Difficulty
Moderate
Steps
2
Time Required
In Progress
This guide is currently being written. Reload periodically to see the latest changes.
Introduction
Coordinate pairs in the form of latitudes and longitudes are often used to record and store the locational component of scientific information. These numbers are scientific measurements in the same sense as other measured variables and customary rules for precision and accuracy of numeric information apply. The table below describes the accuracies associated with a range of coordinate pair precisions. Examples of applications that employ particular precisions are listed as well
| Precision - Latitudes & Longitudes Expressed As | Latitudinal Accuracy at All Latitudes | Longitudinal Accuracy at the Equator | Longitudinal Accuracy at 45° Latitude | Application Example |
|---|---|---|---|---|
| Whole degrees (ddd) | ± 110 kilometers | Same as latitudinal | ± 80 kilometers | Ridiculously rough mapping |
| One-place decimals (ddd.d) | ± 11 kilometers | - - ditto - - | ± 8 kilometers | XX rough mapping |
| Two-place decimals (ddd.dd) | ± 1 kilometer | - - ditto - - | ± 800 kilometers | X rough mapping |
| Three-place decimals (ddd.ddd) | ± 110 meters | - - ditto - - | ± 80 meters | rough mapping |
| Four-place decimals (ddd.dddd) | ± 11 meters | - - ditto - - | ± 8 meters | Some natural resources mapping |
| Five-place decimals (ddd.ddddd) | ± 1.1 meters | - - ditto - - | ± 0.8 meters | Most natural resources mapping |
| Six-place decimals (ddd.dddddd) | ± 0.11 meter | - - ditto - - | ± 0.08 meter | Some natural resources mapping |
| Seven-place decimals (ddd.ddddddd) | ± 0.011 meter | - - ditto - - | ± 0.008 meter | Cadastral mapping |
| Eight-place decimals (ddd.dddddddd) | ± 0.0011 meter | - - ditto - - | ± 0.0008 meter | Cadastral mapping |
| Nine-place decimals (ddd.ddddddddd) | ± 0.00011 meter | - - ditto - - | ± 0.00008 meter | Some survey control |
| Ten-place decimals (ddd.dddddddddd) | ± 0.000011 meter | - - ditto - - | ± 0.000008 meter | Some survey control |
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The precision of the numbers used to record locational information dictates the best-case accuracy of the information.
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Misleading (and even silly) information sometimes gets recorded because the principles discussed here are not widely understood. Good science demands that the precision and accuracy of numbers that communicate location be consistent with good practice.
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The first screen shot shows a web mapping application reporting the location of a mouse click to thirteen places (one ten-trillionth of a degree). That precision implies an accuracy of plus or minus 0.0000001 meters. Yet we know that the accuracy of the background aerial photo is probably no better than plus or minus 10 meters. Kinda silly, huh?
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