Section 5: GPS Surveying
Anchor: #i1028044General Information
In general, GPS applications can be categorized as follows:
- Autonomous—Immediate positions determined without the aid of post processing or differential corrections. Low-end handheld GPS receivers acquiring only this type of position may be used for finding monuments or rough positions within about 30 feet.
- Static—Long observations establishing long baselines for the purpose of determining survey-grade coordinates for control of projects or intermediate points for extending the National Spatial Reference System. The data from these observations are post-processed in a network which is adjusted using a least squares method.
- Fast Static—Similar to Static with the exception of the length of time of observation – the receivers have the capability of determining (from user set parameters) the time necessary to acquire sufficient data for post processing a position. Time usually ranges from 8 to 20 minutes for an observation. When done with single frequency equipment, this type of survey is usually limited to about 12 kilometers. Even with dual frequency equipment, it should be limited to about 20 kilometers for design grade surveys. Accuracy degrades quickly, but predictably, at longer distances. In a VRS Network, this accuracy will increase.
- Post Processed Kinematic—Used for higher production – occupation times of well under a minute yet closed loop accuracy when two or more base stations are used. Single frequency data is collected using distances of less than about 10 kilometers.
- Real Time Kinematic (RTK)—Used for topographic surveys, staking out, and other applications, where radial baselines are acceptable. Accuracies of about 2 cm horizontal and 3 cm vertical are attainable at distances of up to about 10 kilometers. Accuracy drops off quickly at longer distances because of atmospheric errors. Many times the communication link between the base station and rover unit will prevent working at these distances. Observation times can be as short as 5 seconds.
- Continuous Kinematic—Used for rapid collection of topo data over large areas not requiring a high degree of accuracy. The rover antenna, usually attached to a vehicle, navigates the roadway or terrain to create trails of points collected at regular intervals without operator intervention. Most often, the driver will follow breaklines or survey chains (top of banks, fences, edge of pavement, etc.).
- Airborne GPS—Used for control for photogrammetry – this reduces the amount of paneling necessary. The system includes receivers at reference stations and rover equipment in the aircraft. The receiver in the aircraft is synchronized with the camera shutter for the geo-positioning of the photos.
- Networked RTK (VRS)—This variation of the standard base-and-rover RTK surveying makes use of a sophisticated network of permanent base stations over a large area at a spacing of approximately 70 kilometers. The stations are connected to a central computer which is accessed by the rover unit (via cellular phone) to communicate a correction to the rover from a “virtual” position within a short distance of the rover. This provides accuracy at the rover that rivals the results of working within a couple of miles of an actual base station.
- Code based data collection—This method of acquiring positions rapidly and with additional attribute information is used for GIS mapping applications where accuracy is not a factor; decimeter to several meters may be considered sufficient. This type of work utilizes code based GPS receivers. The methods of GIS data collection are not addressed in this guide.
TxDOT Levels of Survey Accuracy for GPS
Seven levels of Global Positioning System (GPS) surveying have been established by TxDOT to aid in maintaining standards of accuracy for different types of GPS surveys. The first four of these levels apply to design grade surveying. The most accurate stations in the state are the Continuously Operating Reference Stations (CORS) of the National Spatial Reference System (NSRS). These stations are overseen by the NGS and their placements are not included in the TxDOT Levels of Surveys. A number of these are maintained by TxDOT and are referred to as regional reference points (see “Datum and Project Control” below).
Anchor: #i1028111Level 1 Surveys
The basic purpose of this highest level of GPS surveys is for setting auxiliary points to densify a network of A and B order points that augment the above mentioned CORS stations. This will allow shorter observation times for performing static surveys in placing all lower quality control points. A typical data sheet for one of these points will show the amount of detail and documentation involved. In the usual seven digit station name, the first three digits represent the county number and the last four digits denote the discreet point number assigned by the district. This level of surveys requires the direct supervision of an Registered Professional Land Surveyor (RPLS) and these points are usually set only on an as needed basis for very large projects.
Anchor: #i1028121Level 2 Surveys
Intended mainly for project control, these points usually include an azimuth mark for use with conventional surveying equipment. The high degree of accuracy is needed not just in relation to each other (local) but also on the High Accuracy Reference Network (HARN) network so that the same points can be used in subsequent adjoining projects years later. A standard naming convention and data sheets are also commonly used at this level. These surveys require direct RPLS supervision.
