Notes and Suggestions for Botanical Garden Surveyors - Part 5
ByWalt Dunlap, Mapping Specialist, The New York Botanical Garden
As a mapping specialist at The New York Botanical Garden and a professional land surveyor for the last 20 years, I am offering consulting services on land surveying to any garden that is mapping or planning to map their collections or garden.
|The Difference between Accuracy and Precision | | Effects of Temperature and Pressure |
The Difference between Accuracy and Precision
It's possible to look up precision in a thesaurus and find accuracy as its synonym; however, there are shades of meaning between the two. Professionals, such as surveyors, seek to preserve this distinction. It's quite possible to have very precise measurements that are wildly inaccurate. Let's say you are measuring with a total station to the nearest 5" horizontally and 0.01' for distance. These are fairly precise levels of measurement, but if you are using the wrong initial azimuth then your results will be totally inaccurate.
Accuracy relates to the exactness of the result, the closeness of the observations to the true value. Precision relates to the operation used to derive the result, such as using a tape measure graduated in meters versus millimeters.
Effects of Temperature and Pressure
A total station, like every EDM, electronic distance measurer, has an allowance for adjustment of distances based on the current temperature and pressure. Most instruments employ a projected infrared beam that is reflected from the prism directly back to the source. It is NOT A LASER. The prism is technically a dodecahedron, and it functions normally within 12.5° of its center point, although the tilting prisms allow better vertical pointing to the instrument. Since the wavelength of infrared is a known constant, the instrument can calculate the distance the wave travels. The only applicable variables are temperature and pressure. The correction is normally a minor one unless you work in extreme conditions. At 59° F and 29.9 in. Hg, the correction is 0.
For a location near sea level a setting of 29.9" for pressure and a daily adjustment for the temperature should suffice for accuracy. If you have doubts, try changing the settings to see how much your average distances are affected or be more scrupulous in your routine to reset the PPM at every reasonable chance based on reported or observed readings. For shots greater than a thousand feet you should always use the PPM correction supplied by the instrument manufacturer.
Leveling Vials - Meaning of Graduations
When I took an Agriculture Engineering course in college, I found out just how many people hadn't the faintest idea how to tune an engine. There's nothing surprising in this, except that I'm absolutely sure there had been lots of bragging and discussion of "tune-ups", "lean and rich mixes", "T.D.C.", etc., going on for years among them. In the same vein, there are uninformed instrument operators who assume that the graduations of the level vial just below the telescope have some relationship to angular measurements. This is reflected in the mistaken belief that the divisions of the vial directly indicate an angular error in proportion to the horizontal angle turned. The listed notations (e.g., 30"/2 MM) are a measure of the arc of the vial and its precision. The accuracy of the leveling does affect angles but not so directly.
- TO BE CONTINUED -
Updated June 16, 1999
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