Survey Procedures

Survey Standards & Procedures City of Oklahoma

City Public Works Department Survey Section

Wednesday, August 13, 2008

Control Surveys

I. GPS|RTK Control Surveys

Each day’s session of control surveys using GPS\RTK shall begin with an observation and calibration to a minimum of one the City of Oklahoma City’s Primary control points, here in referred to as Primary Control. All observations on secondary control points and calibration points shall be set for 180 measurements, or 3 minute duration. The operator shall verify at each observation that the calibration is with in the tolerances specified for the project or as established by the Chief Surveyor. Each survey session shall end with a check shot to a Primary control point. It may be necessary to perform a second calibration to project specific or the most relevant control to the survey to be performed to supplement or replace the initial calibration. It is the duty of the crew leader to find and use this most relevant control, or if he\she isn’t sure, he\she should ask for direction from the Chief Surveyor.

Localization\CalibrationA calibration should be performed to the most relevant and local control to the project and survey at hand.Examples of relevant control;

1. A plan benchmark(s) or control points from the project plans to be surveyed. 2. Points established on the ground, representing a baseline, construction reference line, Right

of Way line, section line, etc. from which the plans are based on.3. Key features and structures at the beginning, along the way and at the end of a specific project

that are critical to project alignment or grade. Sanitary Sewer manholes, existing pavement, etc. at the beginning and end of a specific project. Checking these key features and structures and the possibility of further calibration(s) is prudent regardless of plan control.

4. Property corners of original tracts, at their actual locations where property is to be subdivided or extended.

5. Where examples 1 through 5 are not relevant or available, the survey shall be calibrated to the closest, reliable Primary Control point.

Determining relevant and local control for the survey should begin well before the field survey begins and may be an ongoing process throughout the survey, requiring subsequent and superseding calibrations. The crew leader should discuss localization and calibrations with the Chief Surveyor before the survey and keep him\her up to date on progress, discrepancies and new findings in the field. A good source for relevant control would be the project manager or designer of a project. He\she may be able to point out the control from which the project was based on.

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City Public Works Department

Survey Section

Procedure for a single point calibrationStep 1. Select and stake out the nearest City of Oklahoma City’s Primary control point. The Primary control point used shall bear the station number assigned by the City of Oklahoma City’s control network. The observed point to be used for a calibration shall be assigned an extension of “_GPS”. In example, if the Primary station number is 400, then the observed point shall be “400_GPS”.

Step 2. Perform the calibration-The operator shall hold the Primary control point as a fixed control point and the observed point as the GPS point to perform a calibration.

Step 3. Initial check-The operator\surveyor shall stake out the original Primary Control point used for the calibration to verify the calibration. The positional deltas at this check should be zero or near zero. Now the survey session and calibration is as good as the point used for the calibration.

Step 4. The operator\surveyor shall perform an independent check to a second Primary Control point. The idea here is to check for a blunder in the original control point\calibration, an error in the RTN solution or poor PDOP and not to find minor discrepancies. It is not uncommon to find 0.10’ or even 0.25’ difference between Primary Control points that are several miles apart and it would be defeating a single point calibration to struggle with these minor differences. If higher order positional tolerances (smaller deltas) are specified for the survey, then a multi-point calibration described below or static survey may be necessary for the survey.

Step 5. Establish control in the vicinity of the calibration with 180 measurements or 3 minute duration for each point collected.Step 6. Post survey Verification Check After establishing the planned control points, the operator shall stake out a Primary control point and store as a checkpoint, with deltas. The original calibration point would be a good choice for this check.

Where a multi-point calibration is prescribed, the operator shall choose 3 or more Primary control points to “box in” the prescribed area for establishing control. The prescribed area for control should be with in and as close to the center of the “box” as possible. The procedure will be much the same as above, however the operator will need to perform successive calibrations to each of the Primary control points used to box in the prescribed area for control. The calibration residuals for each control point shall be analyzed and those with residuals outside of the specified tolerance shall be discarded and alternate Primary points shall be selected for calibration until the residuals are acceptable.

II. GPS|RTK Control Surveys on theWestern Data Systems VRS\RTK Network

All procedures for control surveys using the VRS\RTK Network are the same as the procedures listed above. When the Network is down and without a VRS solution, then a single base solution broadcast from the Network, using a single Network Reference station may be used to set control with the

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Survey Sectionpermission of the Chief Surveyor. The baseline length from the used Reference station shall be considered and recorded.

