CHAPTER 4 GPS SURVEYS

 

4.1

General

 

4.2

Introduction to GPS

 

4.3

Differential GPS Error Sources

 

4.4

GPS Surveys

 

4.5

GPS Specifications for Photogrammetric Control

4.1 General Back to top

Navigation Satellite Timing And Range Global Positioning System (NAVSTAR GPS) is a space-based radio-positioning and time transfer, passive, all weather, 24-hour global navigation satellite system operated and maintained by the United States Department of Defense (DoD). GPS as utilized for positioning is a three-dimensional (3-D) measurement system based on the observations of radio signals of the NAVSTAR GPS. The GPS observations are processed to determine station positions in Earth-Centered Earth-Fixed (ECEF) Cartesian Coordinates (X, Y, Z), which are centered on the World Geodetic System 1984 (WGS-84) reference ellipsoid (the best mean fit to the Earth). In turn, the (X, Y, Z) can be converted to geodetic coordinates (latitude, longitude, and height-above-reference ellipsoid).

Currently there are several worldwide GPS augmentations to the NAVSTAR system. These additional systems will add additional observables to the solutions. This Global Navigation Satellite Systems (GNSS) incorporate the NAVSTAR GPS Modernization (newer generations of satellites), GLONASS enhancements and GALILEO (European Union) constellations.

With adequate connections to vertical control network points and the determination of the geoidal separation, orthometric heights or elevations can be computed. GPS can be also be classified as a linear measurement technique that determines vectors between points, instead of just a direct distance. A vector has a magnitude and a direction (orientation) in space, whereas, a direct distance is only the magnitude component of a vector. Thus, a GPS vector contains a horizontal and vertical angle, and a distance between two points.

This chapter is a continuation of Chapter 3, except that it is devoted to GPS. GPS is a current state-of-the-art surveying technology with its prominence in contemporary and future surveys. GPS has several major advantages such as both day and night operation, intervisibility between stations is not required, mostly weather independent, geodetic accuracy, and is very productive with a possible one man operation. GPS is becoming the first choice surveying technique for all types of surveys. GPS is the “tool of choice” for surveyors, except in those circumstances where it cannot be utilized due to obstructions or other restricting factors.

In this chapter, GPS will be discussed with the same approach that other surveying techniques were discussed in Chapter 3. The discussion will include a short introduction to GPS, error issues, and field procedures.

GPS surveying is an evolving technology. As GPS hardware and processing software are improved, and state-of-the-art new techniques are developed, new guidelines and specifications will be considered. The specifications contained herein are referred to as of the date of this issue.

Disclaimer: The specifications included in this chapter are general recommendations. Contractors should follow their specific contractual agreement with NJDOT.

4.2 Introduction to GPS Back to top

4.2.1 GPS Background

The system was originally designed for military use only, providing sea, air and ground forces of the United States and NATO with an all weather, high-precision, worldwide real-time positioning system. NAVSTAR GPS is maintained by the United States DoD. Though initially designed as a military system, it became freely available for international civil use with certain restrictions to civilians for positioning. The U.S. Air Force Space Command (AFSC) formally declared the GPS satellite constellation as having met the requirement for Full Operational Capability (FOC) as of April 27, 1995. The system has at least a complete set of at least 24 satellites orbiting the earth in a carefully designed pattern.

4.2.2 The Fundamental Components of GPS

The NAVSTAR GPS has three basic segments: Space, Control, and User.

4.2.3 GPS Limitations

Though GPS can provide a worldwide 3-D position, 24 hours a day, in any type of weather, the system does have some limitations. First, there must be a (relatively) clear "line of sight" between the receiver's antenna and several orbiting satellites. Buildings, trees, overpasses, and other obstructions that block the line of sight between the satellite and the observer (GPS antenna) make it impossible to work with GPS. Anything shielding the antenna from a satellite can potentially weaken the satellite's signal to such a degree that it becomes too difficult to make reliable positioning. Any obstruction that can block or interfere with a radio signal can effectively block or interfere with GPS signals.

