As one measures the coordinates of a point or the length of a line, the imminent query that crops up is how good is the measurement. What is the error? Is it precise? What is the accuracy? Can you remeasure and get the same results? etc.
Let’s revisit some of these familiar but important terms and try to interlink them where possible.
To start with, error refers to the difference between the measured value and the correct(true) value.
Accuracy
Accuracy refers to how close a measurement is to the true or accepted value.
Lets say we are measuring a length of a study table whose actual length is 100 cm. Accuracy is how close the measurement is to the actual length. 98 cm and 102 cm (±2 cm) are more accurate than 90 cm and 110 cm (±10 cm) respectively.
Quite often in mapping, we hear the term relative accuracy. It is the degree to which a given point on a map is accurate relative to other points within that same map.
Consider the longitudinal coordinate of all map features are erroneous by 200 metres. The relative accuracy (i.e. the distances among the features) is not affected.
Precsion
Precision refers to how close measurements of the same item are to each other. Precision is independent of accuracy. That means it is possible to be very precise but not very accurate, and it is also possible to be accurate without being precise.
Let us think the measuring tape (1–metre long) itself is short by 2cm due to shrinking. Hence, always you end up with a value of 102 cm against a true value of 100cm. The precision in this case is 2cm.
Precision includes:
• repeatability — the variation arising when all efforts are made to maintain conditions constant by using the same instrument and operator, and repeating during a short time period;
• reproducibility — the variation arising using the same measurement process among different instruments and operators, and over longer time periods.
Let us note that the best quality measurements are both accurate and precise.
A classic way of demonstrating the difference between accuracy and precision is with a dartboard. Consider the bulls-eye (i.e the centre of the circles) of the dartboard as the true value. The closer the darts land to the bulls-eye, the more accurate they are. There are four possibilities:
A - The darts are neither close to the bulls-eye, nor close to each other.
B - All the darts land very close together, but away from the bulls-eye.
C - The darts are all about an equal distance from bulls-eye and spaced equally around it. Statistically speaking, this is encouraging as the average of the darts‘ postions is at the bulls-eye.
D - The darts land close to the bulls-eye and close together.
A - Inaccurate and imprecise
B - Precise but inaccurate
C - Accurate; but imprecise
D - Accurate and precise
Does Precision influence accuracy?
A measurement system can be accurate but not precise, precise but not accurate, neither, or both. For example, if an experiment contains a systematic error -such as a weighing scale in a weight-loss centres :) , then increasing the sample size generally increases precision but does not improve accuracy.
Map accuracy:
The closeness of results of observations ( or computations, or estimates) of graphic map features to their true value or position. Map accuracy is verified by comparing the positions of map point elevations or locations with corresponding positions as determined by ground surveys of a higher accuracy. However, they differ by the statistical means and methodology utilized in presenting the measurement errors.
In India, the map positional accuracy is represented as fraction of map scale viz., 0.25mm * map number. Example: in 1:25000 scale maps, the accuracy is 6.25 m (= 0.25 mm * 25000). And the vertical accuracy is expected to be half of the contour Interval.
In GIS data, accuracy can be referred to a geographic position, but also to its attributes, or conceptual accuracy. Precision refers how exact is the description of data. As discussed earlier, precise data may be inaccurate, because it may be exactly described but inaccurately gathered. The possibilities include:
Position is correct but attributes are wrong. Example: The location of a lamp-post is correctly depicted but attributed as telephone pole.
Position is inaccurate but attributes are correctly described: Example: If you move meridians in a map by an arc-second, all the features are measured wrong by about 30 meters in longitude.
Both position and attributes can go wrong. Example: A spot height wrongly located and attributed with a wrong elevaation value.
ISO (International Organization for Standardization) is an international standard-setting body composed of representatives from various national standards organizations. Visit their wiki page for elaborate treatment on these. Also refer to mapping standards of NMAS, ASPRS, EuroSDR( erstwhile OEEPE), NNRMS,.. for eloborate guidelines of mapping standards.
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