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Surveying & Prospection for Archaeology & Environmental Science Spatial sampling & soil properties Phil Buckland.

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Presentation on theme: "Surveying & Prospection for Archaeology & Environmental Science Spatial sampling & soil properties Phil Buckland."— Presentation transcript:

1 Surveying & Prospection for Archaeology & Environmental Science Spatial sampling & soil properties Phil Buckland

2 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.

3 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)

4 Soil chemistry & properties... Biological proxies - fossil insects, plant macrofossils, molluscs, tree rings... Chemical proxies - phosphates, oxygen isotopes, carbon isotopes ( 14 C), other isotopes & ratios Physical proxies - organic content, magnetic susceptibility, colour (full spectrum), dust, particle size, sedimentation, raised beaches

5 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’

6 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 (PO 4 ) 2) ions bind to soil particles distance phosphates Archaeological site? Background level e.g. waste, manuring, food storage - past & present (pollution)

7 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

8 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)

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

10 Samples Examples: PopulationSample All people in SwedenEvery 100th person in Sweden A 10 hectare meadow100 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 dice100 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 careful consideration - and can result from good project design.

11 Samples Examples: PopulationSample All people in SwedenEvery 100th person in Sweden A 10 hectare meadow100 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 dice100 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 careful consideration - and can result from good project design.

12 Samples Examples: PopulationSample All people in SwedenEvery 100th person in Sweden A 10 hectare meadow100 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 dice100 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 careful consideration - and can result from good project design.

13 Samples Examples: PopulationSample All people in SwedenEvery 100th person in Sweden A 10 hectare meadow100 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 dice100 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 careful consideration - and can result from good project design.

14 Samples Examples: PopulationSample All people in SwedenEvery 100th person in Sweden A 10 hectare meadow100 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 dice100 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 careful consideration - and can result from good project design.

15 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

16 Samples & variation Discrete sampling of continuous variables. The things we measure vary in different ways... low resolutionhigh resolution

17 Samples & variation & interpolation The things we measure vary in different ways... low resolutionhigh resolution Interpolation allows us to simulate/approximate the original variation...by assuming things about the real distribution.

18 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)

19 Sampling Strategies Methode.g.pro’scon’s Grid Random Strategic grids 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 higher resolution detail biased (stat’s unsound) tends to prove nothing can support other proxies 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

20 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

21 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’

22 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

23 Interpolation A 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 map showing % of beetle species (in a sample) that tolerate different temperatures. Summer temperature Temperature range

24 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

25 Interpolation methods - example Prospection area in Skåne - ca. 600x200m Easting Northing Sample grid - semi-regular (sub-regular)

26 Interpolation methods - example Topography - Interpolation by Ordinary Kriging

27 Interpolation methods - example Topography - Interpolation by Inverse distance weighting

28 Interpolation methods - example Inverse distance weightingKriging Appears smootherAppears blotchy, unrealistic? Ridge vs. mound

29 Interpolation methods - example Inverse distance weighting 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

30 Interpolation methods - example Inverse distance weightingKriging omit (mask) unsampled area probably use Kriging (but may have to adjust parameters) Implications?

31 Interpolation methods - example Phospates (total phosphates - P tot ) Major anomalies Minor anomalies Human occupation sites?

32 Interpolation methods - example Loss On Ignition (% - weight loss after burning) High organic content: peat bog (mire)? Low organic content: erosion? mineral soil?

33 Interpolation methods - example Compare proxies... identify similarities in patterns... Some similarities in lows & highs: variables support each other? or autocorrelation - variables influence each other?

34 Interpolation methods - example Magnetic Susceptibility (MS) Prehistoric fireplaces? Low values due to bog? (waterlogged - reduced iron)

35 Interpolation methods - example Probable area of past human occupation

36 Considerations when interpreting Farm Bog Erosion Deposition Colluviation - translocation of sediments by gravity Sediments move with time - so signals may be displaced

37 Considerations when interpreting Farm Bog Erosion Deposition Must be considered when interpreting proxy indicators Sediments move with time - so signals may be displaced Phosphates Occupation phase Present day

38 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 deposits Wind deposited sediments -> Aeolian deposits Gravitationally 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!

39 Integration of maps

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41 Rockart


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