Bouncing of the signal off nearby objects may present another problem, that of distinguishing between the signals coming directly from the satellite and the "echo" signal that reaches the receiver indirectly. This phenomenon is referred to as multipath. Multipath refers to the existence of signals reflected from objects in the vicinity of a receiver's antenna that corrupt the direct line-of-sight signals from the GPS satellites, thus degrading the accuracy of both code-based and carrier phase–based measurements. Particularly difficult is close-in multipath in which the reflected secondary signals arrive only slightly later (within about 100 nanoseconds) than does the direct-path signal, having been reflected from objects only a short distance from the receiver antenna. In areas that possess these types of characteristics, longer observational times or traditional surveying techniques must be used instead of GPS positioning or to complement the GPS positioning.

The receiver must receive signals from at least four satellites to be able to make reliable position measurements. In addition, these satellites must be in a favorable geometrical arrangement. The four satellites used by the receiver for positioning must be fairly spread apart. In areas with a relatively open view of the sky, this will almost always be the case because of the way these satellites were placed in orbit.


Figure 4.1. Position Determination by Measuring Distances to Satellites

4.2.4 GPS Point Positioning

The position of a point is determined by measure distances or pseudo ranges from the receiver to at least four satellites (figure 4.1). The GPS receiver "knows" where each of the satellites is at the instant in which the distance was measured. These distances will intersect only at one point, the position of the GPS receiver (antenna). The receiver "knows" the position of the satellites, because this information comes from the broadcast ephemeris that is downloaded when the GPS receiver is turned on. The GPS receiver performs the necessary mathematical calculations, then displays and/or stores the position, along with any other descriptive information entered by the operator from the keyboard.

The way in which a GPS receiver determines distances (called pseudo ranges) to the satellites depends on the type of GPS receiver. Basically, there are two broad classes: carrier phase based and code based.

4.2.4.1 Carrier Phase Receivers

Carrier phase receivers, mainly used in surveying, are capable of centimeter (cm) accuracy or better. These receivers measure distances or pseudo ranges to visible satellites by determining the number (N) of whole wavelengths (l) and measuring the partial (phase) signal wavelength (F) between the satellites and the receiver's antenna. Once the number (N) of wavelengths is known, a pseudo range may be calculated by multiplying 'N' by the wavelength of the carrier signal (L1 and/or L2, 19cm and 24.4cm respectively) plus the partial wavelength. Figure 4.2 illustrates this ranging method.  It is then a straight forward, albeit complex task to compute a baseline distance and azimuth between any pair of receivers operating simultaneously. With one receiver placed on a point with precisely known 3-D position and with the calculated baseline (distance between 2 points), the coordinate for the unknown point may be determined.


Figure 4.2. Carrier Phase Based Ranging

Carrier phase receivers come in two models: single-frequency (using L1 only) and dual-frequency (using both, L1 and L2). Dual-frequency receivers are required for all NJDOT projects where observations are greater than 10kms. Dual-frequency receivers are the preferred type of receiver for ALL observations, regardless of length.

4.2.4.2 Code Based Receivers

Code based receivers use the speed of light and the time interval that it takes for the signal to travel from the satellite to the receiver to compute the distance to the satellites (figure 4.3). The time interval (Dt) is determined by comparing the time in which a specific part of the "bar code" left the satellite with the time it arrived at the antenna. The pseudo range is computed by multiplying the time interval by the speed of light constant (c=299,792 Km/second). Pseudo ranges from at least four satellites are needed in order for a receiver to perform essentially a triangulation and produce a position fix. Position fixes are made by the receiver roughly every second, and the more advanced receivers enable the user to store the position fixes in a file that can be downloaded to a computer for post processing.


Figure 4.3. Code Based Pseudo Ranging

4.2.5 Advantages of GPS

Probably the most advantageous aspects of GPS are its accuracy, precision, and efficiency, particularly in survey tasks that span a large area. Traditional terrestrial survey activities that would have normally taken months to complete, now take only a few days utilizing GPS. Real-time applications of GPS enable surveyors to perform quick data collection and construction stakeout activities.  In fact, a complete construction design plan can be loaded into a receiver and staked out by following the guidance of the receiver.

GPS is used exclusively to densify control networks. The New Jersey Geodetic Survey (NJGS) Unit of NJDOT has been actively using GPS since 1989 to develop a geodetic control network which contains approximately ±3,000 GPS suitable horizontal control stations in New Jersey which can be utilized for GPS surveying activities.

