilizer application where you would need "sub-meter accuracy". You should also be aware that GPS receivers, when reporting a position, despite what you may see on your GPS display, can not tell you the accuracy of the reported position. GPS receivers will instead report the Estimated Position Error (EPE), which is an estimate of the likelihood that, some percentage of the time, the actual location of the reported position is within some distance of your current location.
How will you use it? - Does the GPS receiver display all the information you want, either on a single screen, or on multiple screens with easy navigation between screens? Are the buttons/controls easy for you to use? Is the GPS receiver's display readable given your eyesight and viewing habits (e.g. in direct sunlight)?
Description of several popular GPS receivers
Trimble Ag 106 – In differential GPS mode using WAAS DGPS corrections, the AgGPS 106 outputs positions with 1-3 meter (3 – 10 feet) accuracy (static HRMS). Connects to Laptop/PDA using a serial connection
Trimble Ag 114 – sub-meter differential
Trimble Ag 132 – position accuracy Static(year to year) – sub-meter differential Dynamic (pass to pass) – 4 – 12 inches (10 – 30 cm)
Trimble Ag 214 – RTK - 1 cm + 2ppm baseline distance horizontal 2 cm + 2ppm baseline distance vertical
Trimble Ag 252 – Horizontal RTK positioning accuracy 2.5 cm (.98 in) + 2 ppm, 2 sigma; vertical RTK positioning accuracy 3.7 cm (1.46 in) + 2 ppm, 2 sigma Sub-meter differential accuracy (RMS), assuming at least five satellites and a PDOP of less than four
Garmin eTrex Legend – Position < 15 meters, 95% typical, Velocity 0.05 meter/sec steady state. WAAS Accuracy: Position: less than 3 meters (10 feet) RMS Velocity: 0.1 knot RMS steady state
Garmin GPS 72 - GPS Accuracy: Position < 15 meters, 95% typical, Velocity: 0.05 meter/sec steady state
Garmin GPS 17 – Position: < 15 meters, 95% typical, Velocity: 0.1 knot RMS steady state, DGPS (USCG) accuracy: Position: 3-5 meters, 95% typical, Velocity: 0.1 knot RMS steady state, DGPS (WAAS) accuracy: Position: < 3 meters, 95% typical, .1 knot RMS steady state
Garmin 18 GPS - In differential GPS mode using WAAS DGPS corrections, the Garmin 18 GPS outputs positions with 1-3 meter (3 – 10 feet) accuracy (static HRMS). Connects to Laptop/PDA using either USB or Serial connection
TopCon Millimeter GPS – Position: millimeter accuracy, Multiple rover support.
TopCon GR3 – Static 3mm +.5ppm horizontal, 5mm + .5ppm vertical, RTK/Kinematic 10 mm + 1 ppm horizontal, 15mm + 1 ppm vertical, DGPS > .25m Post Processing, < .50m Real time
Magellan MobileMapper CE – Real-time accuracy – sub-meter, Post-mission accuracy - < 30 cm, Bluetooth, USB host and slave, RS232, Waterproof to one meter depth for 30 minutes.
Magellan MobileMapper Pro – Real-time accuracy – 2-3 meters, Post-mission accuracy – Sub-meter, Connector – Serial data cable, Waterproof to one meter depth for 30 minutes.
Lowrance iFinder Expedition C – WAAS and GPS Accuracy to 7 meters, Waterproof to IPX7 standard, Built in microphone for recording voice notes to waypoints, Built in Compass and barometric altimeter with weather predictions.
AgLeader GPS 5100 – Differential position accuracy – less than one meter(3.28 ft) horizontal is all of the following criteria are met: At least five satellites, PDOP <4, RTCM SC-104 corrections, Standard format broadcast from a Trimble MS750, AgGPS 214, or equivalent reference station. RTK position accuracy 2.5 cm + 2 ppm, 2 sigma, and vertical 3.7 cm + 2 ppm, 2 sigma.
AgLeader GPS 1100 – Accuracy with WAAS/EGNOS Diff: two meter RMS, Accuracy GPS only: four meter RMS, Output NMEA-0183.
Ag Leader Technology, Inc. – 2202 South Riverside Dr. Ames, IA 50010 (515)232-5363.
Garmin International Inc. – 1200 East 151st Street Olathe, KS 66062-3426 (913)397-8200.
Lowrance Electronics, Inc. – 12000 East Skelly Dr. Tulsa, OK 74128 1-800-324-1356.
Magellan Navigation, Inc. – 471 El Camino Real Santa Clara, CA 95050-4300 1-800-707-9971.
Topcon America Corp. – 37 West Century Rd. Paramus, New Jersey 07652 1-800-223-1130.
Trimble Navigation Limited – 935 Stewart Dr. Sunnyvale, CA 94085 1-800-874-6253.