LTPP Manual for Profile Measurements and Processing

CHAPTER 4. PROFILE MEASUREMENTS USING THE ROD AND LEVEL

4.1 INTRODUCTION

The rod and level can be used to accurately measure the profile of a pavement. These data can be used to evaluate roughness of a pavement by computing a roughness index such as the IRI. In computing roughness indices, only relative elevations and not absolute elevations are needed. The guidelines in this section can be applied to conventional survey equipment such as an optical level and graduated staff, which requires readings to be manually recorded, as well as automated equipment that are capable of automatically storing measured data.

4.2 OPERATIONAL GUIDELINES

4.2.1 General Procedures

Detailed scheduling and traffic control at test sites must be coordinated by the RSC. However, all traffic control activities at test sites will be performed by personnel from either the State or provincial highway agency. Layout of the test site should not be undertaken until all applicable traffic control devices are in place.

4.2.2 Equipment Requirements

The rod and level used in routine surveying and road construction will generally not have the resolution needed for pavement profile measurements. Precision leveling instruments are required for pavement profile measurements. Instruments used for profile measurement should satisfy resolution criterion given in table 2, which was obtained from the ASTM Standard on Measuring Road Roughness by Static Level.⁽¹⁴⁾

The rod used should be equipped with a bubble level so that it can be accurately held vertically. A suitable base may be selected for the rod in order to reduce sensitivity to small variations in rod placement. For smooth textured pavements, any type of base is suitable. For textured surfaces, a circular pad with a diameter of at least 20 mm is recommended.⁽¹⁴⁾ As only relative elevations are required for computing roughness indices, no correction is required if a pad is attached to the bottom of a rod.

4.2.3 LTPP Procedures

4.2.3.1 Maintenance of Records

The operator is responsible for forwarding all data collected during testing to the RSC.

4.2.3.2 Accidents

In event of an accident, the operator will inform the RSC of incident as soon as practical after the mishap. Details of the event shall subsequently be reported in writing to the RSC to assist in any insurance claim procedures.

4.3 FIELD TESTING

4.3.1 General Background

The following sequences of fieldwork tasks are required:

1. Task 1: Personnel Coordination.
   1. Personnel for rod and level survey.
   2.  
   3. Traffic control crew supplied by highway agency or traffic control contractor working for the highway agency.
   4.  
   5. Other LTPP, State or Provincial highway agency, and RSC personnel (they are observers and are not required to be present).
   6.  
2.  
3. Task 2: Site Inspection.
   1. Assess general pavement condition (within test section limits).
   2.  
   3. Identify wheel paths.
   4.  
4.  
5. Task 3: Rod and Level measurements.
   1. Mark wheel paths.
   2.  
   3. Obtain rod and level readings.
   4.  
6.  

To measure the pavement profile using a rod and level, two persons are needed. One person is needed to hold rod (rod person) and another to operate the level and take readings (instrument operator). If the level is not capable of automatically recording readings, having an additional person to record readings (record keeper) will make the process quicker.

According to the ASTM Standard on Measuring Road Roughness by Static Level,⁽¹⁴⁾ an experienced crew of three would require less than 10 seconds to obtain one reading. This involves positioning the rod by the rod person, reading the level by the instrument operator, and recording the measurements by the record keeper.

4.3.2 Site Inspection and Preparation

The two wheel paths in the outside travel lane should be marked using the following procedure:

1. Clean the area of both wheel paths of loose stones and debris to prevent slippage of rod during measurements.
2.  
3. Identify the location of two longitudinal elevation survey lines 0.826 m from the center of the lane.
   1. Case I: If the wheel paths are easily identified, the midway point between two wheels paths should be used as the center of the lane.
   2.  
   3. Case II: If the wheel paths are not clearly identifiable, but two lane edges are well defined, the center of travel lane is considered to be midway between the two lane edges.
   4.  
   5. Case III: If the wheel paths are not apparent and only one lane edge can be clearly distinguished, the center of lane should be established at 1.83 m from that edge.
   6.  
4.  
5. Mark these locations at intervals equal to the length of the chalk line used for marking. Use the chalk line to mark a straight line between previously established points.
6.  
7. Note the method by which the wheel paths were located in the comments field of the Rod and Level Data Collection form (see appendix F). This will help in locating wheel paths used for profile measurements at a future date.
8.  