GPS is a positioning system that can also be used as a real world digitizer for mapping point and line features, such as roads or wetland boundaries. However, for large volume data collection which includes measuring many points, mapping, contouring, etc., one should consider photogrammetry as a more efficient data collection tool.

4.3 Differential GPS Error Sources Back to top

GPS has inherited errors that are unique to this technology. GPS, as a surveying tool, has many standard (traditional) surveying errors as well. For example, an instrument setup error applies across the board to all surveying measurements regardless of the instrument or technique used. It does not make a difference what instrument is used, if a GPS receiver, a total station or a range pole is on the wrong point, or not properly set up on the proper point, the survey is erroneous. There are two major categories of error sources in GPS surveys. The first is System Errors and the other is Operational Errors.

4.3.1 System Errors

System errors are those errors that will affect every positioning activity regardless of the specific location of a particular receiver. System errors originate from inaccuracies in the positions of the satellites, the GPS signal and the propagation of the signal through the earth’s atmosphere. Most of these errors can be eliminated if GPS positioning is performed in a relative mode and with dual frequency receivers. GPS surveys are always made relative to a known control point, thus, many system errors cancel out. System errors include:

An electromagnetic signal propagating through the neutral atmosphere is affected by the constituent gases. The effect of the troposphere on GPS signals appears as an extra delay in the measurement of travel time from the transmitter to the receiver. This delay is caused at the lower part of the atmosphere and is a function of the atmospheric conditions such as barometric pressure, temperature, and humidity. GPS signals are refracted due to moisture in the lower atmosphere. Reasonably established models for the index of refraction and other atmospheric models can correct this error.


Figure 4.4. Sunspot Predictions

4.3.2 Receiver Dependent Errors

4.3.3 Errors Due to Point Selection

The selection of points to be measured with GPS is not a trivial matter. The rules of point selection in traditional surveying, mainly maintaining line of sight, do not apply in GPS surveys. Since the direct line of sight has to be with the satellites, points have to be selected in such a way that the clearest signal is received at that point. The following are errors that can impact the results of GPS surveys:


Figure 4.5. Multipath Example

4.3.4 Operation Errors

Dilution of Precision (DOP) is a purely geometrical contribution to the uncertainty in a position fix. It is a unit-less number that indicates the error in position determination as a function of the relative satellite geometry. GPS derived positions uses four parameters: position (X, Y, Z) and time t. The DOP for all four parameters is called Geometric DOP (GDOP). The DOP for a three coordinate position (X, Y, Z) is called PDOP, the DOP for two horizontal coordinates (X, Y) is called the HDOP, and for vertical position only is called the VDOP. For surveys, it is recommended to use GDOP as the indicator for favorable observation geometry. Observations should be done at times when the GDOP is less than 4. One should never perform GPS surveys when GDOP is more than 8.

GDOP information is computed by an observation planning software. It is based on the predicted location of the satellites relative to the observer. One should be aware of two factors which may change the actual value of GDOP at the time of observation. These factors are obstructions blocking the satellite signals and unanticipated operational problem of satellites. If obstructions are not mapped properly, the software assumes that all the satellites are visible and the computed GDOP may not reflect the actual situation. Similarly, if the observation sessions are planned in advance and one of the satellites has since stopped operating properly (or temporarily turned off), the actual GDOP at the time of observation may not be the same as predicted. Always check the actual GDOP, as indicated on the receiver, to ensure quality observations.

4.3.5 Data Processing Errors

Data processing errors are those errors which can be identified only when the field work has been completed. During the processing of the field data certain “poor” observations have to be filtered while others can be corrected with the software.

4.4 GPS Surveys Back to top

4.4.1 Network Design and Connections

In accordance with contract documents, policy and procedures, and articles, prior to contract award, physically contacting NJGS must be done before starting any GPS-type survey for a NJDOT project. Additional information for these publications can be obtained from the NJDOT Division of Procurement/ Bureau of Professional Services at (609) 530-2452. Project specifics and any additional up-to-data survey related (horizontal/ vertical) information by NJGS, (609) 530-5642, fax: (609) 530-3689, can be utilized if available.

