Section 6: Field Survey Operations and Procedures
Anchor: #i1011688Overview
Field survey operations should be performed using the manufacturer’s recommended receiver settings and observation times. Operations under adverse conditions, such as under a tree canopy or around urban environments where multipath conditions are high, may require longer observation times than specified by the manufacturer.
Anchor: #i1011700TxDOT Survey Levels
As noted in Chapter 3, TxDOT has divided its design grade GPS surveying into seven (7) levels to facilitate understanding of the type and scope of the survey, and to aid in developing specifications.
Anchor: #i1011712Level 1 Surveys
Level 1 is the highest order of TxDOT GPS survey. It is reserved for multi-project points and other statewide or district-wide densification of the FBN and CBN points. Since it is tied to the NSRS, these marks will augment the CORS stations and will aid in control of all subsequent TxDOT survey levels. The department will not usually set these on the speculation that they might be needed, but usually as an adjunct to a major project.
Level 1 points should be established by static survey methods. These points may be established at the same time as other survey levels are being performed. However, the points, and resulting baseline vectors used in the network should be processed to derive the baseline solutions and be adjusted by least squares, independently of the other survey ties. The horizontal quality of these points would be comparable to first order or better conventional points and up to B order GPS quality.
The Level 1 points will place accurate control closer to the project, shortening subsequent observation times and improving accuracy. It will offer the surveyor more flexibility for using rapid static/faststatic, kinematic and RTK survey methods for the other aspects of the survey. It provides an adequate amount of reference (base) station locations, ties subsequent points together, allows for expanding the area of the survey, and provides accurate checks throughout the survey project.
All Level 1 points should be referenced (tied) to at least four (4) FBN/CBN/CORS stations, two (2) of which should be CORS stations.
The current national reference datum is the North American datum of 1983. TxDOT places all new projects on this datum and it is recommended that the HARN adjustment, noted by NAD83 (2003), be used. Be sure the adjustment specified by the district survey coordinator is used – some situations dictate previously used datum adjustments.
All Level 1 points should conform to the requirements outlined in Tables 4.1, 5.1, 6.2, 6.6, and 6.8 of this manual to include the following requirements:
- referenced to at least the two (2) closest CORS control stations, located in two quadrants, relative to the survey project area and additional FBN or CBN published horizontal control stations for a total of at least 4 stations – one or more in each of at least three (3) of the quadrants of the project area
- referenced to five (5) or more published vertical control stations of second order or better, located in all four (4) quadrants, relative to the survey project area if vertical is required for the survey
- all new stations are established by two (2) or more independent baselines
- all stations must be occupied a minimum of two (2) times
- baselines should have a fixed integer double difference solution or adhere to the manufacturer’s specifications for baseline lengths, exceeding the fixed solution criteria (i.e. float solution may be the best solution for baselines in excess of 100 km, depending on manufacturer specifications and recommendations)
- any station pair used as azimuth or bearing reference for use with conventional survey measurements during the course of any other survey level should be included in a network or measured as a radial line with a minimum of two (2) independent baselines
- the district network must be a geometrically closed figure; therefore, single radial (spur) lines or side shots to points are not acceptable. (Radial lines are acceptable only for setting azimuth marks).
Level 2 Surveys
This level includes primary project control and control for airborne aerial photography or LiDAR data gathering. These points in the project area are tied to the NSRS through Level 1 control points and/or CORS and FBN/CBN points, if they fall within about sixty (60) miles of the project. A primary project control network usually is established by the static survey method. The primary project control network may be established at the same time the other survey Levels are being performed. However, the points and resulting baseline vectors used in the primary project control network should be processed to derive the baseline solutions and be adjusted by least squares, independently of the other survey ties.
Primary project control network is designed to meet the following purposes:
- provides a framework to reference the survey to a datum, a mapping projection, and the NSRS
- serves as the basis for all lower survey levels
- allows for the use of conventional survey equipment by always providing an azimuth or intervisible point
- provides control points to serve as geodetic control in a TxDOT project.
A well-designed primary project control network will offer the surveyor more flexibility for using kinematic and RTK survey methods for the other aspects of the survey. It provides an adequate amount of reference (base) station locations, ties subsequent points together, allows for expanding area of the survey and provides accurate checks throughout the survey project. It may be as simple as two well-placed, intervisible points for use with conventional equipment.