Measurements have to be taken along the wheel paths at 0.3 m intervals. The locations at which readings are to be taken can be determined by either of the following methods:

1. Lay surveyor's tape along the chalk line, with the zero mark of tape at station 0+00. Station 0+00 is at the at the leave edge of the white stripe at the beginning of the section. Mark the distances on the pavement at 0.3 m intervals using a suitable marker. Markings have to be made along entire length of section on both wheel paths.
2.  
3. Place surveyor's tape at a slight offset from the wheel path so it will not interfere with rod placement. The zero mark of tape should be at station 0+00. Station 0+00 is at the leave edge of the white stripe at the beginning of the test section. Secure both ends of the tape, as well as several intermediate points on the tape with adhesive tape. Distances along the section can be referenced from the tape. After the distance corresponding to length of tape is leveled, the tape will have to be repositioned.
4.  

4.3.3 Longitudinal Profile Measurements

The first reading taken after the level is set up is referred to as a backsight, while the last reading taken at that setup before the level is moved is referred to as a foresight. Other readings taken in between a backsight and a foresight are referred to as intermediate sights. The procedure to be followed for measuring longitudinal profile is as follows:

1. Complete the required header information in the Rod and Level Data Collection form (forms are included in appendix F).
2.  
3. Set up the level at a suitable location, taking into account the range of the level. With some instruments, it might be possible to cover the entire test length from one instrument setup, located near center of test section. The level should be set up at a position where it will not be disturbed due to passing traffic. In addition, it should be set up at a stable location that will not settle. Set the tripod as low as is practical to reduce error caused by the rod not being held exactly vertical. Thereafter, level the instrument using leveling screws.
4.  
5. Have the rod person place the rod at the zero position of the section and, using the bubble level attached to the rod as a reference, hold the rod vertically. Once the rod is held vertically, the rod person should signal to the instrument operator to take a reading. If readings are recorded manually by a third crew member, the instrument operator should call out the reading to the record keeper. Readings should be recorded in the form included in appendix F. If an automated system is being used, the instrument operator should make sure that reading is saved.
6.  
7. Have the rod person place the rod 0.3 m away from the initial reading, and record a new reading. This process should be continued until either the entire test section is surveyed or the horizontal range or vertical range of the level is exceeded. The horizontal range of the level is exceeded if the distance between the level and the rod is too short or too long to focus properly. The vertical range of level is exceeded if the rod cannot be read due to the slope of the road. When the range of the level is exceeded, the level has to be relocated.
8.  
9. Relocate the instrument if the range of the level is exceeded during measurements. Mark the location at which the rod is to be held for the last reading. This position is called a pivot point. Thereafter, place the rod at the location where the first reading was taken with the current setup of the level and take a reading. Compare this reading with first reading that was taken at this location. If they do not agree within resolution of the instrument, all readings taken from the current level position have to be repeated. If the readings agree, place the rod on the pivot point and take last reading from the current setup of the instrument (foresight). Then set up the instrument at the new location. Place the rod at the pivot point and take the reading (backsight). Continue the leveling process as before, taking readings at 0.3 m intervals. If the instrument has to be repositioned again, the procedure has to be repeated.
10.  
11. Use one of the following procedures when the end of the test section is reached, depending on how the measurements were performed:
    1. Perform the entire survey with one instrument setup: Place the rod at the zero position (point from which survey was initially started) and take a reading. This reading should agree with first reading taken at this location at the start of the survey within resolution of the instrument. If readings do not agree, the profile measurement has to be repeated.
    2.  
    3. Reposition the instrument during the survey: Place the rod at the last pivot point and take a reading. This reading has to agree with the earlier reading taken at this position within resolution of instrument. If they do not agree, the profile has to be measured again from the last pivot point to the end of the section.
    4.  
12.  

The measurement of pavement profiles using a rod and level is labor intensive and time consuming. Therefore, it is advisable to check the accuracy of measured data at regular intervals. This can be performed by establishing a set of control points along the wheel path, for example at 30-m intervals starting from the beginning of the section. After leveling a distance of 30 m, the rod has to be placed at the previous control point and another reading must be taken. This reading has to agree with the previous reading taken at this control point within resolution of the instrument. If readings do not agree, the distance between the control points has to be measured again. This procedure can be used if instrument setup is not changed between two control points being considered. If instrument setup is changed between two control points, the above procedure can still be applied by treating the pivot point as a control point.

