In Case You've Wondered

My blog is where my wandering thoughts are interspersed with stuff I made up. So, if while reading you find yourself confused about the context, don't feel alone. I get confused, too.

If you're here for the stories, I started another blog:

One other thing: sometimes I write words you refuse to use in front of children, or polite company, unless you have a flat tire, or hit your thumb with a hammer.

I don't use them to offend; I use them to embellish.

Thursday, November 17, 2011

Measuring Things

When I first started in the construction business, the initial layout, and controls were placed by a survey field party. This usually consisted of 3 or 4 men that used a multitude of different instruments to place offset points and information on location and elevation.

Usually, there was a party chief, that coordinated the work, recorded data in a field book and used the transit to establish lines and elevations. The rest of the crew had various tasks, including hacking through brush, or wading swamps to establish open areas for sighting and placing controls. In some locations, the crew would wear a pistol, or carry a small shotgun to supplement their brush hooks and machetes. This allowed protection from the "critters" that included snakes, feral animals and alligators. When the day was over, the crew may return home with whelps from insects, a substantial accumulation of filth and the knowledge that the next day may be worse; it all depended on the location. Horror stories were told by all. Cotton mouths, alligators and red wasp nests left memories that most would want to forget.

Measuring distances and angle would usually involve a "chain" and a transit (I describe these at the bottom of this post). The crew chief would use both to determine locations. The information was either recorded or used for stakeouts. If a stakeout was involved, the crew chief used trignometry and calculus to determine the correct distances and angles. The old timers did most of their calculations using tables, a pencil and a scratch pad. Whatever they did was recorded in their field book. Laying out a curve involved pointing the transit in a certain direction, calculating the angles and distances to the points and the crew pulling the chain. When the point was determined, a stake was driven and a nail placed in the top for future reference.

Today, a single person can do everything a field party could do with one instrument. A GPS surveying instrument, when it's used in open areas, can accurately record, or determine any point needed. Satellites are used to determine the points and the results are accurate to within a quarter inch, or better. They do have limitations, especially around structures or heavy growth. The required number of satellites to accurately determine a location requires a fairly open unobscured view of the sky. Poor weather, inversions and satellite problems can cause problems.

If enough points are known, or a single GPS transceiver can be placed in an open area, a total station can be used. The lower end total station requires an operator and a rod man. The rod man has a prism pole, which is the reflector for the total station. The higher end total stations are "robot guns" These total stations will seek the rodman, who has instumentation on the rod. The rod instrumentation is coordinated with the total station. A single person can use the instrument to complete surveying. Information is sent from the total station, which reflects off the prism and is returned back to the total station. There's even combination GPS robot guns that use the best of both technologies. The highest end is a three dimensional instrument. Instead of one point being shot and recorded, thousands are recorded and an accurate true three dimensional image can be recorded without a prism. 

For years, a total station was on my "wish list" of things I felt were necessary to purchase. Unfortunately, it took a long time until the right job happened and the instrument, and software were necessary. Since then, I've learned to use the total station and the software involved. It's been a learning experience. Where in the past, I would have to spend hours using everything from mutliple measuring tapes, trigonometry and some creative methods to find points, I can now use the total station to gather, or transfer information in minutes.

More information:

A chain is a ribbon of hardened steel with babbit placed every foot of the length. A mark is scribed in the babbit, which reflects the even foot that was calibrated where the chain was made. At a certain temperature, and measured poundage of pull, the accuracy is certified. If a measurement is incorrect, the user either didn't use the right data to determine how hard to pull the chain, or the wrong temperature. Lengths can be up to 1000 feet, which means careful rolling when finished and a cleaning when necessary. Measurements require determining the right pull for the temperature and using a scale attached to the chain to pull the correct amount.

A transit is a telescope, with crosshairs, that measures azimuth and vertical angles in degrees.  A compass in the middle determines the direction, and tables are used to determine the deflection of magnetic North to true North.  Directions are in degrees, minutes and seconds. The markings on the better instruments requires a small magnifying glass to accurately determine the angles. When placed over a point, the tripod is manuevered and leveled over the point using a plumb bob. Setting up over a point can be an almost flawless manuever or a frustrating repeat of the steps until the tripod, and transit are in a position where when it's leveled, the plumb bob is right over the point.  

A total station has an electronic distance meter, records angles and software for interpretation. Information is either collected or produced by the operator. The prism is on a special pole that can be leveled over a point. The rod height, which is adjustable is recorded by the operator during the data collecting. When being set up, known points are recorded for the instrument to "know" where it's located or new points are collected and preserved for future reference. The telescope on the total station is used to sight the prism, so the information is all coordinated. Since the total station is only as smart as the operator, recording the wrong rod height, or height of the sighting point on the instrument can lead to innacurate readings. There is an optical plumb bob for setting up over a point.

I'll stop here. I could go on for many more paragraphs, but that will have to wait for future posts.


  1. Very interesting. Before I retired, I was the Quality Manager in a metrology lab (metrology being the science of weights and measurements). We specialized in the weights side of the science, calibrating anything that you could use to weigh something on. This included analytical balances used in the manufacture of vaccines up through railroad scales used to weigh train cars. There are bigger scales, that can weigh entire ships or airplanes, but they were out of our scope. As the Quality Manager, it was my job to see that all the technicians performed their calibrations using the same process; using verified and traceable test weights; and recording their results in the same format. In other words, it didn't weigh a pound until I said it weighed a pound (or a kilogram, as the case may be). It's interesting reading about measurements and calibrations on something other than weight!

  2. Many measurements I deal with are in decimal feet. A foot is still the same distance, but it's broken down into decimal increments: even feet, tenths and hundreths. Over time, the conversions have been burned into my brain. An inch is 0.083 feet, two inches is .166 feet, etc. Usually, for rough measuring, I'll round to the hundreth, which usually doesn't make any difference, although over long distances, or several conversions, the rounding can lead to unacceptable variances.

    It's rare to see architectural and engineering measurements mixed, but I've seen where different plan sheets have different increments. When that happens, I usually have to spend some time doing conversions for checking to see if there are inconsistancies in the dimensions. Making the mistake of not checking may lead to problems.