Surveying & Prospection for Archaeology & Environmental Science

Spatial sampling & soil properties

Phil Buckland

Contents

• Soil chemistry & physical properties as proxy data sources

• What are (spatial) samples - why are they taken?- how are they taken?- what can they tell us?

• Sample data - examining,- manipulating,- interpreting.

Soil chemistry & properties...

Proxy indicator: a measurable variable that tells us about conditions or changes in the past that we cannot directly measure.

Commonly used in environmental archaeology, quaternary geology, environmental change analysis (& monitoring)

Soil chemistry & properties...

Biological proxies - fossil insects, plant macrofossils, molluscs, tree rings...

Chemical proxies - phosphates, oxygen isotopes, carbon isotopes (14C), other isotopes & ratios

Physical proxies - organic content, magnetic susceptibility, colour (full spectrum), dust, particle size, sedimentation, raised beaches

Soil chemistry & properties...

phosphates (P) - element (phosphorus) - organic and inorganic- measure amount & ratios in sediments (citric acid extraction) using spectrophotometer

organic content = Loss On Ignition (LOI)- ratio of organic:inorganic matter in sediment- measure by burning and calculating weight loss

magnetic susceptibility (MS) - ability of material to sustain an applied magnetic fields.- measure induced magnetic field in sample compared to applied field

Clark, A (1990/2000) ‘Seeing beneath the soil’

Soil chemistry & properties...

phosphates (P) - Phosphate degrees P°

- increased amounts often indicate human activity- linked to decay of organic materials (organisms)- Decay leads to: 1) release of phosphate ions (PO4)

2) ions bind to soil particles

distance

phos

phat

es

Archaeological site?

Background level

e.g. waste, manuring, food storage - past & present (pollution)

Soil chemistry & properties...

organic content = Loss On Ignition (LOI) - %

- increased amounts often indicate human activity- linked to decay of organic materials (organisms)- accumulations of organic matter lead to increase

e.g. waste, manuring, food storage

distance

LOI

Archaeological site? Bog (mire)?

Confirm with macrofossil & insect analyses

Soil chemistry & properties...

- dependent on iron content of soil- heating increases MS due to oxidation of iron- erosion, ploughing etc. can expose different materials

e.g. fire, industry, pollution - past & present

magnetic susceptibility (MS) - SI (no units)

distance

MS

Archaeological site? Road (modern)

Samples

More simply:A sample is that part of reality that we actually measure

’A sample is that part of a population which is actually observed.’

www.wikipedia.org

Sample:

’...a set of potential measurements or values, including not only cases actually observed but those that are potentially observable’

Population:

www.wikipedia.org

Samples

Examples:

Population SampleAll people in Sweden Every 100th person in Sweden

A 10 hectare meadow 100 randomly placed 1m squares in the meadow

Phosphate levels in an area 1km around an archaeological site

Soil samples taken at 20m intervals throughout the area

An infinite number of rolls of two dice 100 rolls of two dice

Fluctuations in heavy metal levels in the water of the Bay of Bothnia

Weekly heavy metal test samples from water 5km East of Holmsund

How well the samples reflect the population requires carefulconsideration - and can result from good project design.

Samples

Examples:

Population SampleAll people in Sweden Every 100th person in Sweden

A 10 hectare meadow 100 randomly placed 1m squares in the meadow

Phosphate levels in an area 1km around an archaeological site

Soil samples taken at 20m intervals throughout the area

An infinite number of rolls of two dice 100 rolls of two dice

Fluctuations in heavy metal levels in the water of the Bay of Bothnia

Weekly heavy metal test samples from water 5km East of Holmsund

How well the samples reflect the population requires carefulconsideration - and can result from good project design.

Samples

Examples:

Population SampleAll people in Sweden Every 100th person in Sweden

A 10 hectare meadow 100 randomly placed 1m squares in the meadow

Phosphate levels in an area 1km around an archaeological site

Soil samples taken at 20m intervals throughout the area

An infinite number of rolls of two dice 100 rolls of two dice

Fluctuations in heavy metal levels in the water of the Bay of Bothnia

Weekly heavy metal test samples from water 5km East of Holmsund

How well the samples reflect the population requires carefulconsideration - and can result from good project design.

Samples

Examples:

Population SampleAll people in Sweden Every 100th person in Sweden

A 10 hectare meadow 100 randomly placed 1m squares in the meadow

Phosphate levels in an area 1km around an archaeological site

Soil samples taken at 20m intervals throughout the area

An infinite number of rolls of two dice 100 rolls of two dice

Fluctuations in heavy metal levels in the water of the Bay of Bothnia

Weekly heavy metal test samples from water 5km East of Holmsund

How well the samples reflect the population requires carefulconsideration - and can result from good project design.

Samples

Examples:

Population SampleAll people in Sweden Every 100th person in Sweden

A 10 hectare meadow 100 randomly placed 1m squares in the meadow

Phosphate levels in an area 1km around an archaeological site

Soil samples taken at 20m intervals throughout the area

An infinite number of rolls of two dice 100 rolls of two dice

Fluctuations in heavy metal levels in the water of the Bay of Bothnia

Weekly heavy metal test samples from water 5km East of Holmsund

How well the samples reflect the population requires carefulconsideration - and can result from good project design.

