Measuring Distance ©2010 Dr. B. C. Paul Note – The techniques shown in these slides are considered common knowledge to surveyors. Figures in the slides may be the authors own work or extracted from Instrument Users Manuals, Surveying by Bouchard, Mine Surveying, or various internet image sources.

How do We Know Distance? Physically Measure It Counting off paces (only good for very loose surveys) Tape Measures For a line a mile long this gets fun fast Optical Techniques Things appear larger or smaller depending on distance Rate that things get smaller is function of focal length of lens If know focal length of lens can determine distance optically by how big an object of know size is

More Knowing Distances Wavelengths of Electromagnetic radiation Send out a light beam and reflect it off of something at point Reflected beam will be out of phase unless the distance is an integer multiple of the light wavelength Amount out of phase is the non-integer part of the distance relative to the length of light wavelength Send out more than one wavelength – distance is the same but that distance will only through several wave lengths out by a certain amount for a unique distance Technique is basis of Electronic Distance Measurement

And More Knowing of Distance If know the coordinates of two points Pythagorean theorem will give you the distance Getting coordinates of points Grid of Global Positioning Satellites above the earth Read the angel and distance to a satellite from an electronic signal it sends down Solve coordinates of any point from satellite position If set up two points, a base point and a point being measured can use ground based stations to compare signals and get mm accuracy Can hit within feet with single back-pack mounted units May know this technology is being put in cars now

Surveyor’s Chains Were Robust Wilderness Ready Tape Measures A link is one of those wire lengths with a loop at each end

Chain Characteristics Chains had 100 links with brass holding handles at the ends. Chains had brass tags every 10 links to speed counting of links on partial chains Two Basic Kinds of Chains Surveyors Chain (also called Gunter’s Chain) Was 66 feet long Engineers Chain Was 100 feet long

Why a Profound Number Like 66 ft Turns out a mile (5280 feet) is 80 chains long Divides nicely for quarter sections or common land divisions Surveyors were laying out blocks of land Also helps explain why no little fractions for inches etc.

The Class Chain Is a reproduction 10 meters long We’ll use it to measure out a baseline on first lab just for nostalgia

Measurement by Tacheometry Surveying Instruments have very precise optics – focal length is very well known You can tell how far away something is by how big it appears in the lens (provided you know the size of the something) That something is the rod used to mark your foresight point

Stadia Measurements That’s what those two funny Short horizontal lines are for Take the upper and Lower rod reading – Now you have an object Of known size in an Interval the instrument Was designed to measure

Reading the Rod Rods are marked in 10ths and 100ths of Feet. (not in inches) This is 6.2 ft.

Reading the Finer Points

Getting the Hang of it! Notice that both The black mark And the white Space between Are 1/100 th of a foot Eyeball limitations – even with a telescope usually limit you to shots Of 300 feet or less.

For Instruments Other Than Very Old Ones Ratio between the stadia intercept distance (which you read from the rod) is 100 Can get distance by

You Think I Blew the Formula Ground Level α α Don’t forget the rod you read is inclined with respect To the perpendicular measurement plane (which is where the funny squared and double Angle stuff came in) I’m not going to do a trig Proof in a 1 hours hands On course (entertain yourself with your Own proof if your going nuts)

Vertical Control with Stadia V in the formula is the vertical change in distance from the height of instrument to the center reading on the rod To get change in ground elevation ΔV = V + Height of Instrument – Center Rod Reading

An Example The upper rod reads 6.5 The lower rod reads 6.1 The intercept is 6.5 – 6.1 = 0.4ft Using a stadia constant of * 100 = 40 feet

Measuring By EDM EDM shoots a lazer Beam into a reflector

Reflects back Out of Phase You remember from physics that out of phase light beams Cancel part of intensity. You can tell how far out of phase by Measuring light intensity.

To Do it. Turn the instrument on using the Switch on the side Our totally digital stations have To be turned on to work at All. Take aim at your target using the telescope

Set Your Units for Distance Measure The ft and M indictor tells You what units Its it. Push the enter button to change it

The Real Tuff Part. Push the Measure Button

I Lied – This is Even Harder Read the Distance

Proficiency #5 Task #1 chain the distance between 2 stakes – use instrument to keep people on line Task #2 measure a distance by stadia Task #3 measure a distance by EDM