Check Point in Surveying
Definition
A check point is a surveying station whose position has been previously established or is known with high accuracy, utilized during surveying operations to verify the correctness of measurements, calculations, and positioning work. Check points serve as quality control mechanisms in surveying projects, ensuring that accumulated errors are detected and corrected before they propagate further through the survey.
Purpose and Importance
Check points are fundamental to maintaining accuracy and reliability in surveying work. They function as reference stations that allow surveyors to validate their work against known positions. When surveyors establish a traverse or perform distance and angle measurements, they periodically occupy check points to confirm that their accumulated measurements align with the established position. This practice is essential in detecting systematic errors, instrumental malfunctions, or procedural mistakes early in the surveying process.
Types of Check Points
Check points can be categorized based on their establishment method. Permanent check points are monumented stations that remain in place for extended periods, often serving multiple projects. These might include benchmarks, geodetic control stations, or previously surveyed corner points. Temporary check points are established specifically for a current project and may be dismantled afterward. Additionally, check points may be classified as either independent verifications using different methods or confirmatory checks using the same surveying technique.
Implementation in Survey Practice
During the execution of a survey, check points are strategically located at regular intervals or at critical junctures within the work. Surveyors establish measurements to check points using the same instruments and methods employed for the primary survey work. If the measured position at a check point differs significantly from its known position, surveyors must investigate the source of error. This might involve re-measuring sections of the traverse, checking instrument calibration, or reviewing calculation procedures.
Error Detection and Management
The comparison between computed and known positions at check points provides quantitative data for error analysis. The discrepancy, called the "check point closure error" or "check point misclosure," indicates the magnitude of accumulated error. Acceptable tolerances for check point closure vary depending on the survey's purpose and required accuracy. Precise surveys may require check point closures within centimeters, while lower-order surveys might tolerate larger discrepancies.
Standards and Best Practices
Professional surveying standards recommend establishing check points at regular intervals, typically every 5-10 stations in traverses or at strategic points in other survey methods. Surveyors should use check points of equal or higher accuracy than the survey being performed. Documentation of check point positions, dates, and closure errors is essential for quality assurance and for communicating survey reliability to clients.
Relationship to Other Control Methods
Check points complement other quality control procedures in surveying, including double measurements, reciprocal observations, and independent survey verification. When combined with proper field procedures, instrument maintenance, and calculation verification, check points significantly enhance the overall reliability of surveying work.
Conclusion
Check points represent a practical and effective method for surveyors to maintain quality control throughout their work. By systematically verifying positions against known stations, surveyors can identify and correct errors promptly, ensuring that final survey products meet accuracy requirements and client expectations.