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Home arrow Archives   The American Surveyor     

Expert of Measurement Print E-mail
Written by Jason E. Foose, PS   
Sunday, 29 June 2014

A 1.350Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

Land Surveyors are trained in the art and science of measuring. Through this virtue I am comfortable with my belief that Land Surveyors can be relied upon as experts of measurement with regard to practicing within particular fields of knowledge. For example a Land Surveyor well versed in Boundary Law would be expected to have a better understanding of the legal interpretations of measurements than a Geodesist whom may have a better understanding of spherical geometry and vice versa. However, both disciplines should be expected to demonstrate a fundamental knowledge to determine, perform and analyze the appropriate measurements commensurate with that which they profess to know.

Steven Latoski, PE, Professional Traffic Operations Engineer (PTOE), and Public Works Director of Mohave County, Arizona, recently charged me with the task of measuring a few "noisy" pavement sections for the purpose of collecting some baseline data for future analysis. The noisy sections were observed to be the product of subsurface undulations reflecting through a surface treatment of chip seal. My task was to identify, locate, and quantify those undulations which physically could be described as being very similar in size and shape to a washboard section dirt road. The undulations were visible to the naked eye, occurred at a very high frequency, and appeared to be on the order of an inch or so in height and/or depth. This should be a simple conventional topo operation, right? Wrong!

Considering the realistic mechanical error associated with total station measurements, the high volume of road traffic, and the gradation of surface chips applied to the roadway surface, it became apparent to me that a few hundredths "here and there" could easily accumulate in our measurements to a tolerance greater than the precision of that which we desire to quantify. That sobering thought prompted me to seek alternative methods to replace our conventional wisdom. It also occurred to me that applying conventional wisdom to an unconventional problem would most likely neither produce an effective and useable set of measurements nor a reasonable solution, consequently the dynamic of this problem shifted from "how I measure" to "what I measure".

Roadway surfaces are measured and evaluated by motorists on a daily basis. Generally the public agency can gauge the road conditions through motorist feedback, which by nature is focused toward the most offensive conditions. Prior to this assignment there were no metrics involved but a rather simple and effective drive-by conclusion of "Yep, it's rough" or "naaawww, it ain't so bad". The Public Works Director recognized that regardless of how accurate the conclusions were, they could lead to a perception of subjectivity without hard data to back them up. The obvious question became "how rough is too rough to motorists?" We all feel these sensations through steering wheel, seats and any other points of contact between the vehicle and our bodies. The ultimate solution to this problem was to measure and quantify the motorist's sensation under normal driving conditions rather than produce a geometric representation of the roadway surface.

"There's an App for that."
A google search of "what device measures vibrations" yields the following results: Every smart phone and tablet nowadays has a built-in accelerometer that enables the display to adjust its position relative to gravity. I use an Android platform so I searched for a free vibration monitoring app through Google Play and found an app called Vibration Monitor developed by http://mobile-tools.eu/

The function of the app is to measure and store motion by recording changes in x, y, z directions. The output file is easily exported to a spreadsheet for further analysis. Files can be uniquely named and will contain many x, y, z readings presented in the same columnar ASCII format as a traditional coordinate data file. The app can be adjusted to read with or without consideration to gravity at 9.8 m/s². In this application it is much more effective to remove the "gravity" and produce readings that float around a zero line. I would like to thank Artur Downar of mobile-tools.eu for sharing his app freely with Google Play. I salute all developers willing to share their tools in the name of open source development.

Putting it all together.
The process of collecting and analyzing the data is straight forward and adaptable. The following suggestions were established from a few trial runs and now serve as our guide for collection and analysis. The cardinal rule is to perform the measurements the same way every time under the same conditions. Consistency should hold precedent over any weight assigned to a particular method. For example, if you select a ¾ ton pickup truck with off road tires as your sample vehicle, always use a ¾ ton pickup with off road tires. The difference between Ford, Dodge, or tire brand is probably insignificant, but the difference between choosing a luxury sedan for one sample and selecting Jeep for another is significant.

