GPS Surveying

1.  GPS Surveying:

GPS Surveying, now commonly referred to within the broader field of Global Navigation Satellite Systems (GNSS), represents a revolutionary approach to determining precise positions on the Earth. It emerged during the 1970s and grew out of the space program.

What is GPS/GNSS?

• GNSS is the term used for the entire scope of satellite systems used in positioning. GPS is the United States’ system. Other systems include Russia’s GLONASS, Europe’s Galileo, and China’s Compass. Receivers that use GPS and another system like GLONASS are known as GNSS receivers.
• GNSS relies upon signals transmitted from satellites for its operation.
• These systems provide precise timing and positioning information anywhere on the Earth with high reliability and low cost.
• They can be operated day or night, rain or shine.
• Crucially, they do not require cleared lines of sight between survey stations, which is a revolutionary departure from conventional procedures that rely on observed angles and distances.

How it Works (Basic Principles)

• Precise distances from satellites to receivers are determined from timing and signal information.
• The satellites become the reference or control stations.
• The ranges (distances) to these satellites are used to compute the positions of the receiver.
• Conceptually, this is equivalent to resection in traditional ground surveying.
• A minimum of four positioning satellites must be observed to solve for position.
• Measurements:
  • Pseudorange measurements are typically used for navigation and situations not requiring typical survey accuracies (e.g., approximate monument location, updating small-scale maps, navigation). The basic measuring unit of the C/A code used in navigation is about 30m.
  • Carrier frequency phase measurements are used for the higher precision necessary in engineering surveys and relative positioning. The L1 carrier is 19 cm, with range measurement to millimeters possible.
• Differencing Data: Techniques like single and double differencing are used to eliminate or reduce errors, such as receiver and satellite clock bias.
• Relative Positioning: Involves two (or more) receivers occupying different stations and simultaneously making observations to several satellites. The vector (distance) between receivers, called a baseline, and its coordinate difference components are computed. This method is based on carrier phase-shift measurements.

Types of GNSS Surveys/Methods

The sources describe several specific field procedures based on carrier phase-shift measurements and relative positioning techniques:

• Static: Involves occupying stations, often forming geometrically closed figures. Provides high accuracy. Used to establish project control. The epoch rate (measurement interval) must be the same for all receivers (typically 15 sec). Repeat baseline observations should be made for checking. Specifications exist for static surveys regarding accuracy orders and redundant observations.
• Rapid Static.
• Pseudokinematic.
• Kinematic: The most productive form of surveying where speed is essential. Provides immediate coordinate values while the receiver is stationary or in motion. Accuracy is typically less than static surveys but adequate for most survey forms. Often used for mapping surveys.
• Real-Time Kinematic (RTK): A type of kinematic survey where a base station broadcasts corrections (differences) to roving receivers simultaneously tracking the same satellites. Provides immediate values to the coordinates of points. Can be used to locate construction stakes.
• Differential GPS (DGPS): Utilizes a less expensive GPS receiver and radio signal corrections from a base station to provide submeter accuracies. Acceptable for mapping and GIS database inventories. Continuously Operating Reference Stations (CORS) are established receiver stations that collect signals continually and provide correction data (real time or postprocessing) via radio, cell phone, or website.

Planning and Field Procedures

• Planning GPS surveys is more critical than for conventional surveys due to the potential economic impact of having to repeat work.
• Planning involves plotting points, noting baseline lengths, and showing existing control. Long baselines might be split to improve accuracy.
• Ensure a clear view of the sky at observation points to avoid multipath effects from nearby buildings. Obstructions like tree canopy or structures impede signal reception.
• When using GPS, field procedures depend on receiver capabilities and the type of survey.
• Key field steps include setting the receiver over the point, measuring and recording the Antenna Reference Height (ARH), verifying session programming, selecting the mode, and verifying satellite lock and position computation. Observations are monitored.
• Field notes are important even with electronic data collection. They should include the ARH, equipment numbers, session times, crew, and job information. Sketches in manual notes are invaluable, especially for topographic surveys, helping with data editing and clarifying ambiguities.
• Data collected electronically can be stored internally or on controllers/memory cards and transferred to a computer for processing.