Classes of Control Surveys

All control surveys shall be checked with and calibrated to the City of Oklahoma City’s Primary Control Network, followed by a check to a separate and independent Primary control point. A minimum of 3 control points shall be set for each survey or project.

Class I control surveys shall be a combination of two static, post processed surveys sessions on separate days using a minimum of 4-hour sessions. The two solutions (minimum) shall then be post processed and adjusted.

Class II RTK\VRS control surveys require that newly established control points be observed on a minimum of two separate occasions, each with an independent initialization. The two independent observations should be at different times of the day with a minimum time separation of 4 hours. The optimum procedure would be to observe all the control points on the first day, and then observe them again on a successive day.

Class III RTK\VRS control surveys require a minimum of 3 observations for a newly established control point. After each observation, the operator shall “dip” the GPS receiver to firmly loose initialization, move 10 to 20 feet from the point and then reacquire initialization for the successive observation. The surveyor\operator shall repeat this procedure, reacquiring initialization in different directions each time until the control point has been observed for a minimum of 3 times. The 3 observations shall be compared and one of the two with the smallest deltas within the specified tolerance shall be used. If no two results are within the specified tolerance, then the procedure shall be repeated until there are two positions with acceptable deltas.

Class IV RTK\VRS control surveys require a single observation following a calibration to a Primary Control point followed by a check to an independent Primary Control point. Class IV

Class IV control surveys

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Survey Section

III. Control Surveys with the Total Station

IV. Bench Loops

V. Topographic Surveys

VI. Hydrographic Surveys

VII. Property Boundary and Right of Way SurveysReal property surveys, as defined here in, includes the establishment of or defining property corners, Right of Way points\corners and in some cases, easements.

All property surveys shall meet or exceed the “Minimum Standards for Land Surveying” as defined by the Oklahoma State Board for Registration of Professional Engineers and Land Surveyors. The text of the Minimum Standards for Real Property Surveys has been distributed to all personnel in the survey section on several occasions and is available at any time. Training and discussion of these standards has been and will continue to be routine for all survey personnel. It is the ultimate responsibility of the Chief Surveyor to insure that property surveys meet these standards. In order for the Chief Surveyor to maintain responsible charge of land surveys as required by law, the following general policies shall be adhered to;

1. It is the duty of each survey crew leader to familiarize him\her self with and follow these standards.

2. The survey crew leader shall follow the Chief Surveyor’s instructions carefully and report and discrepancies, if any, and all findings to the Chief Surveyor.

Property boundary or Right of Way surveys shall be by a closed traverse using a calibrated total station. The maximum closure for all property surveys shall be 1:20,000 or higher order closures may be directed by the Chief Surveyor.Setting property corners or collecting corner evidence with GPS\RTK equipment on properties or Right of Ways with any single boundary line of less than 1000 feet shall not be acceptable. GPS\RTK equipment may only be used on properties with all boundary lines that exceed 1000 feet with the approval of the Chief Surveyor. The positions of corroborating corner evidence off of the property to be surveyed and not a part of the property may be collected using the RTK\VRS network under favorable GPS conditions and using Class III survey procedures listed above.

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Survey Section

VIII. Equipment Care

Prism\GPS poles should be checked for plumb daily and adjusted if necessary.

VIIII. Survey TermsDELTAS-The positional distance between two or more observations of a single pointGPS-Global Positioning SystemIndependent Check-A check to a second, known control point, independent of the starting control point.PDOP-Positional Dilution of PrecisionPRIMARY CONTROL-City of Oklahoma CityRTK-Real Time Kinematic (survey) where corrections are broadcast to the rover in real time, not requiring post processing.RTN-Real Time Network is the same RTK Network. A series of base or reference stations that broadcast to a central processor, which in turn broadcast corrections to the rover(s) in real time.SECONDARY CONTROL-Control set using Primary ControlSTATIC SURVEY-A GPS receiver collects satellite data which must be post processed.VRS-Virtual Reference Station is a process developed by Trimble for use in a RTN where-as the processing software plots a virtual point near the site of the survey, reducing the length of the base line to the rover. This improves the accuracy of a RTK survey.

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Documents

Survey Procedures