E. Systematic Errors

1. Causes

Systematic errors are generally the result of equipment maladjustment or its reaction to environmental condition changes. For example, a steel tape changes length based on temperature. We can compute a correction as long as we know the steel’s physical characteristics, a calibration temperature and length, and the temperature at measurement.

Systematic errors originating from equipment condition can be resolved by procedure or adjustment. Specific compensatory procedures are described in chapters and sections on instrument use. Topic IV. Equipment Checks and Adjustments describe mechanical compensation for different instruments.

The presence of systematic errors can lead to a measurement set which is precise but not accurate. Recall the Precise but not accurate shot group from before, Figure E-1.

Figure E-1 Systematic error effects

While the shot group is precise, something is consistently happening which prevents hitting the bull's-eye. The shooter is apparently using correct  procedures, but the maladjusted sights on his gun are affecting his results.

2. Compensation

Because they conform to a mathematical or physical law, we can eliminate systematic errors either mathematically or, in many cases, procedurally. Another option may be adjusting the equipment used. Consider the shot group in Figure E-1. Most shooters will recognize that the sights on the gun are out of adjustment. The shooter can:

• aim above and right of the bull's-eye (oricediral compensation)
• adjust the gun sight's windage and elevation (mechanical compensation)  

In this case, a mathmatical compensation doesn't make sense.

When you input the temperature and pressure in a total station, you are providing it the information to mathematically compensate for atmospheric effect on distance measurement. Entering a prism offset compensates another systematic error.

In some cases, we don’t need to know the amount of systematic error if we can just get rid of it. Equipment maladjustment is a generally a systematic error. Being surveyors, and a clever group at that, we use specific measurement procedures which allow maladjustments to cancel. The procedures vary based on the type of instrument and measurement made. We'll get into specifics in subsequent topics on field operations.

Learning how to use surveying equipment is more than becoming familiar with controls and which buttons to push. It includes learning specific measurement procedures which help compensate systematic errors as well as trap mistakes.