Samples & variation

The things we measure vary in different ways...

Continuous variables:- Vary continuously- Often MEASURABLE

Discrete variables:- Stepwise, or non-continuous variation- Often COUNTABLE

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Samples & variation

Discrete sampling of continuous variables.

The things we measure vary in different ways...

low resolution high resolution

Samples & variation & interpolation

The things we measure vary in different ways...

low resolution high resolution

Interpolation allows us to simulate/approximate the original variation

...by assuming things about the real distribution.

Sampling Strategies

Must consider:

• Project aims and how they can be achieved

• Variables to be measured and how they behave in reality

• Scientific theory (& statistical ground rules)

• Avoid bias

• Encompass areas outside of the immediate area of investigation (background/reference samples)

Sampling StrategiesMethod e.g. pro’s con’s

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Strategic grids

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• statistically robust(no intentional bias)• stratified sampling is scalable

• can (randomly) miss areas of interest• difficult to implement*

• practical in field• uniform coverage• good statistics

• may miss higherresolution detail

• biased (stat’s unsound)• tends to prove nothing

• can support otherproxies from samples• easy for archaeologists

• can target interest• good compromise between detail and statistical robustness• easy to cover back-ground & features

• can be interpolation problems (worst case =undetected)• some bias possible

*without total station or good GPS

Sampling strategies

Good sampling strategy can allow:

• a good level of realism in models (reconstructions/interpretations)

• measure and control of errors

• valid use of summary and advanced statistics

• results that stand up to rigorous interrogation

• useful models for interpretation

Interpolation

Translating sample point data into continuous surfaces

‘interpolation is a method of constructing new data points from a discrete set of known data points’

www.wikipedia.org

Interpolation

A surface is a 3 dimensional representation of the values of any variable in two dimensional space (at an instance in time).

e.g.

• ground temperatures at a specific time

• phosphate levels in soil

• the ground surface = topography

• the sea surface

… although the two dimensional space does not have to be geographical… e.g. climate space

InterpolationA surface is a 3 dimensional representation of the values of any variable in two dimensional space (at an instance in time). … although the two dimensional space does not have to be geographical…

Climate space mapshowing % of beetle species (in a sample) that tolerate different temperatures.

Sum

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ture

Temperature range

Interpolation methods

Numerous methods exist.Deterministic methods: ‘assign values to locations based on

the surrounding measured values and on specified mathematical formulas that determine the smoothness of the resulting surface’ ESRI. E.g.

• Spline• Inverse distance weighted

Geostatistical methods: ‘are based on statistical models that include autocorrelation (the statistical relationship among the measured points)’ ESRI. E.g.

• Kriging

Most methods can be tuned to application

Interpolation methods - example

Prospection area in Skåne - ca. 600x200m

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Easting

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Sample grid - semi-regular (sub-regular)

Interpolation methods - example

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Topography - Interpolation by Ordinary Kriging

Interpolation methods - example

Topography - Interpolation by Inverse distance weighting

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Interpolation methods - example

Inverse distance weighting

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Kriging

Appears smoother Appears blotchy, unrealistic?

Ridge vs. mound

Interpolation methods - example

Inverse distance weighting

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Kriging

Kriging - uses relationship between data values of each point to every other point to construct values for missing points.

Inverse distance weighting - missing values are a simple mathematical function of the value of the nearest point.

More info: ArcGIS help files;Internet; Recommended literature

Kriging identifies a gradient W-E and applies it to the missing values

IDW missing values fall off with distance from known points

Interpolation methods - example

Inverse distance weighting

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Kriging

• omit (mask) unsampled area• probably use Kriging (but may have to adjust parameters)

Implications?

Interpolation methods - example

Phospates (total phosphates - Ptot)

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Major anomaliesMinor anomalies

Human occupation sites?

Interpolation methods - example

Loss On Ignition (% - weight loss after burning)

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High organic content:peat bog (mire)?

Low organic content:erosion?mineral soil?

Interpolation methods - example

Compare proxies... identify similarities in patterns...

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Some similarities in lows & highs:• variables support each other?• or autocorrelation - variables influence each other?

Interpolation methods - example

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Magnetic Susceptibility (MS)

Prehistoric fireplaces?Low values due to bog?(waterlogged - reduced iron)

Interpolation methods - example

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Probable area of past human occupation

Considerations when interpreting

Farm

Bog

Erosion

Deposition

Colluviation - translocation of sediments by gravity

Sediments move with time - so signals may be displaced

Considerations when interpreting

Farm

Bog

Erosion

Deposition

Must be considered wheninterpreting proxy indicators

Sediments move with time - so signals may be displaced

Phosphates

Occupation phase

Present day

Considerations when interpreting

Other considerations:• Ploughing, digging & erosion may expose or mix subsoils

with different properties• Water & wind erosions & associated deposition may cover or

destroy evidence - leaving an incomplete record

Water deposited sediments -> Alluvial depositsWind deposited sediments -> Aeolian depositsGravitationally deposited sediments -> Colluvial deposits

• Proxies may interact - i.e. values may be related by physical & chemical processes - ‘autocorrelation’ in statistics

• Rates of decay, weathering & transportation will vary depending on climate and sediments/bedrocks

• Geostatistics may give false positives if not used properly!

Integration of maps

Integration of maps

Integration of maps

Rockart