4.3.4 Factors to Be Considered During Survey

The following factors have to be considered when performing profile measurements with rod and level:

• If the level is sensitive to temperature variations, it might need to be covered with an umbrella to protect it from direct sunlight.
•  
• If conditions are windy, profile measurements should be avoided, as movement of the level could occur.
•  
• If the level has to be set up at more than one position during profile measurements, make the length to the backsights and foresights equal. This will eliminate errors due to curvature and refraction at turning points.
•  
• If the rod is not exactly vertical, there will be errors. Set up the level as low as possible to reduce error caused by the rod not being exactly vertical.
•  

4.3.5 Profile Computation

During profile measurements in the field, the crew is only expected to record the readings of the level using procedures described in section 4.3.3. Computation of the elevation profile from these data will be done in the office. This section briefly describes how data recorded in field are used to obtain longitudinal profile of pavement.

For profile computations, the elevation of the location where the first reading (first backsight) was measured is needed. However, as only relative elevations are needed to compute roughness indices (such as IRI), an arbitrary value can be selected for the elevation of this point. Relative elevation of any point measured from initial instrument setup can be obtained from the following equations:

   (1)

Where:

IS = Initial instrument height,

BM = Elevation of point where first backsight was taken (assume any value e.g. 30 m),

RR1 = Rod reading at first backsight, and

RR = Rod reading at any point from initial instrument setup.

Once position of level is changed, instrument height will also change. New instrument height can be obtained from the following equation:

   (2)

Where:

Nht = New instrument height,

Oht = OLd instrument height,

BS = Backsight at pivot point, and

FS = Foresight at pivot point.

The relative elevation of the points measured from this new instrument location can be determined by using equation 4.2 and using new the instrument height (Nht) instead of IS.

4.3.6 Quality Control

A quality control check must be performed in the field to ensure that no movement of the level took place during current setup of the level. This check must be performed every time before the level is moved and when the end of the test section is reached. Once the last reading at the current location of the level has been taken, before moving the level, place the rod at the location at which the first reading was taken with the current setup of the level, and take a reading. Both readings have to agree within resolution of the instrument. This check can also be performed at regular intervals by establishing a set of control points as described in section 4.3.3.

4.4 CALIBRATION AND ADJUSTMENTS

The user manual of the level should be consulted about how to perform adjustments to the instrument. Different makes and models of levels will require different adjustments. The user manual should be consulted to determine if the following adjustments are needed for the level being used and, if so, how to perform the specific adjustment. The following are some common adjustments that are required in levels in order to obtain accurate measurements:

1. Make the axis of the level bubble perpendicular to the vertical axis. After setting up the level, center the bubble. Move the telescope 180 degrees about vertical axis. If the bubble moves, the instrument needs adjustment.
2.  
3. Adjust the horizontal cross hairs. This adjustment will ensure that the horizontal cross hairs are truly horizontal when the instrument is leveled.
4.  
5. Adjust the line of sight. This adjustment will make the axis of the sight perpendicular to the vertical axis and also make it parallel to the axis of the level. The method of adjustment for this error is commonly referred to as the two peg method.
6.  

The rod has to be checked to verify the accuracy of the markings. A standardized tape should be used for this.

4.5 EQUIPMENT MAINTENANCE

Shockproof packaging should be used when transporting the instrument. Always clean the instrument after measurements are completed. Before cleaning the lenses, blow dust off the lenses, then clean the lenses using a soft cloth. Lenses should not be touched with fingers. If the instrument becomes wet in the field, make sure that it is completely dry before packing. The tripod should be inspected regularly to ensure that the connections are not loose.

4.6 RECORD KEEPING

The Rod and Level Data Collection form (see appendix F) should be used to record readings when profile measurements are performed using a rod and level. A comment should be made on this form as to how the wheel paths were located. All items in this form should be completed by the instrument operator or record keeper. Location information in this form should be completed even when an automated instrument that is capable of saving data is used.

4.7 DATA BACKUP

Backup copies of the Rod and level Data Sheets must be made without exception as soon as possible. A minimum of two copies must be made for each data sheet. One copy should be transmitted by mail to the RSC office, while the second copy should be retained by the operator in case first copy is lost in mail.