All GPS network survey projects will be established horizontally to National Spatial Reference System (NSRS) stations with minimum accuracy of a First-Order (1:100,000). Typically, orthometric vertical control values are a same-survey requirement. Usually, NJDOT projects are linear or corridor in character. As a result, the proper geometric locations of the NSRS GPS control stations, in particular benchmarks, with respect to the project would stabilize any possible rotational error. Specific locations of the new control points depend on the optimum layout to carry out the required needs of the survey.

Checks shall be made to ensure that no existing network control points have been moved or disturbed. If any are doubtful, additional existing points shall be tied into the network. All control stations which are in agreement for the intended accuracy will be used as constraints in the final adjustment. If any control station does not fit, then a report of the reason(s) for not constraining it must be highlighted in the project report.

The minimum number of horizontal network constraints is stated herein, with their location being distributed in at least three different quadrants relative to the center of the project. Where existing National Geodetic Survey (NGS) published horizontal control on a common datum is present, all such stations lying within two kilometers of the survey’s boundaries must be included in the survey.

Horizontal control networks shall be based on the North American Datum of 1983 (NAD83). All stations must be published with a common datum tag and agree with each other for the required accuracy of the survey. The use of eccentric points, without direct connections to the NSRS, as horizontal control stations is not permitted. The project horizontal network will be connected to the NSRS in one of the following methods:

The A/B-Order FBN/CBN, in New Jersey, as it currently stands, contains about 117 stations. The spacing of these stations was originally designed to be within 20-25 km of each other. This allows for any GPS survey, utilizing dual frequency receivers, to be easily connected to the NSRS via this route.

Vertical control networks shall be based on the North American Vertical Datum of 1988 (NAVD88) and connected to a minimum of four NGS stations, with published NAVD88 orthometric elevations, preferably a minimum of Second-Order accuracy or higher. Three vertical control stations (benchmarks) determine the plane of the geoid but provide no redundancy. The fourth bench mark shall be included to provide this redundancy.

Direct GPS connections must be made to a minimum of two NSRS benchmark stations. One highly recommended vertical connection is to a FBN/CBN station. Communication with NJGS must be made before surveys are started because of the possibility of additional benchmarks that may be available due to on-going level projects being surveyed by NJGS to establish new NAVD88 heights within the State. If authorized from NJGS and used, these “benchmarks” must be documented in the final project report. The use of eccentric vertical stations is permitted provided they are located within 100 meters of the original mark and three wire/digital leveling is used to determine the elevation of the eccentric point(s). All eccentric vertical connections must be documented.

GPS derived ellipsoidal height differences, when combined with geoidal height differences, can give usable orthometric height differences. The latest version of the NGS provided geoid model (currently GEOID03) must be used and documented in determining the orthometric heights for any points established by GPS done for NJDOT. Figure 4.6 illustrates the large number of existing NSRS benchmarks used to model GEOID03 in New Jersey.


Figure 4.6. 2003 GPSBM Control Data Used To Create GEOID03

The use of GPS for vertical network surveys requires an understanding of the relationship between conventional and GPS height systems, and of problems unique to the vertical component of a GPS measurement.

Conventional leveling measures the relative elevations of points above an undulating equipotential surface called the geoid, which is close to, but not the same as, “mean sea level.” An approximate example of a geoidal reference surface is NAVD88. The leveling observations used by NGS in NAVD88 were corrected for rod scale and temperature, level collimation, and astronomic, refraction, and magnetic effects. Geopotential differences were generated and validated, using interpolated gravity values based on actual gravity data. An orthometric correction is then applied to the elevations to correct for the error introduced by the fact that level surfaces at different elevations are not exactly parallel.

In contrast, GPS measures the relative elevations of points above a smooth, mathematically simple surface called an ellipsoid. An example of an ellipsoidal reference surface is GRS80, the defining surface for NAD83. Elevations derived from GPS measurements are ellipsoidal heights

The ellipsoidal (h) and orthometric (H) heights are closely related by the geoid height (N), the separation between the two reference surfaces. Geoid heights can be derived from GPS observations on bench marks, where both the ellipsoidal and orthometric heights have been measured for the same point. A network of GPS bench mark observations, gravity observations, and elevation models are used to develop a geoid model, from which geoid heights at other points in the area can be estimated. The accuracy of these geoid heights is dependant upon the accuracies of the various measurements used to construct the model.

The height equation h=H+N is only an approximation, as orthometric height is measured along a curved plumb line normal to the geoid surface, while the ellipsoidal and geoid heights are measured along straight lines normal to the ellipsoid surface.


