imaging of geochemical data, by bgs ni
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2010
Imaging of geochemical data by interpolation within geological boundaries
Don Appleton (British Geological Survey)
Presented by Claire McGinn (Geological Survey of Northern Ireland)
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2010Acknowledgements
This work is based on research using a VB.Net tool compiled for ArcGIS 9.3 by Keith Adlam (BGS)
Data for this work were provided by the Tellus Project, funded by the Department of Enterprise, Trade and Investment of Northern Ireland.
The contributions of GSNI and BGS staff who collected, prepared and analysed the Tellus soil samples is gratefully acknowledged.
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2010
Area: 14,000km2
Northern Ireland
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Tellus Project: Northern Ireland Soil data
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Who uses Tellus data?
• Environmental Agencies• Commercial Industries• Marine Scientists• Agricultural Departments• Universities• Local Councils• Regulatory Authorities
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Potentially Harmful Elements (PHE)
• Arsenic (As)• Cadmium (Cd)• Chromium (Cr)• Lead (Pb)
Nickel (Ni) in soils
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Gridding by Inverse Distance Weighting: Chromium
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Simplified Bedrock Geology Map
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Inverse Distance Weighting
versus
Soil Parent Material (i.e. within geological boundaries)
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Bedrock geology boundaries
Superficial geology boundaries
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3
2
2
34
2
44
43
4+4+4+3+2=17
17/5=3.4
4+3+3+2+2=14
14/5=2.8
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4+4+4+3+2=17
17/5=3.4
4+3+3+2+2=14
14/5=2.8
1-2.93-4.95-6.97-8.9
TalkingWith Maps
2010
4+4+4+3+2=17
17/5=3.4
4+3+3+2+2=14
14/5=2.8
1-2.93-4.95-6.97-8.9
TalkingWith Maps
2010
Step 1: Geology polygons are intersected with the 1km GridStep 2: Centroids are created for each geological polygon within each 1km polygonStep 3: VB.Net Tool identifies the nearest 5 soil sites to the centroidfor the Parent material and superficial geology
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Rasterised image of the centroidsinterpolated within parent material boundaries
Sample points and centroids(Parent material method )
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Chromium in soils
New Parent Material mapping
Old method using IDW mapping.
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Parent Material soil chemistry maps: optimum number of samples for Parent Material mapping
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 5 10 15 20 25
Number of neighbouring samples
MSD
As AVCr AVNi AVPb AVAverage of As, Cr, Ni, Pb AVOverall average ALR-GALA
( )2
1
measured estimatedn1 MSD ∑
=
−=n
i
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Evaluation of Parent Material and IDW soil chemistry mapping using holdout validation
Element Spatial analyst IDW
Parent Material mapping (GM)
Parent Material mapping (IDW GM)
As 0.18 0.15 0.16Cr 0.43 0.31 0.31Pb 0.20 0.19 0.21
Mean squared deviation (MSD) for As, Cr and Pb for the three mapping methods based on combining the results for two 10% holdout subsets. Lowest MSD values for each element indicated in bold type. (Lne data).
( )2
1
measured estimatedn1 MSD ∑
=
−=n
i
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Chromium in soils – all NI
New regional Parent Material mapping.
Old version using IDW
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Nickel above Soil Guideline Values (SGV)Residential 130 mg/kg; Allotments 230 mg/kg
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Arsenic above Soil Guideline ValuesResidential 32 mg/kg; Allotments 43 mg/kg
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For more information
Contact GSNI: [email protected]
GeoIndex:http://maps.bgs.ac.uk/gsni_geoindex/