Errors in Measurements
There are two types of errors that concern us the most in geospatial data generation, and those are random error and systematic error. The third type, which is what we call blunders, is not considered an error, but we need to understand it and deal with it appropriately.
Random Error (or accidental error) is the type of error that randomly happens in nature due to our, or the instrument’s, incapability in realizing the true value. The true value in any measurement process is elusive to us and is beyond our metaphysical power. In a measuring process, we are only estimating the true value. Random error can be reduced by training, experience, and improved quality, but it cannot be eliminated.
Systematic Error: Is the error that has a repeated constant value and follows a mathematical logic. It can be reduced through calibration.
Blunders: A blunder is not an error; it is a mistake resulting from carelessness or negligence that resembles error. Common causes of blunders in surveying and mapping are:
- Measurement taken incorrectly
- Values misread from the measuring device (i.e. screen)
- Number transposed as they are recorded (696 vs. 969)
- Miscounting grids ticks
- Handwriting that is hard to read
- Values entered incorrectly into the computer
- Using the wrong datum and/or coordinate system
- Using the wrong units (meter versus US or international foot)
- Rounding numbers in recording the data
Facts on Error and Normal Distribution:
- Errors are unavoidable, but controllable;
- Any mapping process will have some variation of errors built in;
- No combination of machine and human can produce a product that is exactly the same each time;
- Biases should be removed prior to analysis;
- Small errors are more common than large errors;
- Errors are just as likely to be positive as negative;
- Large errors seldom occur and can only be so big. Blunders can be large.