Download - Using ArcGIS to Study the Correlation between Lightning Strike Density and Terrain Elevation
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Using ArcGIS to Study the Correlation between
Lightning Strike Density and Terrain Elevation
Alex DeCaria and Michael Babij
Department of Earth Sciences
Millersville University
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Raster vs. Vector
Raster data
• Gridded data
• Attributes are assumed valid over entire cell
• ArcGIS stores in GRID files
Vector data
• Uses points, lines, and polygons
• Attributes are assumed valid only on the given shape
• ArcGIS stores in shape files
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Raster vs. Vector
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Raster vs. Vector
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Example of Attribute Table
Point ID Temperature Pressure
1 262 19
2 188 15
3 267 12
4 278 12
5 280 18
6 238 12
7 253 21
8 265 23
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Lightning Data
• Lightning strike positions for 1995-2001 acquired from the National Lightning Detection NetworkTM (NLDN)
– NLDN detects only cloud-to-ground lightning
– Positions are lat/lon on WGS84 datum
– Positions are accurate to within 1 km
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Formatting Data
• Data came as ASCII file with each record representing a single lightning strike.– Lat/lon– Time– Signal (+ or )
• Wrote Fortran90 program to parse data into separate years and specific location.
• Data files then read into Microsoft Access database program and converted into ArcGIS shape files (one for each calendar year)
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Lightning Strike Positions for 2001
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Calculating Density
• First converted data from lat/lon coordinates to meters easting and northing
• Spatial Analyst then used to calculate density.
– The density calculation creates a raster from the vector (point) lightning locations
– Need to specify search radius and cell size.
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How ArcGIS* Calculates Density
2rcount r = search radiusd = cell size
* Need Spatial Analyst extension
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Effect of Search Radius on Density Calculations
Radius = cell size Radius >> cell size
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Calculating Density
• First converted data from lat/lon coordinates to meters easting and northing
• Spatial Analyst then used to calculate density.
– The density calculation creates a raster from the vector (point) lightning locations
– Need to specify search radius and cell size.
– Need to specify “simple” or “kernal” for calculations
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“Simple” vs. “Kernal” for Density Calculations
Simple Kernal
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Calculating Density (cont.)
• Density data are adjusted based on estimated detection efficiencies– Efficiency was 61% in 1995 and increased to
100% in 1998 and later
• Adjusted yearly densities were averaged to calculate annual average flash density map
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Annual Average Strike Density 1995-2001
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Elevation Data
• Used USGS National Elevation Data set (~30-meter horizontal grid).
• Converted to 1-km horizontal grid using Raster Calculator feature of Spatial Analyst extension
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Low-resolution Terrain Elevation
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Joining Density and Elevation Data
• Used Raster Calculator to re-map density and elevation data sets onto their shared domain.
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Comparision of Strike Density and Terrain Elevation
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Creating Joined Elevation/Density Data File
• Converted both the elevation and density data sets into vector (point) format using the “convert” feature of Spatial Analyst.
– ArcGIS only converts integer rasters to vector form.
– Used Raster Calculator to convert real raster to integer raster
– Needed to multiply density by 10 to preserve information in decimal place.
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• Joined the two vector data sets using the “Join” feature of Spatial Analyst.
• This creates a joined “attribute table” that contains both density and elevation information at each point.
Creating Joined Elevation/Density Data File (cont.)
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Statistical Analysis
• Exported joined attribute table to ASCII file and then imported it into a Microsoft Excel worksheet.
• Used Microsoft Excel to create scatter plot and perform linear regression.
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Conclusions
• There exists a weak, yet statistically significant negative correlation between terrain elevation and annual average lightning strike density for SE Pennsylvania.
• Suggests that orography is not the dominant factor in thunderstorm formation in this region.– Consistent with Orville and Huffines (2001)*
• Next step: Analyze data over Rio Grande Valley in New Mexico, where there appears to be a positive correlation between flash density and elevation.
* Orville, R.E. and G.R. Huffines, 2001: “Cloud-to-ground lightning in the United States: NLDN results in the first decade, 1989-98, Mon. Wea. Rev., 129, pp. 1179-1193
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Acknowledgements
• Ken Cummins, Väisälä-GAI, Inc.– Provided lightning position data free of charge.
• Millersville University Faculty Grants Committee– Funded student research hours.
• Tom Whitfield, PA Geological and Topographic Survey– Provided topographic data and advice.