Data and Excavation
Define datum.
- A point of known global coordinates (X,Y,Z)
All unit, finds, features, etc. are mapped relative to datum
What is LiDAR and how does it work?
● Li = Light - D = Detection - And - Ranging
● Time-of-flight laser scanners
– LiDAR device knows its own exact location and orientation (dGPS + IMU)
– LiDAR device emits laser beam, times bounce back (“return”) to calculate scanned surface(s)
● Terrestrial LiDAR: mounted on the ground at a static datum (i.e. on a tripod over a surveyed point)
● Aerial LiDAR: carried on an aircraft (plane, drone, etc)– Emit thousands of pulses per second in a “sweep” pattern– Some pulses get through any foliage to reach the ground– Last return is usually “ground”
● Filter vegetation points for a “bare earth model”
True or False: Stonehenge does not bare any markers of intentional alignment.
False. Stonehenge has lunar and solstice alignments.
What are the features of an excavation unit?
- Defined shape (square)
Straight vertical side walls, multiple adjacent units may be excavated to form a trench
- Defined size (i.e. 1x1m)
- Oriented to N (usually)
What are the benefits of LiDAR?
Benefits of aerial LiDAR:
– High-precision 3D spatial data
– Can “see through” vegetation
– Digital methods can enhance subtle features (invisible on the ground)
– Ease and efficiency of survey!!
● Large area coverage
● Impervious to difficult terrain & veg
True or False: Isotope analysis contributed to the understanding of Ötzi's diet.
True.
What is excavation procedure?
- Excavate carefully in controlled vertical contexts
~ Layers: natural sediment strata
~ Levels: arbitrary strata of a given depth (i.e. 10cm)
- Document and collect in situ finds
- Screen backdirt (sift excavated sediment through mesh)
- Soil sample: bulk collection of sediment for flotation analysis
- Map each level "floor"
- Document the side wall stratigraphy
- Take copious and detailed notes. "NOTES NOTES NOTES!"
What are the drawbacks of LiDAR?
Drawbacks of aerial LiDAR
– Expensive
– Large datasets
– Specialist skills required
● Piloting ● Data processing
True or False: Unmarked graves are a kind of subsurface unmarked feature.
True.
What is The Recovery Revolution?
“a dramatic refinement of field and laboratory methods that has produced infinitely more fine grained data.”
(Fagan and Durrani 2023)
- Mid-20th century onward, ongoing/accelerating
- Focus on multidisciplinary scientific approaches
Ex: flotation sampling
What do Remote Sensing Technologies do?
Bonus: Name three.
RST let archaeologists "see under ground". They are minimally invasive, reduce destructive excavation, and help focus research.
Includes:
Electrical Resistivity Tomography (ERT)
Magnetometry
Ground-penetrating Radar (GPR)
What is magnetometry? How does it work?
Magnetometry measures and maps variations in the Earth's magnetic field.
– If a material has a strong or altered magnetic signature, it stands out from the background. Burned, heated, or disturbed soils can hold a stronger magnetic signal. (hearths, kilns, pits, ditches)
– Undisturbed, natural sediments have a weaker magnetic signal. (compacted floors, natural ground)
– Metal and fired materials create strong magnetic anomalies. (iron objects, brick walls, baked clay)
The basic premise: The Earth has a weak natural magnetic field. A magnetometer detects tiny variations in magnetism caused by archaeological features. High magnetic contrast between a feature and its surroundings helps identify buried structures.
● Strong anomaly = highly magnetic material (e.g., burned clay, metal, filled ditches).
● Weak anomaly = little or no disturbance (e.g., undisturbed ground, compacted floors)
What is the process of flotation sampling?
● Bulk sediment sample placed in water, agitated
● Non-organic sediment (sand, rocks, etc.) will settle to the bottom
● Organic materials will float
● Seeds ● pollen ● Charcoal ● Any other floral remains
● Skim the floaters off with a fine mesh
● Dry the samples
● Examine under a microscope
What is Electrical Resistivity Tomography? How does it work?
ERT measures and maps electrical resistivity in sediment.
– Loose, moist sediment is an OK conductor (low resistivity). (e.g. ditches, pits)
– Dense, dry sediment is a poor conductor (high resistivity) (e.g. floors, dirt walls, roads)
– Rocks are poor conductors (high resistivity) (e.g. stone walls, structures)
The basic premise: An electrical pulse of known current (amperage) is passed between two probes. In a perfectly conductive environment, the current should flow easily.
● A separate pair of probes measures the voltage difference in the ground.
● Using Ohm’s Law (Resistance = Voltage / Current) the apparent resistivity of the subsurface is calculated.
● low_V / C = high resistivity ● high_V / C = low resistivity
What is GPR?
How does it work?
Ground Penetrating Radar (GPR) sends radar pulses into the ground to detect buried features.
– Different materials reflect the radar waves differently. Strong reflections = big changes in material (e.g. walls, stones, voids). Weak reflections = gradual changes (e.g. compacted soil, sediment layers).
– Works best for detecting structures, voids, and layering.
The basic premise: A radar pulse is sent into the ground. It bounces back when it hits a buried feature. The time it takes to return tells us the depth and type of material. A buried wall or void shows up as a strong reflection; soft soil or sediment shows a weaker reflection.