Back to basics
June 1, 2006
When I was a young boy, I recall many times hearing the phrase "the rule of thumb is..." or "as a rule of thumb." I'm sure I originally looked at my thumb and wondered what a "rule of my thumb" could mean.
As with many of us who are now sporting gray hair, I was familiar with a ruler on my fingers by one or more of my teachers. Ouch, that got my attention. I'm sure after a while I realized that rule of thumb meant a rule that could generally be used as a standard to obtain a quick solution to a situation.
With a little research, I found that the expression "rule of thumb" has been historically recorded since the year 1692 and probably was used decades or even centuries before then. It meant then what it means now: some method or procedure that comes from practice or experience, or a rough and useful principle or method based on experience rather than precisely accurate measures.
It should be noted that rules of thumb are only rough descriptions. Rules of thumb are intended to simplify procedures or processes that may be complex. So, in a way, rules of thumb are sometimes inappropriate when precision is required. The term "rule of thumb" or a similar expression exists in many languages and cultures. Its likely origin is that the thumb is often used for rough measurement by carpenters, seamstresses, artists and many others. In fact, the measurement of an inch is believed to have been derived from the distance between the tip of the thumb and the first joint.
Rules of thumb can be one word, short statements or long sentences. The most fundamental rule of thumb for surveying is just one word or expressions of that one word. It is "Check!" Or maybe "Always check!" Or "Check, check, check!" You get the point. The No. 1 rule of thumb for surveying is what we must always do with every measurement or calculation we make: check!
Non-Surveying Rules of ThumbThere are hundreds of rules of thumb that we follow in our daily lives. The following is a short list of some common ones:
- Dress in layers in cold weather.
- Never download E-mail from someone you don't know.
- Most poisonous snakes have a triangular-shaped head.
- When driving, follow two car lengths behind for every 10 miles per hour. Or, stay two seconds behind the car ahead.
- In carpentry, measure twice and cut once.
Surveying Rules of ThumbThere are also many rules of thumb used in surveying. Needless to say, there isn't enough space in this article to list them all. Following are just a few of many. Hopefully, you will be able to list many more and will send them to me for a follow-up article on this topic. General
- Do it twice (repeat all procedures).
- When leveling, the bubble follows the left thumb.
- The head of a tripod should be horizontal.
- Always rotate an instrument 180 degrees to check level.
- Don't ever erase in a field book.
- Measure distances by chaining forward and reverse.
- Be familiar with the chain being used.
- Don't ever cut a foot.
- Forward and back distances should agree within 0.01 feet.
- Double all angles for critical control work.
- Double-center all very important layout points.
- The last movement with a tangent screw should be clockwise.
- Confirm your backsight and check it.
- Check the prism offset daily.
- Read the owner's manual!
- Chain distances of less than 100 feet. Use the total station for distances greater than 100 feet.
- Obtain a Height of Instrument (HI) from two bench marks on jobsite work to check the bench mark, the reading and the arithmetic.
- Always make sure the compensator is working on an automatic level.
- Always close a level loop.
- Balance backsights and foresights.
- Look long and go short (long backsights and short foresights).
- Always lay out to better than 1/2 of your final tolerance.
- Always tie into existing layout.
- In building layout, start at the center and work out to minimize errors!
- Stringlines don't lie. They are straight.
- Check 90s with the 3/4/5 method.
- Check diagonals whenever possible.
- Avoid cumulative error. Measure short distances along the chain; do not measure in short increments.
- Avoid having two lasers set up in the same area of a jobsite.
- Set up high to avoid ground temperature effects.
- A minimum of five satellites is needed to obtain precise location.
- Keep field procedures as simple as possible.
- Choose the highest point for a base station.
- Do all calculations at the beginning of the day before leaving the office.
- Avoid jobsite calculations-that is where mistakes are prone to happen.
Make Fewer MistakesRules of thumb provide guidance and stability to the methods we use everyday in surveying practice. The more rules of thumb we know and apply, the easier it will be to teach others about the standard methods of surveying, and hopefully everyone will make fewer mistakes.
Do you have some additional rules of thumb that I have not listed? If so, please send them to me by E-mail to firstname.lastname@example.org. If I get enough suggestions, I'll do a follow-up article to list more rules of thumb that all of us can follow and improve our work.