Figure 4.6. Surface Relationships

DRAFT Guidelines for Establishing GPSderived Orthometric Heights (pdf 901k)

When comparing the GEOID03 model with GPS ellipsoidal heights in the NAD83 reference frame and leveling in the NAVD88 vertical datum, it is seen that GEOID03 has roughly a 2.4 cm absolute accuracy (one sigma) in the regions of GPS on benchmark coverage. For further GEOID03 information see the NGS reference link: US Geodetic Survey

The height component of a GPS survey measurement is also affected by relatively poor geometric strength for trilateration, as the earth blocks all satellite signals from the hemisphere below the horizon. This imbalance makes height measurements more susceptible than horizontal measurements to signal path errors from multipath or atmospheric conditions. Accordingly, GPS height accuracies for a survey are typically 1.5-3 times worse than GPS horizontal accuracies, depending on data quality and base line length.

Depending on the project requirements, intervisible pairs can be established by GPS procedures. Station pairs (station and reference station) are provided for any terrestrial survey needs. These azimuth pairs shall be established at regular intervals in order to provide sufficient control along the project length with spacing, if possible, of 100-400 meters between stations so as to be accurate to within fifteen seconds (±10”) of arc.


Figure 4.7. Guidelines For Minimum Spacing of Intervisible Pairs

All station pairs must receive double simultaneous independent occupations. An independent occupation is defined as an occupation whereby the 2-meter pole, bi-pod, or tribrach is physically moved and releveled. The use of 2-meter fixed height poles is recommended for all observations. Substantially permanent, durable and recoverable physical marks should be established for these intervisible pairs.

4.4.2 Reconnaissance

A key to any successful GPS survey is comprehensive reconnaissance. The basis for survey control will be the NSRS as contained in the NGS Integrated Data Base (IDB). These existing published survey stations (horizontal (NAD83) and vertical (NAVD88)) are required by NJDOT projects to ensure proper integration of the GPS project within the accepted data base. The recommended site for published survey data is the US Geodetic Survey. The NJGS can also be contacted for further directions.

Proper reconnaissance consists of appropriate station selection, identification of existing stations, monumentation or marking of new stations, estimation of travel time between stations and scheduling. A careful reconnaissance at the initiation of the project will save a lot of resources, as well as make the field and office operation much more efficient.

Good reconnaissance plans include, but are not limited to the following procedures:

4.4.3 Field Procedures

4.4.3.1 General

As discussed in the previous section of this manual, the precision of the GPS vector baseline results depends (among other) on the following:

The number of possible observing sessions per day is a function of the required survey accuracy, satellite availability, type of survey method utilized, and project logistical considerations, such as travel and set up time required between observing sessions.

The specifications for field procedures depend on the survey methods utilized. In general, there are two general methods, static and kinematic, using carrier phase tracking. Both methods have similar observation and initialization requirements, and differ mainly in their initialization procedures and whether the positional computations are preformed in real-time or post-processed. Three techniques under the static method are Classical Static, Rapid Static (Fast) Static, and Static Reoccupation. Three techniques used under kinematic surveying are, Stop and Go Kinematic, Continuous Kinematic, and Real-Time Kinematic (RTK). Regardless of the particular observation method selected, the following specifications must be met:

Each one of these methods has advantages and disadvantages for different types of surveys.

4.4.3.2 Observation Methods

4.4.3.2.1 Static Observations

4.4.3.2.1.1 Classical Static Technique (Typically For Baselines > 20 km)

Static GPS survey procedures allow various systematic errors to be resolved when high accuracy positioning is required. Static procedures are used to produce baselines between stationary GPS units by recording data over an extended predetermined period of time during which the satellite geometry changes. This is the classical GPS survey method for long lines and higher accuracy, typically 1:100,000 or greater. In order to resolve integer ambiguities between satellite and receiver, points are occupied for long sessions (30 minutes to 6+ hours), depending on the number of visible satellites, baseline length, intended accuracy, etc. Baselines are re-observed in different sessions according to a predetermined observation scheme.

