Map Projections and Coordinate Systems 1

Instructor: Ling Bian                                                                                          LabA: Tue:   5-6:20pm, W145, Chunyuan diao
Office: 120 Wilkeson                                                                                        LabB: Thur: 6:30-7:50pm, W145, Tong Sun
Office hours: Tu Th 2-3pm  or by appt.                                                             LabC: Fri:  10-11:20am, W145, Tong Sun

A method used in surveying that uses a constellation of satellites orbiting the earth at

very high altitudes.

The GPS technology allows accurate geodetic surveys by using specially designed receivers

that, when positioned at a point on the earth, measure the distance from that point to three

or more orbiting satellites.

Through the geometric calculations of triangulation, the coordinates of the point on the

surface of the earth are determined.

NAVigation Satellite Timing And Ranging Global Positioning System)

Developed by U.S. Department of Defense
A constellation system of 24 satellites orbiting the earth a a high altitude twice a day

The basis of GPS is triangulation

Satellites: continuously broadcasts time and its location

GPS receiver: receives the signals and calculates the distance between the satellite and the receiver

Distance = velocity x travel time

Determination of location
3 or more simultaneous distance measurements are needed to determine the location of the receiver

Coordinate systems
A GPS provides its position in geographical/UTM/State Plane coordinates and altitude

Radio channels: one or more channels to receive signals of
(1) the time a signal is sent, and

(2) a pseudorandom code to identify the signal

Internal clock:
synchronized with the satellite in order to calculate the signal travel time precisely

    Computer:
        calculates distance and location
        stores location readings

calculates the satellite availability (the quality of recording depends on the clustering of satellites)

Autonomous GPS: one receiver unit

Selective availability interference
The US government used to insert random errors in the signals in order to maintain optimum military effectiveness of the system. It was turned off in 2000.

    Differential GPS:
           two receiver units, with one placed at a known location to detect the inserted errors and calculate the needed corrections. The correction
           information is used by another receiver to overcome its selective availability interference

WAAS (Wide Area Augmentation System)
is based on a network of approximately 25 ground reference stations that cover a very large service area. Signals from GPS satellites are received by wide area ground reference stations (WRSs). Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist. These WRSs are linked to form the U.S. WAAS network. This service is primarily for aviation purposes.

6. Static vs. Kinematic GPS
Static: two or more receivers receive data for a lengthy period of time at a fixed location

Kinematic: one receiver is placed at a known location, while one or more receivers move from point to point

7. Advantages and limitations
Ad: fast, more accurate, lower cost than manual approaches