Anchor: #i1028039

Section 5: GPS Surveying

Anchor: #i1028044

General Information

In general, GPS applications can be categorized as follows:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.).
  7. Airborne GPS — Used for control for photogrammetry. Airborne GPS 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.
  8. 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.
  9. 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.
Anchor: #i1028099

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 subsection Datum and Project Control within this section).

The following information provides GPS positioning specifications for TxDOT.

Anchor: #i1249345Table 3.6 TxDOT GPS Positioning Specifications

 

Level 1

Level 2

Level 3

Level 4

Typical job type

RRP’s, CORS or major control densification

Primary project control points

Property corners, secondary project control, flight panels

Topo surveys, and non-critical stakeout

Type of GPS

static

static

fast static or RTK

RTK

GPS Positioning Relative to Other Points (Local Accuracy)

Instrument setup error

2 mm for B order monuments-RRP/CORS have zero setup errors

2 mm

3 mm

5 mm

Total baseline length error at 2 sigma

8 mm + 1 ppm

8 mm + 1 ppm

12 mm + 1 ppm (see Note1)

20 mm + 1 ppm (see Note 1)

Maximum baseline length for referencing from CORS station

200 km

200 km

N/A

N/A

Maximum baseline lengths between points on a project

100 km

25 km

5 km (no limits within a VRS cell)

5 km (no limits within a VRS cell)

Minimum time per occupation

2 hrs + 1 min per km baseline

1 hr + 1 min per km baseline

180 epochs with rod rotated 180 degrees between observations

3 epochs (see Note 2)

Minimum number of occupations for static network (see Note 3)

2 for B order monuments - N/A for RRP/CORS stations

2

2

N/A

Minimum time between occupations

3 hrs

2 hrs

1 hr

N/A

GPS Positioning on the State Plane Grid (Geodetic Accuracy)

Horizontal accuracy at 2 sigma

12 mm

20 mm

25 mm

45 mm (see Note 4)

Elevation

Accuracy at 2 sigma (assuming 100% perfect geoid model

22 mm

25 mm

30 mm

40 mm (see Note 5)



Note 1 - RTK baselines are measured from base station to rover point.

Note 2 - This should not be confused with 3 seconds - depending on conditions, it usually takes approx 5 to 10 seconds for 3 useable epochs.

Note 3 - A complete new setup is required for each occupation.

Note 4 - This should not be confused with project (local) accuracy which is 20 mm + 1 ppm in relation to radius points, traverse points and other secondary control as shown in above “Local Accuracy” specifications.

Note 5 - The vertical component is not acceptable for most stakeout operations.

Anchor: #i1028111

Level 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 (7) digit station name, the first three (3) digits represent the county number and the last four (4) 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: #i1028121

Level 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: #i1028131

Level 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, or two (2) observations from a networked RTK connection.

Anchor: #i1028141

Level 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 or a networked RTK connection.

Anchor: #i1028151

Level 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: #i1028161

Level 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: #i1028171

Level 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: #i1012100

Data Collection Forms

A sample GPS log sheet is shown on the next page. Please use this form or a similar one. A way to link the form to the data file is important to the processing person. In this case, the form requests the GPS receiver’s default file name on the line 8-digit filename.

A form can be tailored to the needs of the survey crew depending on their experience and proficiency. The collection of weather and meteorological data may be necessary if the project worked on is to be included in the NSRS (bluebooked).

Below is a sample GPS Log Sheet. For a .pdf of the log sheet, click here.

GPS Log Sheet

Operator Name

 

Observation Date

 

Station Name

 

8 digit File Name (if known)

 


Antenna Height

Meters

 

1st Measurement

 

Survey Ft.

 

2nd Measurement

 
   

3rd Measurement

 
   

Average

 


Measured to:

 

Bottom of Notch

 

Top of Notch

 

Antenna ref. point

 
   

Antenna Type

         
   

Actual Start Time

         
   

Actual Stop Time

         
   

4 Digit Receiver #

         


Notes

 
 
 

 
 

If adjustable height tripods are used, the height of the antenna above the mark should be measured. This measurement should take place at a minimum of three (3) locations around the ground plane, in two (2) separate units, at the beginning of the observing session, and again at the end of the observing session. The H.I. must be recorded in a field book or on log sheets for every occupation.

Anchor: #i1012140

Static Observation Field Procedures

All control stations and boundary corners should be occupied a minimum of two times during the course of a survey. Table 3.11 Minimum TxDOT Network Design Specifications, in this chapter outlines how those occupations should be accomplished.

The normal collection rate (epoch) is 5 seconds for static observations, but for long observation times of more than about 3 hours, 15 second epochs are acceptable. For observations of less than half an hour, 5 second epochs are preferable. For fast/rapid static observations, 5 seconds is required. RTK is done at 1 second.