Specifications:

4.4.3.2.1.2 Fast-Static Technique (Typically For Baselines 0-20 km)

Fast-Static GPS surveys are similar to static GPS surveys, but with a shorter observation period and usually with a faster epoch rate. This technique utilizes shorter observation times for shorter baselines with accuracies usually < 1:100,000. It can be done with a classical network design or as radial surveys.

Specifications:

4.4.3.2.1.3 Reoccupation Technique

Reoccupation surveys are similar to Fast-Static, except that the length of each session is short. Each point is re-visited after at least an hour, when the geometry of the satellites becomes significantly different.

Specifications:

4.4.3.2.2 Kinematic Observations

4.4.3.2.2.1 Stop and Go Technique (For Baselines < 10 km)

A temporary reference station is tracking constantly. The roving receivers are initialized to establish an accurate relative position between them and the reference station. Following the initialization each new point is occupied for a minimum of 10 minutes. Lock on the satellites must be maintained at all times or a new initialization must be performed.

Specifications:

4.4.3.2.2.2 Continuous Kinematic Technique

The reference receiver and rovers are initialized as in stop and go. After initialization the rover is constantly moving and measuring positions. Lock on the satellites must be maintained at all times or a new initialization must be performed.

Specifications:

4.4.3.2.2.3 Real-Time Kinematic (RTK) Techniques

RTK is rapidly becoming widespread and the most commonly preferred GPS positioning technique. RTK is an advanced form of relative GPS carrier-phase surveying in which the reference station transmits its raw measurement data to the rover(s), which then compute a vector baseline from the reference station to the rover. The computation is done nearly instantaneously, with minimal delays between the time that the reference station measures, and the time these data are used for baseline processing at the rover. RTK is only suitable for environments with reasonably good GPS tracking conditions (limited multipath, obstructions and RF noise), and with continuously reliable base station to rover communication. To achieve real-time positioning, a radio or other electronic data transmitting device (i.e. CDMA, cell phones) link between the reference receiver and the rovers must be established. RTK surveying techniques are an ever-changing, dynamic technology.

4.4.3.2.2.3.1 Conventional (Single Base Station) RTK Surveys (Baselines<10 km)

Conventional RTK surveys, by nature, result in a radial pattern of baselines from the reference station to the rover(s). A delta X, delta Y, and delta Z are produced from the reference station to the rover unit on the WGS84 datum. From these values, the coordinates of the occupied points by the rover are produced. This does not produce strong network geometry. Depending on the distance from the reference station, the positions as determined may result in poor local accuracy between the two closely established stations if a direct connection is not observed between the two. If a second base station is observed, two additional baselines should be formed to each RTK point. This will add redundancy and improve the positional reliability.

There are numerous and widespread applications for RTK-type GPS surveying. Typical applications include, but are not limited to hydrographic surveys, location surveys, real-time topographic surveys, construction stake out surveys, and photo control surveys.

Specifications:

4.4.3.2.2.2.3 Real-Time Network (RTN) RTK Surveys

Real-Time Network RTK surveys are similar in principal to conventional RTK surveys. Both are dependent on observations from base stations. Typically, ground monuments can be thought of as “passive” system of control. These control stations are needed to be field recovered and verified before they can be utilized as constraints for the RTK project. In addition, there is a need to set up a receiver and broadcaster at the selected reference station(s). Many times because of security issues, the reference station needs to be manned with an observer rather than standing alone.

RTN control is an “active” type of system. Ideally, these RTN master stations would be part of the NSRS as National and/or Cooperative CORS. If so, then they are monitored continuously and their integrity/ accuracies are checked. The data is made readily available at anytime. Also, because these stations are connected to the NSRS, any datum changes or readjustments of these master stations are handled seamlessly. The network administrators who then broadcast correctors to the rover(s) have the optimum values and performance of these master stations. There is no need to man a ground based reference station, thereby freeing up an additional RTK observer.

The method used to determine the position of the rover(s) depends on the configuration of the RTN system. The derived solution may be a single baseline vector from one reference station or the multiple baseline solution resulting from a combined network solution broadcasted from a central computer.

According to NGS, (2/2007) there exists no cohesive national policy, guidelines, standards, specifications, metadata archiving or database management for RTN. The NJDOT does not currently have any guidelines for RTN.