All primary project control networks should be referenced (tied) to at least three (3) horizontal stations, two of which should be the close CORS/FBN/CBN points. Any available statewide/district-wide densification points qualify as holding points for the primary project control points.
All Level 2 networks should conform to the requirements outlined in Tables 4.1, 5.1, 6.2, 6.6, and 6.8 of this manual to include the following requirements:
- referenced to three (3) or more CORS/FBN/CBN or Level 1 control densification horizontal control stations, located in three (3) or more quadrants, relative to the survey project area
- referenced to four (4) or more benchmarks (second order or better) in four (4) quadrants for orthometric heights
- all new stations are established by two (2) or more independent baselines
- all stations must be occupied a minimum of two times
- baselines have a fixed integer double difference solution or adhere to the manufacturer’s specifications for baseline lengths exceeding the fixed solution criteria (i.e., float solution may be the best solution for baselines in excess of 100 km depending on manufacturer specifications and recommendations)
- any station pair used as azimuth or bearing reference for use with conventional survey measurements during the course of any other survey level should be included in a network or measured radially with a minimum of two (2) independent baselines
- the primary project control network must be a geometrically closed figure; therefore, single radial (spur) lines or side shots to points are not acceptable (radial lines are acceptable only for distant azimuth marks).
Level 3 Surveys
This level of surveying work and points includes photogrammetric control panels, boundary corners, right of way (ROW) corners, local control, setup points for topographical surveys, laser-scan control points, azimuth marks less than one half mile from the station and other points requiring similar accuracy.
Level 3 stations can be tied to TxDOT Level 1 network stations, TxDOT Level 2 network stations or NSRS stations first order or better.
In some situations, these points can be surveyed using any one of the GPS survey methods described herein.
All Level 3 points should conform to the requirements outlined in Tables 4.1, 5.1, 6.2, 6.6, and 6.8 of this manual to include the following requirements:
- referenced to two (2) or more TxDOT Level 1 or Level 2 stations, NSRS FBN or CBN published horizontal control stations, located in two (2) or more quadrants, relative to the survey project area (post-processed RTK is acceptable when done from two (2) separate base stations)
- all new stations are established by two (2) or more independent baselines
- all stations must be occupied a minimum of two times
- baselines should have a fixed integer double difference solution
- any station pair used as azimuth or bearing reference for use with conventional survey measurements during the course of any other survey level should be included in a network or measured with a minimum of two independent baselines
- Level 3 control must be part of a geometrically closed
figure; therefore, single radial (spur) lines or side shots to points
are not acceptable
- radial lines are only acceptable and required for station pairs where one of the stations will be primarily used as an azimuth mark
- all stations tied with RTK should be measured twice with
a new initialization in between the two ties.
- at the minimum time difference specified in Chapter 5, Table 5.1, a second set of measurements should be taken
- this will yield a total of four sets of independent observations (two pair) at each point
- in most cases, each pair will be measured from different base station locations.
Level 4 Surveys
Level 4 survey work includes wing panels for horizontal position only (see Chapter 3, Table 3.4), side shots for topographical survey and stake-out. Survey methods used can be RTK or fast/rapid static observations from higher-level points.
All Level 4 points should conform to the requirements outlined in Tables 4.1, 5.1, 6.2, 6.6, and 6.8 of this manual to include the following requirements:
- Be referenced to one or more TxDOT Level 1, 2 or 3 stations, NSRS FBN or CBN published horizontal control stations.
- At least 10% of the Level 4 points should be checked with a second observation after a new initialization.
- Each time an initialization is done for the first time or as a result of a complete loss of lock on the satellites, the previous station surveyed must be surveyed a second time with a new initialization.
Level 5 Surveys
Level 5 surveys include mapping grade work that is held to sub-meter network accuracy. Examples of this type of work include locative and inventory data gathering for a GIS system.
Equipment used will generally be code based GPS receivers, enabled to receive real-time corrections from base station positions. In many cases, Coast Guard beacons, OmniStar, Racal and other third party providers of differential corrections, will provide the necessary component for spatial data for other than design and construction use. If the situation warrants, data can also be post-processed.
