Backsighting
Definition
The process of orienting a total station by pointing it at a known reference point (backsight) after setting it over the instrument station. It establishes the angular reference frame so that all measurements are consistent.
- Field
- Crime scene surveying, total-station work
- What it establishes
- Angular reference frame of the instrument
- Step order
- Performed after setting up over the instrument station
Common questions
What goes wrong if backsighting is skipped or done to the wrong point?+
Every subsequent angle and coordinate the total station records is measured relative to the backsight direction, so an incorrect or skipped backsight shifts or rotates the entire set of recorded scene measurements, which can misplace evidence points relative to each other and to fixed scene features.
Does backsighting also establish the scale or only the direction of measurements?+
Backsighting on its own establishes angular orientation; distance and coordinate scale come from the instrument's own electronic distance measurement to known points, so a correct backsight combined with accurate station and backsight coordinates is what lets the survey tie into a consistent scene coordinate system.
Why is a known reference point required rather than any visible landmark?+
The backsight point must have previously surveyed or otherwise fixed coordinates so the total station can calculate its own orientation angle from the known geometry; sighting an arbitrary landmark with unknown coordinates would not let the instrument compute a correct reference frame.
Related terms
- Datum Peg
- A fixed ground marker with precisely recorded coordinates that serves as the origin point for a site grid. All spatial measurements on...
- Resection
- A method of determining the instrument station's own coordinates by measuring angles and distances to two or more points of known position....
- RTK-GPS
- Real-Time Kinematic Global Positioning System: a satellite-positioning technique where a static base station transmits correction data to a roving receiver in real...
- Site Grid
- A rectangular coordinate system established across a site, usually with the origin at a datum peg and axes aligned to magnetic north...
- Total Station
- An electronic instrument that simultaneously measures horizontal and vertical angles and distance to a reflective prism, computing three-dimensional coordinates directly. Typical angular...