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GEOL 452/552 - GIS for Geoscientists I
Lecture 20 - Chapter 8 (Raster Analysis, part1)
• 4 lectures on rasters - but won’t cover everything
• (Raster GIS course: Geol 588: GIS II (Spring 2011)
• Today:
• Raster data, Spatial Analyst setup, Boolean raster analysis, raster calculator, reclassification
• Ch 8 Tut 1 - 18
• HW 12: Ch 8 ex. 1,2, 5, due Nov. 12
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• Elevation data (DEM)
• Visualize/analyze continuos data
• Topographic functions
• Boolean and Suitability analysis
• Distance (straight line and cost-of-travel) analysis
Some Raster GIS tasks
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• Identify the best (shortest)path between locations
• Statistical analysis within predetermined zones
• Interpolate data values for a study area based on samples
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• cells (pixels) in rows (vertical) and columns (horizontal)
• 2D matrix, 2D array
• resolution: cell width/height
• Extent (real world)
• “Local” coordinates (counting # of cells) vs. geo-location (UTM, lat/long)
Raster data
Local extent? Real-world extent?
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• Continuous Data:
• Surface (elevation, rainfall, pollution, ...)
• sampled at center of cell
• no discrete boundaries
• usually floating point value (can be integers)
• if floats: no attribute table
Smooth (Stretched) coloring (really: 256 colors)
Note: elevation values are “chopped off” here
(they are integers)...... but this really is
continuous data
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• discrete data (“classes”)
• value is mean as index only
• each cell filled with whole number
• stored as integer data
• Has “attribute table” (Usually: Value, Count)
• sometimes: also stores meaning (here Vegtype attribute)
• special case: binary grid (0,1 only)
Value VegType Count
1 Fir 6
... ... ...
6 Walnut 10
3 1 46 2 15 4 3
3 1 4413
4
3 1 4 4
124
1
1
30m
30m
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Spatial Analyst Setup• Spatial analyst (SA) = ArcGIS raster
module
• Copy follow along\Ch8A_class_ex and start ArcMap with Ch8A_class_ex.mxd
• We use Spatial Analyst as part of ArcMap(also: SA tools in ArcTools)
• 2 steps needed before you can work with Spatial Analyst in ArcMap
• 1) Activate module (GIS lab: needed every time) via Tools Extensions
• 2) Get Spatial analyst toolbar via View - Toolbars - Spatial Analyst
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Look at different types of raster data
• .img file = ERDAS IMAGINE format (special raster type “image”)
• vegetation: discrete data (index to vegetation type)
• Look at attribute layer:
• FORMATION = what type?COUNT = how many pixels per type?
• dem_float.img (elevation in feet)
• floating point, no attribute table
• symbolize as stretched (compare to Classified?)
• Stretched - Color ramp Elevation #1 (Right-click - Graphic View to switch between text and color ramp)
• precipitation.img: rainfall (in cm?)
• Note min/max of both floating point rasters
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Reclassify elevation and precipitation rasters into binary
• Binary raster: only possible values are 1 (True) or 0 (False) in each cell
• Encodes a condition: “is <something> True or False in this cell here?”
• Let’s make 2 new binary rasters: Spatial Analyst - Reclassify
• dem_float: if Value > 2300 set to 1 (otherwise: 0)
• Add Entry: 1600 - 2300 becomes 0 ; 2300 - 2600 becomes 1
• Output raster: high_elevation.img
• precipitation: Value < 30 set to 1 (otherwise: 0)
• Add Entry: 20 - 30 becomes 1 ; 30 - 35 becomes 1
• Output as low_precip.img
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Save as ERDAS .img file into your folder!
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1
0
0
1
1
1
1
1
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Raster overlay (AND)• Which cells have high elevation AND low rainfall?
• Use raster calculator to:
• AND both rasters
• - or -
• multiply both rasters
1 × 1 = 11 × 0 = 00 × 1 = 00 × 0 = 0
1 AND 1 = 11 AND 0 = 00 AND 1 = 00 AND 0 = 0
both conditions
are True here
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Raster calculator• Important:
• In the GIS lab do not use the C: or D: drive as Working directory for temp rasters (as they will be deleted there at logout!)
• Instead set your Working directory (folder) to your student folder (Spatial Analyst - Options)
• [high_elev.img] AND [low_precip.img]
• [high_elev.img] * [low_precip.img]
• Makes a new raster called Calculation (a temporary raster)
• Always make temp rasters (“Calculation”) permanent!
• R-click on raster, Data - Make Permanent, save in your folder as “both_conditions.img”
• Rename layer to both_cond in Table of content
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• Result: cells with 1 have high elevation (>2300 ft.) AND low rainfall (< 30 cm)
• What % of the total area is that?
• What area for each cell?
• Look at Properties - Source
• How many cells with VALUE=1?
• Look at COUNT in Attribute table
• here: 3908 cells of a total of 33174 (194 columns by 171 rows) = ~ 11%
result of AND
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Wrap up
• Lab: Ch 8 Tut 1 - 18
• Tutorial will prep for HW
• HW 12: Ch 8 ex. 1,2, 5, due Nov. 12
• (based on same data as used in ex_8a.mxd)
• Straight line distance function in: Spatial Analyst - Distance ...
• Remember to set your Working directory (SA - Options) to your folder!
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