Introduction
Overview
Before describing the ROMDAS and its features, it is important to appreciate some of the principles of road measurement. A proper understanding of these principles will ensure that the ROMDAS system is used to its full potential.
Terminology
The following terminology is employed in this User’s Guide:
Chainage: the location along a road from a start point (in m).
LRP: Location Reference Point. A permanent marker or feature adjacent to the road used as a reference point for surveys.
Keyboard rating: recording events with the PC (or external rating) keyboard.
Continuous event: an event on, or adjacent to, the road which applies over a section of the road
Transverse profile: the pavement profile across a lane.
Longitudinal profile: the pavement profile along a lane.
Location Reference Points
Using LRPs
When conducting a survey, the most important single consideration is the location referencing system.
The location referencing system used in ROMDAS is a linear system i.e. the survey starts at a given point and progresses along the road. The survey chainage increases as you drive away from the start point and decreases when heading back along the same road towards the start point.
Surveys are always done between a start and end point. The common practice is only to record these two chainages, however, this is inadvisable. No matter how well calibrated a distance/speed sensor is, there will be variations in the lengths recorded between different surveys on the same road. This will create problems in reconciling data.
A better approach is to have regular Location Reference Points (LRPs) along the road. These can be existing km stones, culverts, buildings, signs, or any physical feature which will not change between surveys. By selecting LRPs at regular intervals, generally 1 km, one minimises the errors between different surveys, particularly those conducted in successive years. This is done by resetting the chainage at each LRP thereby expressing all data in terms of the offset from the last LRP.
To illustrate the importance of using LRPs and resetting the chainages at each LRP, consider Figure 1.
This consists of a road which has been accurately surveyed and has LRPs at 1000 m and 2000 m.
4 Principles of Road Measurement
Figure 1: Implications of LRP Resets on Survey Chainages
No matter how well calibrated the odometer is, it will never read exactly the same in two surveys of the same road. This applies not only to the ROMDAS, but to any distance measuring device. Proper calibration limits these effects, but they can never be eliminated. Thus, in each subsequent survey the sections will not be identical, with the chainage errors accumulating as one travels along the road.
In Case 1, the odometer is underestimating the distance. As a consequence, the actual sampling intervals are greater than the target 250 m. At the first LRP there is only a small difference, however, as one continues along the road the errors accumulate so one eventually has the target segments completely out of synchronisation with the actual segments.
However, were one using LRP resets the errors would not accumulate and would only pertain to the last sampling interval. This is illustrated in Figure 1 under “Case 1 - Reset” where the final sampling interval before the LRP reset is shorter than the others. At each LRP the data are resynchronised thereby ensuring that the errors are confined to each section. It is also important to note that the data, for example the roughness, is calculated on the actual distance measured so the value will be representative of that actual segment, even though they are measured on a shorter segment.
Case 2 arises when the odometer is over-estimating the chainage. As in Case 1, the error accumulates so the sections are soon unsynchronised. In this instance, ROMDAS synchronises the sections depending upon the distance after the last sampling interval when the LRP is recorded. Two situations arise:
the user can specify a distance after the LRP to increment to the next LRP. By default, this is set at 100 per cent of the sampling interval. If the LRP is recorded within this zone, the data will be stored as in “Case 2 - Reset A”;
if the LRP is not recorded within this zone, the data are recorded as “Case 2 - Reset B”.
Implications of Odometer Error
To illustrate the importance of resetting chainages at LRPs, consider the following example from a typical (and real life) odometer calibration. Three runs were made with the vehicle over a 200 m section.
The number of odometer pulses recorded were 973, 975 and 977 over the section. These corresponded to 4865, 4875 and 4885 pulses per km. Assuming that the mean of 4875 is appropriate, the following is the distances that would have arisen with each of these values:
The above data show that the small difference of 2 pulses over 200 m can translate into an error of 100 m over 50 km. Over short distances, such as are used with LRP resets, the error is small enough to be ignored.
Importance of LRP Resets
The importance of using LRP resets cannot be overemphasised. Highway agencies which do not use these invariably have problems reconciling their data from year to year. Many have to resort to sophisticated (or not so sophisticated!) processing algorithms (“rubber banding”), while others simply give up in frustration. ROMDAS has been designed in such a way that you can easily avoid these problems. Because of that we STRONGLY recommend the use of LRP resets.
ROMDAS has been designed to make full use of LRPs. The software will let the user supply a file containing a list of LRPs and their chainages, as recorded in a previous survey. As the survey progresses, the ROMDAS informs the operator that the vehicle is approaching an LRP established in a previous survey along with a description of the LRP. The operator will then press the ESC key when the vehicle is adjacent to the LRP. The recording is then reset thereby ensuring that the data corresponds exactly to the measurements made in previous surveys. The end chainages can be synchronised by replacing the surveyed chainage with the measured chainage from the LRP survey. It is also possible to insert new LRPs into an existing file or even to create an entirely new LRP file during a survey.
It is recommended that the chainages of the LRPs be established either in a separate survey or during the first roughness survey. These LRPs can then be used in all future surveys to ensure sampling consistency.