Necessary Equipment:
1. A vehicle that exhibits extra sensitivity to surface conditions. A straight axel ¾ ton pickup is a better choice than a smooth riding sedan however a sedan will work fine if that is what is available.
2. A tablet or Smart phone with the Mobile Tools-EU Vibration Monitoring app installed.
3. Spreadsheet Software package.
4. Pencil and paper for recording notes, conditions, and observations.

The Method
Select a one mile long segment of roadway as a sample section. Any distance will suffice as long as it is a "known" distance. The device does not record linear positioning data therefore the data must be scaled to that known distance.

1. Record Metadata, notes, photos, observations. Write down and record any and all observations until you develop a feel for what is pertinent to your operation. It's easier to discount extraneous notes and info during analysis than it is to try to recreate or remember details after the fact. The operator should develop a sense of required data very quickly. Times, dates, vehicle type and number, weather, traffic conditions and operator are all important notations to consider.
2. Simply select a "free floating" place in the cockpit of the vehicle to lay the tablet or smart phone flat. Consider that it must be conveniently located so that the passenger operator can start and stop recording, it must not slide around, and should be a placement that can be repeated. Record and photo log your setup for future reference.
3. Drive the sample segment in both directions at a consistent and even speed. Avoid interruptions and peak vehicular/pedestrian periods. Traveling speed is a very important "constant" to maintain. Resampling is easy. It's better to have two good samples than one questionable sample. Multiple samples of the same section can provide strength to statistical analysis.

Data Processing and Analysis
The Vibration Monitoring App produces x, y, and z data which represents the recorded movements "back and forth"(y), "side to side"(x), and "up and down" (z). The data is easily exported in the spreadsheet as a space delimited file. The "x" and "y" data serve no useful purpose in this exercise and may be ignored. The "z" data should contain values centered around zero and roughly half should be negative. This range is produced by disabling the "gravity" function in the apps setting prior to recording measurements. This zero baseline range produces measurements that fall within a positive and negative range of the zero base line. A key step in producing a useable benchmark value is to isolate the magnitude of all readings. This is accomplished by taking the absolute value of all positive and negative readings. The mean the absolute values produces a value that represents the crests or top values of the measurements whereas the mean of the raw positive/negative data set will produce a value close to zero. The Absolute Value Example demonstrates that "Gravity" based data will benchmark at a mean very close to the value of gravity (9.8m/s²) and zero based readings will naturally produce a mean base line of approximately zero. The mean of the absolute values however represents a mean of the extremes or "crests" which produces a strong baseline metric of the sensation perceived by the motorist through the steering wheel and seats. The spreadsheet software can be used to create scatterplots of the absolute values and statistics as well as any other analysis' tailored to the user's purposes.

Conclusions
The traditional role of the Land Surveyor performing measurements to quantify land is as familiar to all as a worn out pair of blue jeans. The arts and sciences behind the profession encompass a very broad spectrum of knowledge. Exercising discipline and professional judgment empowers the Land Surveyor to expand beyond the traditional role of "geometres" to that of a noble problem solver. The process of measuring "rideability" is evolving and has provided Mohave County Public Works with a strong analytical tool to quantify our motorists concerns. The ability to quantify subjective observations, establish base line metrics and monitor "rideability" over the course of a surface treatment's lifespan is proving to be a valuable asset in the pavement management process. Jed Noble P.E. Engineering Manager, Mohave County Public Works has embraced this process and refined the concept into a functional element of his Pavement Preservation Program. Public Works Director Steven Latoski has embraced the effort and development from concept to implementation. I offer a special thanks to both for their collaboration in the effort. For further details regarding the Vibration Monitoring app go to mobile-tools.eu or Google Play store and search vibration monitoring. Feel free to email me with any question, comments, concerns or improvements.

Since 2012, Jason Foose has served as Arizona's Mohave County Surveyor. He is licensed in Arizona, Colorado, and Nevada and has enjoyed a full time career in Surveying since 1993. Prior to that he worked part time as a rodman, and full time for a title insurance company running chains of title in the dusty old Victorian courthouse in Medina County, Ohio. He owned and operated a small surveying practice on the Colorado Front Range and accumulated 12 years of private sector experience before accepting a position as a staff surveyor with Mohave County. This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

A 1.350Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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