4.4.3.3 Independent Reoccupation of Stations

GPS surveys require redundancy of observations which are used to detect blunders and to obtain statistically sound results. Redundancy is achieved by reoccupying some points in different sessions with different geometric combinations. The following criteria pertain to static, rapid static and reoccupation procedures for network adjustments:

4.4.3.3 Making the Observations

Pre-Observational Check-List

Every day, before leaving the office, the following equipment must be checked for inventory, as well as for proper functioning:

Receiver

On-Site Occupational Procedures

4.4.3.4 Monumentation

Where feasible, control points which are used as origin, intermediate check, and closure points such as but not limited to, photo control stations, intervisible station pairs and/or RTK base station points are recommended to be permanent in nature. Examples include and are not limited to, poured-in-place concrete monuments, meeting NJDOT specifications, with uniquely stamped disks, feno rods with disks, steel rods/ re-bars with caps uniquely identifiable, and drill holes in solid foundations.  The above installed stations must be either sketched to scale with recovery ties or contain descriptions in a text format to fully identify the station. If a permanent-type monument is not used for these stations, then prior authorization from NJDOT is needed.

Semi-permanent points, typically PK nails, can be used for all other recoverable survey points (i.e. photo targets and intermittent traverse points).

4.4.4 Office Procedures

4.4.4.1 General

Generally, there are two types of classifications for a GPS survey. The first is based on the internal consistency of the observed vectors and their local resultant positions. The method to assess these internal consistencies is to use a free adjustment, thereby holding only one arbitrary 3-D position and evaluate the vector derived positions. The second classification would be a fully constrained adjustment holding all pertinent 3-D published NSRS control stations.

Loop closures and differences in repeat baseline measurements are to be assessed to check for blunders and to obtain initial estimates for internal network consistencies.

Software used for processing the raw data must be capable of producing results that meet the accuracy standards specified for the survey.

The software must be able to produce, from the raw data, relative position coordinates and corresponding variance covariance statistics, which, in turn, can be used as input to three dimensional network adjustment programs.

The most current version of the geoid model as furnished by NGS shall be used for elevation determinations.

Evaluate a minimally constrained 3-D adjustment holding the latitude and longitude and the ellipsoidal height of one NSRS station. This will be used to analyze the internal quality of the field work.

A fully constrained 3-D least squares adjustment, at the 2-sigma (95%) level shall be made to provide final adjusted coordinates of the GPS network control stations. All NSRS control stations are to be held in the adjustment; unless it is proven that the station does not meet the required accuracy.

4.4.4.2 Loop Closure Analysis

The error of closure is the ratio of the length of the line representing the equivalent of the resultant errors in the GPS baseline vector components to the length of the perimeter of the figure of the analyzed survey loop. A loop is defined as a series of at least three independent, connecting baselines, which start and end at the same station. Each loop shall have at least one baseline in common with another loop.  Each loop shall contain baselines collected from a minimum of two independent sessions. The following minimal criteria for baselines loops shall be used for static and rapid static procedures for First-Order classification (1:100,000) network adjustment:

4.4.4.3 Repeat Baseline Difference

4.4.4.4 Project Documentation

The following information should be documented and filed upon completion of the horizontal and vertical control work:

4.5 GPS Specifications for Photogrammetric Control Back to top

4.5.1 Introduction

The specification details are referenced in NJDOT publication: Minimum Guidelines for Aerial Photogrammetric Mapping, BDC98PR-009, Chapter 4-09.2. (Photogrammetric Manual with Appendices).

The guidelines for photogrammetric consultants performing mapping for NJDOT projects for the four levels of feasibility assessment mapping as currently issued by the New Jersey Bureau of Scope Development are under section 4.5.2.

Recommended methods, such as RTK, for photogrammetric target positioning are previously outlined in Chapter 4.4.3.2.2.2 and Chapter 4.4.3.2.2.3. These RTK techniques, as an example, based on established control points are used for photo target points and other survey points as required. With a good geoid model (the latest NGS version) and good NSRS control stations for checks; this technique becomes an efficient way to survey, provided proper redundancy is maintained.

The horizontal coordinate reference datum shall be NAD83. The vertical reference datum shall be NAVD88 unless the project has special requirements. The latest NGS issued geoid model shall be used accordingly.