Errors in GPS/GNSS

Sources of errors exist. These can include atmospheric delays (tropospheric and ionospheric), satellite and receiver clock biases, and multipath effects (signals reflecting off surfaces before reaching the antenna). Differencing techniques help to minimize or eliminate some errors. Dilution of Precision (HDOP, VDOP, PDOP) is a factor related to satellite geometry that affects accuracy.

Applications of GPS/GNSS in Surveying and Geomatics

GPS/GNSS surveys are being used in all forms of surveying:

• Control Surveys: Used for establishing precise horizontal and vertical positions of reference monuments. They have largely replaced traditional triangulation and trilateration for establishing basic control over large areas. They can bring state plane coordinates into any region with satellite visibility. Primary and secondary control points can be established using GPS. Can be used to establish NAVD 88 control.
• Topographic Surveys: GNSS receivers specially designed for this work are small and portable. They can determine and store coordinates in real time, often as a one-person operation. Kinematic methods are most often used. Data can be downloaded for automatic map drafting. Can plot contours. Useful for locating boundary and control markers covered by snow using navigation functions.
• Construction Surveys: Any GNSS method could be used. Static surveys establish project control, kinematic surveys produce maps for planning, and RTK surveys locate construction stakes. Particularly useful for staking widely spaced points or in rugged terrain.
• Boundary Surveys.
• Hydrographic Surveys.
• Mapping. GPS can be used for mapping surveys. Data can be added to mapping or GIS databases.
• Geographic and Land Information Systems (GIS/LIS): Surveyors play a major role in developing and implementing GIS/LIS. GNSS is ideal for the collection of spatially related data due to speed and accuracy. Kinematic mode is particularly well-suited for collecting vector data like pipeline routes, property boundaries, and pavement edges. GIS relies fundamentally on accurate positional data, which surveyors provide using methods including GPS. Differential GPS (submeter accuracy) is popular for GIS data collection and inventory.
• Machine Guidance and Control: Earthmoving and grading plant are being controlled in three dimensions using GPS in RTK mode. Can guide the operator or directly control machine hydraulics. Can replace traditional methods like sight rails or batter boards.
• Navigation: Code-based receivers are widely used for navigation in transportation and shipping. Surveyors may use navigation functions to locate control monuments.
• Other applications: Include fleet management, precise docking, recreational craft navigation, precision farming, and scientific studies like meteorology, oceanography, and geophysics.

Integration with Other Technology

• The demand for spatial data has led to the integration of surveying technologies.
• Many manufacturers allow switching between GNSS receivers and total station instruments using the same data collector or survey controller. This is useful in areas where GNSS surveys are not practical due to obstructions.
• Some total stations have integrated GPS receivers. This allows the advantages of both systems. For instance, a total station can be set up at an uncoordinated location, and the integrated GPS can quickly determine its position in RTK mode, allowing surveying to commence without occupying a known control point. Handheld total stations can also collect data directly into a GPS data collector, extending surveys into obstructed areas where GPS signals are blocked.

Advantages

• Rapidly replacing other systems due to range, accuracy, and efficiency.
• High accuracy is achievable, especially with carrier phase measurements and static methods.
• Speed and productivity, particularly with kinematic methods.
• Operates day or night, rain or shine.
• No need for cleared lines of sight between survey stations.
• Reduces or eliminates the need for traditional control networks in some applications as the orbiting satellites provide the control.
• Can significantly reduce the time needed for establishing control and collecting data.
• Data is often in electronic form, easily compiled for use in GIS.

Limitations

• Requires an obstruction-free view of the satellites.
• Does not work well or at all close to buildings, indoors, underground, or underwater.
• Susceptible to multipath effects, especially in urban environments.
• (Mentioned as a factor in planning in one source, but may be outdated) Present high cost of GPS hardware and software could make repeating a survey economically prohibitive.

Conclusion

GPS/GNSS technology has had a revolutionary impact on all aspects of surveying. Its capabilities in precise positioning, speed, and efficiency make it a fundamental tool in modern surveying (Geomatics) practice. While it has limitations, its integration with other technologies like total stations continues to expand its applicability.