In Lab 2 we identified the importance of geo-spatial data and worked with two sources of general utility, geo-spatial data: DOQs and DRGs. Information systems supporting natural resource decision-making will also typically require organization- or problem-specific geo-spatial data. For example, an hydrologist may want point data representing locations of storm sewers along a waterway. Or a park manager may want line data representing trails in a park. Or a forester may want polygon (area) data representing particular stands of trees. The spatial (coordinate) component of data like these is commonly collected using global positioning system (GPS) technology.

The U.S. Department of Defense maintains 24 satellites, orbiting the earth at an altitude of approximately 12000 miles, which are the basis for GPS. A person utilizes the system via a GPS receiver unit. The GPS receiver accepts signals from the satellites and, through basic triangulation calculations, uses the signals to determine position.

A wide price/capability range of GPS receivers are available. All receivers use the same satellites, though sometimes in different ways. Greater positional accuracy generally requires receivers of greater capability (price). Receivers costing a few hundred dollars are often adequate for many natural resources mapping applications, with positional accuracies better than 15 meters. High-end “survey grade” receivers can obtain accuracies better than a centimeter.

Utilizing a GPS receiver under tree cover, for example, can interfere with satellite signal reception. In an urban setting, tall buildings can also be a problem. Since the receiver units compute distance (used for triangulation) by timing satellite signals, the state of the atmosphere through which the signals are traveling also impacts system accuracy. Higher capability units or peripherals can be used to address the first two problems. Methods, collectively referred to as differential GPS, are also available for addressing issues related to changing atmospheric conditions. Prior to May, 2000 the Department of Defense intentionally degraded GPS accuracy in a non-random, yet unpredictable manner. This source of system error has not been a consideration since that time.

GPS receivers can typically provide positional readings in a number of formats. Two common formats are latitude/longitude and Universal Transverse Mercator (UTM). When using a GPS receiver with a map it is also important that the datum being used by the receiver to display position is the same as the datum used to construct the map.

For most natural resource science and management applications of GPS it is necessary that the GPS receiver unit is able to store positional readings; readings taken in the field are subsequently uploaded to a desktop GIS. The reason for this should be obvious when considering the collection of line and polygon data. For point data, most GPS receivers also have the ability to take multiple readings at the point and average the readings to obtain a more accurate positional fix.

To obtain a better idea of how GPS works, refer to Trimble’s excellent, animated tutorial. Trimble sells many higher-end GPS receiver units. An accessible written description of GPS is provided by Garmin. Garmin sells many popular lower-end GPS receivers. You will be using a Garmin 76 or Garmin 12CX receiver for the laboratory exercise this week; hard copies of the receiver usage instructions will be provided to you during lab.

Link to the assignment for this week (0.9MB)


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