Anchor: #i1028131Level 3 Surveys
Still sometimes held tightly but can be relaxed enough to use faststatic or kinematic methods with two, or more, higher level reference stations. Appropriate for use mostly for surveying photogrammetric center line panels, property corners and base stations for topographic surveys. RTK will require the use of two or more base stations.
Anchor: #i1028141Level 4 Surveys
Least stringent design level allowing radial baselines for kinematic surveying. Mainly for topo work, registering data and for continuous kinematic. This level requires use of a temporary base station.
Anchor: #i1028151Level 5 Surveys
Level 5a - Includes mapping-grade (GIS) work that is held to 30 cm accuracy. Generally, this level requires a real time correction or post-processing from a base station. Surveys of this level are limited to horizontal accuracy, with vertical positions used for informational purposes only due to their poor accuracy.
Level 5b - Includes mapping-grade (GIS) sub-meter network accuracy. This is probably the most used level of accuracy for GIS work and can be accomplished with the largest variety of equipment. Surveys of this level are limited to horizontal accuracy, with vertical positions used for informational purposes only due to their poor accuracy.
Anchor: #i1028161Level 6 Surveys
This level includes mapping-grade (GIS) work that is held to within a 5 meter accuracy. A Satellite Based Augmentation System (SBAS) is commonly used for this level of accuracy. Surveys of this level are limited to horizontal accuracy. The vertical component is generally of very poor accuracy.
Anchor: #i1028171Level 7 Surveys
This category includes locative work for rough positioning using autonomous positions. An accuracy of 10 meters is required, which is achievable with most consumer grade handheld units. This data is limited to horizontal accuracy with vertical positions seldom included.
Anchor: #i1028181Requirements and Specifications
Only general procedures for performing GPS static and real time kinematic surveying are outlined in this chapter. The specific requirements and specifications are outlined in the TxDOT GPS User’s Manual, located on the Internet at www.dot.state.tx.us under the “Business” heading, select “Online Manuals.”
Anchor: #i1028193Equipment and Software
TxDOT offers support to its surveyors for Trimble receivers and Trimble processing and adjustment software. Technology Services Division (TSD) maintains a list of recommended equipment and software, in the Procurement and Justification System (PJS), which includes GPS items.
A request for additions can be made on GPS equipment available for TxDOT procurement using the Request for PJS Catalogue Addition Record Form 2157. This catalogue is accessible to the district information resources administrators. Procedures for use of the equipment are outlined in Chapter 4 of the TxDOT GPS User’s Manual and procedures for processing and adjustment of GPS data are included in Chapter 6 of the same manual.
Anchor: #i1028208Datum and Project Control
The reference system for horizontal control in the United States is the North American Datum of 1983 (NAD 83). The reference system for vertical control is the North American Vertical Datum of 1988 (NAVD 88). Surveys are referenced to these datums through measurements to control points of the National Spatial Reference System (NSRS). The NSRS is referenced to a nationwide network of Continuously Operating Reference Stations (CORS). There are approximately 70 of these NSRS stations in Texas. A densification network of A and B order points supplement the network.
Primary project control points should be surveyed in from the CORS stations with ties to additional A and B order points as needed to provide shorter distances and proper geometric network configuration (see the section on “Level 2” surveys in the TxDOT GPS User’s Manual, Chapter 6, Section 6 “Field Survey Operations and Procedures”). It should be noted that HARN coordinates are computed as of the date of the HARN survey (1993). On the other hand, CORS coordinates are computed as of the observation date. As a practical matter, there is not enough movement over time for points in the eastern US to be significant at magnitudes of less than one centimeter accuracy. However, users should examine differences between HARN and CORS coordinates to determine if there has been significant local or regional movement over time.
Primary project control points should be positioned and spaced so that they can be used for both conventional and GPS work. An azimuth mark should be visible from the station for surveys with conventional equipment. A data sheet form (see Chapter 6, Section 3 of the TxDOT GPS User’s Manual) to record new control point metadata should be documented for use with new GPS control points. Generally, more reference stations and ties are given than for conventional surveys and an ellipsoid height is added.
Figure 3-7. TxDOT Regional Reference Points (RRP).