Longer baselines will require longer observations on end points. Minimum observation times for Levels 2 and 3 are listed in Table 3.8 GPS Static Observation Planning. Allowances should be made for difficult setups that may have less satellite visibility or high PDOP.

Level 1 surveys usually involve long distances and will almost always require observation times of 4 to 6 hours and at least two occupations.

The following table illustrates minimum observation times:

Anchor: #i1008595Table 3.7 Minimum Observation Times for Survey Levels 2 and 3

Length of Baseline

Minimum observation time *

less than 10 km

45 min

10 to 40 km

1 hr

40 to 100 km

2 hr

100 to 200 km

3 hr

more than 200 km

4 hr or more



* Assuming at least 5 satellites and PDOP of less than 6.0.

Anchor: #i1012181

RTK Field Procedures

Real-time kinematic (RTK) allows close-in surveying without the requirement of line of sight to the control point. This is very cost effective for Level 4 surveys, and with more stringent requirements RTK can be used for Level 3 surveys.

Set up the base station on a control point with known x, y, z coordinates (all control points must have GPS-static quality horizontal values and differential leveled vertical values). The selection of the base station sites during the project planning phase will greatly affect the success of the RTK observations. If a poor base station site is selected, there will likely be problems throughout the entire survey.

The following information identifies parameters of base station sites:

  • Select a site with good sky visibility down to (ten) 10 degrees from the horizon.
  • Be aware of high power transmitters, such as microwave, TV stations, military installations, high voltage transmission power lines, etc.
  • Be aware of multipath caused by radio wave reflective objects, such as trees, buildings, large signboards, and chain link fences, etc.

If there are no useable control points in the immediate area, or much is to be gained by setting a new control point for the base station, a position can be obtained for the base station setup by means of a calibration from other control points. The survey may be started on just the autonomous position after setting up the base station on the newly placed mark.

With a successful initialization at each of at least three (3) control points, perform a forced coordinate position or calibration by keying in the proper coordinates for each point. This will propagate the correct coordinates to the base station. The calibration control points must be within about three (3) miles of the base and in at least two (2) separate quadrants.

The calibrated base station coordinates will only be as good as the quality of the chosen calibration points and are a poor substitute for the assurance of a pre-surveyed control point.

The surveyor logs the following base station setup into the field notes:

  • station name and/or number
  • receiver and antenna type
  • antenna measurement method (i.e. bottom of notch, bottom of antenna, etc.)
  • record antenna H.I. measurements at the beginning and end of each setup
    • if using a fixed height tripod, make and record a measurement to verify that the fixed height has been checked
  • record the local time that the base station is started and stopped
  • record any problems encountered during the course of the survey with the base station.

The TxDOT UHF transmitters should be two (2) watts. The private sector does not have this 2-watt restriction on their itinerate frequencies. The FCC radio license is for data transmissions. This means TxDOT’s radios have to stop transmitting when voice transmissions are being made. Be sure the transmitter is equipped with a blocker. Have the proper license and carry a copy with the equipment.

Anchor: #i1012287

Rover Settings

Configure equipment settings for the type of project to be surveyed. It is a good idea to have all the possible options available while collecting data. In many cases, not every option is used. However, if needed, they will be available.

Some options are to:

  • store raw observables at the base to allow for post-processing of the base position should the need arise
  • store vector information to allow the RTK data to be adjusted with least squares should the need arise
  • set up the survey to allow for post-processed kinematic data should the radio link be lost on a few shots.
Anchor: #i1012317

Rover Initialization

There are several ways to initialize a kinematic survey. They can include a known baseline; use of an initializer bar; a new point; and an on-the-fly (OTF) initialization.

After the first OTF initialization, observe a point. This can be a temporary mark or a point in the survey. Discard the first OTF initialization and OTF re-initialize with the H.I. changed by more than two feet or move more than forty feet away from the point to be used as a check.

After the new OTF initialization has been accomplished, return to the point being used as a check and re-shoot it. Compare the first and second shots. Are they within an acceptable tolerance?

If the points check, proceed with data collection with the confidence in surveying with a correct initialization. If the error between the two points is beyond the expected error, one or both of the OTF initializations used for a check are incorrect. OTF re-initializations at any of the positions previously used cannot be reused.

The location must change by a difference of more than two feet of H.I. or, more likely, move more than forty feet away in a different direction. This will usually provide enough information to identify the OTF initialization that is incorrect. Once the problem is solved begin the survey. This procedure must be repeated with any loss of initialization.

Each time a re-initialization is done as a result of a complete loss of lock on satellites, the first station surveyed thereafter must be surveyed a second time with a new initialization from which the survey can continue if the two initializations agree.