Unless otherwise specified, the de facto standard unit of measurement for any NJDOT photogrammetric project shall be U.S. Survey Foot. Currently, New Jersey has not legislated a specific conversion value between the meter/foot. The relationship of the meter/foot that will be used will be:

1 foot (U.S. Survey) = 1,200/3,937 meters (m) exactly.

1 meter = 3.28083333333333 U.S. Survey Feet.

4.5.2 Feasibility Assessment Levels Mapping

NJDOT BUREAU OF SCOPE DEVELOPMENT
MAPPING FOR FEASIBILITY ASSESSMENT (FA),
FINAL SCOPE DEVELOPMENT (FSD) and DESIGN DEVELOPMENT (DD)

Feasibility Assessment is considered to be the preliminary development of different engineering solutions to transportation problems. The concepts are to be developed on a mapped plan and the level of mapping required may differ for different projects for various reasons. Final Scope Development will further advance the selected concept, and Design Development will develop the construction documents. Below is a list of different levels of Feasibility Assessment mapping, a detailed description of the mapping and relative costs.

Level 1 - Aerial Photography (FA):

New aerial photography will be flown and obtained of the project area. The photography will be flown at an altitude allowing for the preparation of digital mapping in the future.

A digital image of the photograph with a minimum scanning rate of 25 microns will be created to plot larger scale prints.

Measurements will be performed if and where required only to scale the photograph, for concept use purpose.

Survey Control is not required, however documentation of the method of the measurements will be prepared. Also, NJ State Plane Coordinates Grids will be added to the Aerial Photography. Grid locations will be approximate and established utilizing USGS Quadrangle maps which are 1:24,000 scale.

All the alternatives will be developed electronically on the larger 1:1000 scale images (or 1:300 scale images if necessary).

Level 2 - Digital Mapping with Limited Ground Survey (FA):

New aerial photography of the project site will be obtained. Digital mapping of the project area will be prepared in MicroStation format conforming to the NJDOT CADD Manual, showing all features visible from above and contours at 1 foot (0.25 meter) intervals in flat terrain, 1.5 feet (0.5 meter) intervals in rolling terrain and 3 feet (1.0 meter) intervals in mountainous terrain. Project scale to be determined based on project specifics.

Horizontal and vertical control will be established for the photogrammetric control and a Primary Survey control report will be prepared in accordance with the NJDOT Photogrammetry Guidelines. Intervisible pairs of control points with 3-D values will be located along the project length so that they are visible between pairs to traverse the project and complete the traverse.

A DTM (digital terrain model) produced for this level of mapping will be sufficient for scoping purposes only.

For bridge projects, the field survey will be sufficient to establish elevations at the bridge abutments in order to obtain clearances below the bridges and other critical elements without performing field survey under the bridges. If directed, the consultant will measure the bridge underclearances.

All mapping information will be based on the U.S. Survey Foot and tied horizontally to the NAD83 and vertically to NAVD88. The mapping will conform to the NJDOT Photogrammetric Guidelines and general conditions such as article 44 and 51.

All the alternatives will be developed on the digital mapping at 1”=30’ (1:300) scale or specified scale.

If directed, the consultant may be required to establish the roadway baselines and obtain drainage information.

Level 3 - Digital Mapping with Ground Survey (FA):

Aerial Photography will be obtained; Digital Mapping and Survey Control will be prepared as described in Level 2.

Field survey shall be performed in the project limits, including a recoverable traverse and level bench run with the addition of a complete field edit to confirm and supplement the photogrammetric survey.

For bridge projects, the field survey will be sufficient to establish elevations at the bridge abutments in order to obtain clearances below the bridges and other critical elements with field survey under the bridge.

All existing visible utility features will be located and mapped accurately. If directed, the consultant will accurately locate the connecting utility lines between utility features.

If directed, the consultant may be required to establish the roadway baselines and obtain drainage information.

Level 4 - Complete Digital Mapping with Complete Field Editing (FA, FSD, DD):

Aerial photography will be obtained; digital mapping and survey control will be prepared as described in Level 2 and Level 3.

A field survey and edit will be performed as described in Level 3 with the addition of the existing roadway baseline and the establishment of existing roadway ROW lines. Additional pavement elevations will be obtained to supplement and develop the design DTM. Location of the existing ROW lines assumes a combination of field and research effort and that record information that is useable and reliable is obtained. Individual property parcel research is not included.


Last Document Correction:
March 8, 2007