Anchor: #i1012036Level 6 Surveys
Level 6 surveys include mapping grade work that may not be sub-meter but is at least within five meters network accuracy. Examples of this type of work include locative and inventory data gathering for a GIS system where positional information is not as critical as for a Level 5 survey.
Equipment used will generally be code based GPS receivers, enabled to receive real-time corrections from base station positions but at farther distances from the base or with less stringent parameters.
Anchor: #i1012051Level 7 Surveys
Level 7 work includes very rough positioning with an autonomous fix, usually using a handheld consumer type GPS receiver. Positions will generally be good to within thirty (30) meters. Raw GPS data cannot be stored and retrieved for post-processing as with equipment used in all other TxDOT levels of surveys.
Anchor: #i1012061Field Quality Control
There are several areas to be addressed as far as quality control for the field data acquisition GPS surveys. Quality control measures ensure that the field measurements are performed correctly. There are three (3) sources for error in any survey, whether it is a GPS survey or a conventional survey. These sources for error are blunders, systematic errors, and random errors.
The goal of quality control is to eliminate the blunders and the systematic errors. Then, all that is left are random errors. With a good network design and a sufficient number of redundant measurements, the random errors can be handled and minimized to yield the best possible final result.
The following table identifies field data collection information:
|
Level of Survey Accuracy**** |
Level 0 |
Level 1 |
Level 2 |
Level 3 |
Level 4 |
|---|---|---|---|---|---|
|
Minimum Elevation Mask in Degrees (Collection) |
10 |
13 |
13 |
10 |
13 |
|
Acceptable Survey Method |
S |
S |
S, R/F |
S, R/F, PPK, RTK |
R/F, PPK, RTK |
|
Maximum DOP Value* |
M |
M |
M |
M |
M |
|
If adj. Tripod, then Minimum. Number H.I. Measurements** |
6 |
6 |
6 |
6 |
6 |
|
Maximum H.I. Height Difference Between Measurements |
3mm |
3mm |
4mm |
4mm |
5mm |
|
Minimum Number GPS Receivers*** |
3 |
3 |
3 |
2 |
2 |
|
Photo or Pencil Rubbing Required for all Stations Except CORS |
Y |
Y |
N |
N |
N |
|
* M denotes Manufacturer recommended value ** Measurements to be divided between beginning and end of session. Measurements should be made in two different units. *** Minimum # of receivers simultaneously logging data during a session not including CORS. **** This chart is not applicable to Levels 5-7 mapping grade surveys. |
|||||
Anchor: #i1012085
Monument Identification
During the reconnaissance phase or in the course of the field survey, a monument could easily be improperly identified. This may result in the wrong monument being observed, or the wrong identifier used in the field notes, or digital data collection file. This error can be controlled by requiring pencil rubbings or photos at the time reconnaissance is performed and during the course of the survey.
Each time a new or an existing station is observed, a pencil rubbing or photo should be taken at the time of the observation for Level 1 surveys. For all other levels, this practice is at the discretion of the party chief.
Anchor: #i1012100Data Collection Forms
A sample GPS log sheets is shown in Figure 6-4 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).
One data sheet per observation must be turned in to the processing person. This is especially true now that most receivers no longer allow for the entering of antenna heights without the use of a data collector.
Figure 6-4. Sample GPS Log Sheet.
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: #i1012140Static Observation Field Procedures
All control stations and boundary corners should be occupied a minimum of two times during the course of a survey. Table 5.1 in Chapter 5 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 6.3. 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:
|
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: #i1012181RTK 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.
TxDOT VHF radios at the base station may transmit at full power but the TxDOT UHF transmitters are restricted to two 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: #i1012287Rover 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.
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.
Figure 6-5. TxDOT Feature Codes.
Figure 6-6. TxDot Feature Codes con’t.
Anchor: #i1012375RTK 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. However, on occasion the elevations are derived from spirit leveling from the control, but horizontal is still asked for. In this case, a Level 4 RTK survey is acceptable.
Anchor: #i1012390RTK 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 (see Chapter 8, Digital Data) and in CAiCE format as txdot2k.ftb. A printed list of the TxDOT feature codes is available and is shown in Figure 6-5 and Figure 6-6.
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.