Total-Station and GPS Survey

Stand a total station over a known point, level it, and orient it by sighting to a backsight. From that moment, any prism you set up elsewhere on the site can be measured to three-dimensional coordinates in a few seconds. The instrument sends out a laser or infrared beam, the prism reflects it, and the return time gives the slope distance. The two encoded circles give the horizontal and vertical angles. The onboard software converts these into easting, northing, and elevation relative to whatever coordinate system you have assigned to the instrument station.

Modern instruments achieve angular accuracies of 2 to 5 arc-seconds and distance accuracies of 1 to 3 mm plus a small parts-per-million component over the distance. For the distances involved in a grave excavation (rarely more than 50 m), the practical coordinate error is well under 10 mm. That is more than sufficient for recording the spatial relationships between a skeleton's elements, a surrounding context boundary, and associated finds.

Data are stored internally and transferred to a laptop by USB, Bluetooth, or memory card. The raw data file is the primary record. It should be preserved in its original form alongside any exported CAD or GIS files. If the data are ever questioned in court, the raw field file is what demonstrates that the derived plans are accurate representations of what was measured.

Before any measurement is taken, the coordinate reference frame must be established. The standard approach is to drive datum pegs into stable ground at the margins of the site, outside any area likely to be disturbed by excavation or vehicle traffic. Two pegs are the minimum; three or four at the corners of a search area give greater resilience. Their coordinates may be assigned in a local arbitrary system (E 1000, N 1000, for example) or tied to a national grid if geodetic registration matters.

1. Set up over a datum peg
   Centre the instrument over the peg using the optical plummet, level it with the tribrach, and enter the peg's known coordinates.
2. Backsight
   Aim at a second datum peg or a clearly identifiable fixed point and set the horizontal circle reading to the calculated bearing between the two points. This orients the instrument.
3. Check a third peg
   Measure a third datum peg and compare its computed coordinates against the known values. The residual error should be within the instrument's specified accuracy. A larger residual indicates a setup error that must be identified and corrected before any scene recording begins.
4. Record setup in the log
   Write the date, time, operator, instrument serial number, datum pegs used, orientation bearing, and the check-peg residual into the survey log. This entry is part of the evidential record.

When the instrument cannot be placed directly over a datum peg, resection is used instead. The instrument is set up at a convenient free station, and distances and angles to two or more datum pegs are measured. The software solves for the instrument position and assigns coordinates to the free station. Resection residuals should be recorded and checked before proceeding.

The total station produces points. The point only has evidential meaning when it is labelled: corner of context 034 grave cut, north-east, depth 0.42 m; left distal femur of skeleton 01, pre-disturbance. The survey record must link every coordinate to an entry in the context sheets or finds log. A coordinate without a label is archaeologically worthless and inadmissible.

• Context boundaries: at least four corners of each cut or layer outline, with additional points on curves. Enough points to reconstruct the plan at 1:20 without interpolation errors.
• Finds: a single point for small objects; three or more points for elongated items (a bone, a weapon) to capture orientation. The point is taken to the top surface of the object as found.
• Skeletal elements: each element given at least one coordinate before disturbance. Articulated skeletons may be recorded at 10 to 20 cm intervals along the long axis to capture body position and orientation in three dimensions.
• Section markers: the top and bottom of each defined stratigraphic boundary on exposed sections, to allow accurate section drawings to be placed in three-dimensional context.
• Datum checks: re-measuring a datum peg at the start and end of each session to verify that instrument orientation has not drifted.

RTK-GPS resolves the efficiency problem of large areas. A base station is set up on a known point and transmits correction data by radio to a roving receiver carried by the surveyor. The rover combines the satellite signals with the corrections and computes its position in real time, typically to within 10 to 30 mm in plan and 20 to 50 mm in elevation. A single operator can cover hectares of open ground in a day, recording probe-positive locations, geophysical anomaly corners, or surface scatter positions directly into a GPS data logger.

In practice, most forensic operations use both: RTK-GPS during the search phase to map surface features and anomalies, and a total station during excavation to record the three-dimensional positions of features and finds at the precision the court requires. The two datasets are registered to the same coordinate system via shared datum pegs, so they form a single coherent spatial record.

Total-station data export formats vary by instrument manufacturer but the most portable are plain text files (CSV or DAT) with point number, easting, northing, elevation, and description columns. These import directly into AutoCAD, MicroStation, QGIS, or ArcGIS. In CAD the points are joined into context boundary polygons and annotated with context numbers, generating scaled plans that match the hand-drawn context sheets. In GIS the same data can be overlaid on aerial photography, geophysical survey grids, and topographic contours to build a complete site map.

For court, plans are printed at a stated scale with a north arrow, scale bar, legend, and a note of the coordinate system used. The plan caption should state the survey method, instrument type, instrument accuracy class, and the dates of survey. A brief statement of the quality-control procedure (datum check residuals within stated tolerance, three pegs used for resection) belongs either in the caption or the supporting witness statement. Judges and juries rarely inspect raw coordinate files, but opposing experts frequently do, so those files must be complete and preserved.

The stated accuracy of a total station is its manufacturer's specification under ideal conditions. In the field, accuracy depends on instrument calibration, atmospheric refraction over long distances, reflector centring error, and the care with which the prism pole is held vertically. Standard practice is to check calibration using a built-in two-face measurement (face left and face right) at the start of each day. Residuals from datum checks should be recorded in the survey log and are the primary field evidence that the data are reliable.

• Calibration: instruments should have a current manufacturer calibration certificate. A forensic surveyor should be able to produce this in court.
• Datum checks: re-measurement of a datum peg before, during, and after each survey session. Residuals are logged with date, time, and operator.
• Atmospheric correction: for distances over 50 m the instrument's EDM applies a correction for atmospheric temperature and pressure. These values should be measured and entered at setup.
• Prism pole plumb: an out-of-plumb prism pole introduces horizontal error equal to the offset times the sine of the tilt angle. Using a circular bubble on the pole, or better a bipod, removes this source of error.

RTK-GPS has additional limitations that matter in forensic contexts. Tree canopy, buildings, and steep terrain reduce satellite visibility and degrade or lose the fix. The RTK correction link can drop if the rover moves out of radio range of the base, introducing undetected errors until the link is re-established. A field protocol that flags and reviews all fixes marked as floating (partial correction) rather than fixed (full correction) is essential.