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# Learning Objectives:

• Introduce different coordinate systems used for mapping

• Define the initial latitude-longitude coordinate system of a dataset, change it to the official

coordinate system of New Jersey (NJ State Plane 1983 NAD)

• Learn to use different data sets (TIGER, geocoded addresses, GPS, air photos) in one view

Purpose: This exercise will introduce you to the basics of Global Positioning System (GPS) digital data collection and differential correction, and also give you experience mapping an area with different data sets from different sources. Working in groups if possible, you will use a GPS receiver to record the latitude-longitude (“lat-long”) positions of the sidewalk entrance to PCTC and the location of each group member's car, bus stop, or other mode of transportation. As a group, you will differentially correct and export the GPS data to ArcMap. Finally, each person will locate and highlight their car and measure its distance from the entrance of your building.

Part I - GPS Data Collection, Correction, and Export as a GIS shapefile Introduction: Satellite positioning with a GPS receiver is based upon a simple equation:

Velocity = Distance / Time

24 U.S.-owned GPS satellites circle the Earth and send signals traveling at the speed of light (186,000 miles/sec) continuously. The positions of these satellites are accurately tracked by U.S. installations around the world. Embedded in the signal sent by each satellite is the time that the signal was sent, and the GPS unit receiving the signal has a clock that records the time the signal was received. Therefore, the above equation can be re-written:

186,000 miles/sec = Distance / Treceived - Tsent

The equation can be re-arranged and solved for distance; thus, the GPS receiver “knows” that it is a certain distance from this particular satellite at this particular time. Visualize the GPS receiver at the center of a sphere whose surface has a radius (“r”):

# Satellite (somewhere on surface of sphere)

r

r

The GPS receiver “knows” that the satellite is somewhere on the surface of this sphere at some distance “r”, but that is all that it knows. More accuracy requires more satellites; in fact, geometry dictates that 4 satellites (at least) are needed, because 4 intersecting spheres are required to produce a point. More satellites typically translate to better accuracy. A Trimble Geoexplorer 3 mapping-grade GPS receiver will be used by each group. Each student is required to actually work the receiver and record his or her name under Comment when taking the position of that student's car or motorcycle or bus stop. However, prior to going out to collect data, you need to check satellite coverage in your area for your collection date by using the Trimble Pathfinder Office software on a computer in the GIS Lab. If satellite coverage is not good (at least 4 or 5 visible satellites), it is EXTREMELY DIFFICULT to get usable GPS data.

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