NOTE: Contact the local district survey coordinator for a copy of the latest version of the TxDOT Feature Code List.

Anchor: #i1012375

RTK for Wing Panels

Before starting to survey the panels, use the rover to check into at least one other control point with known x, y, z coordinates (all control points must have GPS-static quality horizontal values and preferably, differential leveled vertical values). These checks should be logged in the field book and in the data collector device. Also make and log checks during the course of the day. If any check shots are greater than 0.10 feet horizontally or 0.12 feet vertically, the problem should be resolved.

Wing panel surveys are in the Level 3 category and must therefore adhere to those requirements including positioning from a second base station.

Anchor: #i1012390

RTK for Topographical Surveys

Connectivity of survey chains is required for topographical surveys and the use of TxDOT feature codes is mandatory. This TxDOT list is available in Trimble format as txdot2k.fcl and in CAiCE format as txdot2k.ftb. A printed list of the TxDOT feature codes is available from district survey coordinators.

At no time should the rover exceed a distance of three (3) miles from the base in a topographical survey using radial baselines.

With RTK topographical surveys, any time initialization is lost and reestablished, a previously occupied point should be redone as a check. At least one in every ten (10) points of the survey should be redone with a new initialization at a later time. This would result in a total of at least 10% of the stations receiving a second occupation.

Anchor: #i1028193

Equipment 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 Catalog Add Request Form 2157. This catalogue is accessible to the district information resources administrators.

Anchor: #i1001353

GPS Receiver

The receivers used for network surveys should record the full-wavelength carrier phase and signal strength of both the L1 and L2 frequencies, and track at least eight (8) satellites simultaneously on parallel channels. L1 only receivers are acceptable only for baselines less than 10 km. Ties to CORS sites should be made with dual-frequency instruments if base lines are longer than 10 km. Receivers should have sufficient memory and battery power to record 6-hours of data at 5-second epochs. Receivers should contain the latest manufacturer’s firmware upgrades.

Anchor: #i1001367

GPS Antenna

The antennas should have stable phase centers and be designed to minimize multipath interference. All antenna models used should undergo antenna calibration by the National Geodetic Survey (NGS). Users should consult user’s manual for other specifications.

NGS Geodetic Services Division maintains a GPS Antenna Calibration Web site for calibrating a variety of antennas.

When processing GPS baselines, the user must apply the appropriate GPS antenna phase center offsets. Inappropriate phase center offsets can introduce up to 10 cm of error in the baseline.

GPS antenna ground planes should be utilized according to manufacturer specifications. Ground planes must be utilized for all stations when performing TxDOT Level 1 and Level 2 surveys. For other surveys, a ground plane must be used at the base station and should be utilized in areas where there might be significant multipath. Many new antenna models have built in ground planes.

Anchor: #i1001401

GPS-RTK Rover Rod

A fixed height rover rod should be used and if possible, it should be the same height as any fixed height tripods on the project; usually 2 meters. Make a physical measurement in the field notes to verify it has been checked. Also, check the level bubble on the rod before and after each project.

Anchor: #i1001411

Tripods

The tripods must facilitate precise offset measurements between the mark datum point and the antenna reference point (ARP). Fixed-height rods or fixed height tripods are preferable and required for certain surveys due to the decreased potential for antenna centering and height measurement errors. All tripods should be examined for stability with each use. Ensure that hinges, clamps, and feet are secure and in good repair. Test the fixed-height tripods for stability, plumb alignment, and height verification at the start and end of each project.

Anchor: #i1001425

Tribrachs

Tribrachs and rod levels should be field calibrated before use on each project and should be checked at the end of the project. Any data not bracketed by a successful calibration check are suspect. Professional Tribrach calibration, usually scheduled once a year with regular use is a reasonable interval for maintaining the accuracy of the instrument.

Anchor: #i1001449

Personnel

All field personnel should be trained in the avoidance of systematic errors during field operations. Field personnel often work alone and must be prepared to make wise, on-the-spot decisions regarding mark identification and stability, equipment use and troubleshooting, and antenna setup. Office personnel should be familiar with geodetic concepts and least squares adjustments. Personnel should participate in any available certification and training activities.

All boundary control survey projects performed for TxDOT will be performed under the charge of a Texas Registered Professional Land Surveyor (RPLS). Personnel requirements for various types of surveys may vary from one TxDOT district to another. The use of certified survey technicians (CST’s) is encouraged not to fulfill any requirements, but to aid in the efficiency of operations with the use of goal-oriented employees.

Anchor: #i1028208

Datum 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.

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 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.

TxDOT Regional Reference Points (RRP),
January 2010. (click in image to see full-size image) Anchor: #i1003708grtop

Figure 3-5. TxDOT Regional Reference Points (RRP), January 2010.

Previous page  Next page   Title page