Distance Measurement Accuracy
Measurements
The above discussion of LRPs touched upon the issue of measurement accuracy. It must be appreciated that the accuracy of your distance measurement is directly proportional to the number of pulses measured per km. With a Proximity odometer vehicles generate 2000 - 7000 pulses/km. This corresponds to 0.5 - 0.14 m/pulse which is adequate for most applications.
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The exception to this is when looking at acceleration behaviour in travel time surveys. Unless there is approximately 5000 pulses/km one finds that the results are insufficiently accurate to get a good estimate of the acceleration behaviour. The ROMDAS High Resolution Distance Measurement Instrument (HRDMI) is available for situations where high resolutions are required.
Visual Keyboard Rating
Principles
The principle behind keyboard rating is to use the survey vehicle to establish the chainage of features, pavement condition or other roadside events.
Types of Events
Before undertaking keyboard rating it is necessary to break down the items to be measured into point and continuous events:
a point event is something which exists at a single point in space, such as traffic signs or LRPs; or,
a continuous event is something which exists over a section, such as pavement condition.
Continuous events have two chainages: a beginning and end chainage.
There is a special type of continuous event called a switch event. This can be understood as a series of continuous events. For example, one may define a ranking for pavement condition from 0 to 5. These are continuous events so one would normally have to press two keys when changing; one to end the previous condition and one to apply to the new condition. Switch events remove the need to press two keys. When the second key is pressed the first event is cancelled2.
Assigning Events
It is necessary to allocate an individual key on the computer keyboard to each event. These should be carefully selected so as to be both easily accessible and easy to remember. It is good practice to tape small labels to the keys identifying the event.
When recording pavement condition it is recommended that the severity of the event also be recorded.
This is done by allocating several keys to the same event, each with differing severities, for example:
Key Description
A No cracks
S Low cracking
D High Cracking
F Extreme Cracking
It is strongly advised that any severity rating system must include a ‘no defect’ condition since many models which use condition data have different functions for ‘no’ versus ‘some’ distresses.
When selecting the number of distresses to record, and their severities, always bear in mind the practical limitations of the operator. The greater the number to record the more difficult it is to get reliable and repeatable results.
2 Normally, one has events that apply continuously along a section of road. For example, there will always be either no cracking or a level of cracking. ROMDAS defaults to having the user switch only between switch events; you cannot have ‘no’ event. However, this can be overruled in the keycode event setup screen.
Rating Keyboards
There are special 20 or 58 ROMDAS rating keyboards available which are designed to facilitate condition rating surveys. Each key can be individually programmed to any key on the computer keyboard. These greatly simplify the visual rating process. The programming of these keyboards is described in Appendix G.
20 Key Keyboard 58 Key Keyboard
For example, one can assign different distresses to the rows and different severities to the columns. This greatly simplifies the correct identification of the key to press for a distress. For inventory surveys the keys could be labelled with different inventory items. The figure is an example of how a 58 key rating keyboard could be labelled to collect different data. Here, the condition is being expressed in terms of the Surface Integrity Index which is a 0 to 5 scale3.
It is possible to record keyboard events before the survey actually starts. This allows the operators to mark the events so that they are recorded at the survey chainage. For further information on Preliminary Keycodes see Section 0
Surveying Adjacent Sections
The Problem
A problem can often arise when measuring two adjacent lanes on a two-lane road.
When the survey of the first lane is completed the vehicle must be turned around to measure the second lane. Roughness is measured over regular sampling intervals but the last interval is usually of a shorter length than the sampling interval. When measuring the roughness in the second lane unless the first interval length is the same as the last interval for the previous lane, the survey results will be staggered and not correspond to exactly adjacent sections. This problem can be visualised as under diagram (a).
3 Paterson, W.D.O. (1993). A Standard Surface integrity Index of Pavement Condition: Definition and Measurement Procedure. Internal Paper, the World Bank, Washington, D.C.
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ROMDAS allows the user to terminate a survey, reposition the vehicle and start the survey on the other lane. It adjusts the first interval length so that it is the same as the last interval for the previous lane. This ensures that the results of a roughness survey apply to adjacent samples. This is illustrated below under diagram (b).
The user does not need to survey the other side of the road immediately. By opening an existing file the operator will be prompted if they want to continue an existing survey or survey the other side. This makes it possible to plan the surveys in the most efficient manner possible.
NOTE: This option cannot be used in conjunction with LRP resets. In such a case there should be separate files of LRPs for each lane. It will also not work if you are manually defining the end of the roughness sampling interval.
Programming a Survey
Overview
To improve the efficiency of the survey programme it is often necessary to interrupt what should be a continuous run to measure roads running off from the primary survey route. An example of this is shown below. The procedure to follow is:
The operators shall terminate the main survey at an LRP or other roadside feature which will be easy to return to (Run 1). It is generally recommended that this be past the point where the other surveys are to commence.
They may then execute the other surveys (Run 2 and Run 3).
They shall return to where they ended the previous survey they enter the same Survey_ID as was used previously. They will then be given the option to Continue the previous run. The data collection will be started where they left off and, once processed, both components of Run 1 will be integrated.
LRP or Roadside Feature
Run 1
Run 2 Run 3
Continuation of Run 1