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Raster Spatial Analysis and Terrain Analysis RESM 440 Monday-Wednesday October 11-13, 2010

2. Overview. Raster analysis: Answering spatial questions using cell-based datasets and modelingAnalysis with grid datasets:Converting to rasterClipping/resamplingReclassifyingDistance surfacesMap algebra and modelingTerrain analysis: Slope, aspect, contours, hillshadeViewshed. 3. Grid e

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Raster Spatial Analysis and Terrain Analysis RESM 440 Monday-Wednesday October 11-13, 2010

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    1. Raster Spatial Analysis and Terrain Analysis RESM 440 Monday-Wednesday October 11-13, 2010

    2. 2 Overview Raster analysis: Answering spatial questions using cell-based datasets and modeling Analysis with grid datasets: Converting to raster Clipping/resampling Reclassifying Distance surfaces Map algebra and modeling Terrain analysis: Slope, aspect, contours, hillshade Viewshed

    3. 3 Grid extent Properties of a Grid: Cell size: Measured in map units (meters), square Extent: Full bounding rectangle for entire Grid dataset

    4. 4 Converting to raster Vector datasets can be converted to raster and vice-versa: Can be useful in modeling

    5. 5 Clipping a Grid Clipping: Changing a grid’s extent and/or area of interest Mask: Your area of interest (can be any polygon) Extent: Rectangular bounding box for grid

    6. 6 Resampling a Grid Resampling = changing cell size Increasing cell size (larger cells = Grid is more generalized) Decreasing cell size (smaller cells = Grid is more specific)

    7. 7 Reclassifying a Grid Definition: Changing input cell values to new output cell values Purposes: Simplify your dataset Group your values Use output in modeling

    8. 8 Computing a distance surface Straight line distance may be computed continuously from each cell to any source Starting points/source: Any spatial dataset of interest (streams, roads, wells, mines, cities etc.)

    9. 9 Grid overlay: Map algebra Map algebra: Combining grid layers together mathematically Formula is applied cell by cell Purposes: Create new data Convert data (e.g. meters to feet) Perform analysis and answer questions Run models

    10. 10 Examples of map algebra

    11. 11 Examples of map algebra, continued NODATA cell value: Special cell value Used to indicate: No value at location No data at location (missing) Data outside study boundary NODATA is not zero! Can affect calculations in ArcGIS leading to blank cells

    12. 12 Map queries with grids Queries: Finding cells that meet your criteria (yes/no) Examples: Which areas are within 200 m of any stream? Which areas have forested land cover? Which areas have elevation over 1000 m? Answers shown as binary grids with 1 (yes) and 0 (no)

    13. 13 Examples of map queries Which areas have elevation greater than 1000m?

    14. 14 Practical example of raster based modeling Habitat analysis:

    15. 15 Examples of combining analyses Example question: Where are coniferous forested areas over 1000m in elevation?

    16. 16 Getting results from raster datasets Basic statistics: Properties of the grid dataset itself Min, max, mean value from entire grid Cell size and extent Summarizing grids: Summarizing zones Tabulating area by zones What are zones? Unique polygon areas for which you would like summary statistics (i.e. counties, watersheds, states)

    17. 17 Summarizing grids: Using zones Summarize by zones: Continuous grids may be summarized numerically Statistics include min, mean, max, variety, range, sum, std dev. Example: Mean elevation by county Example: Minimum elevation by county Tabulate by zones: Categorical grids may be summarized by area Area of land cover, by type, by watershed

    18. 18 Terrain/surface analysis Surface: Representation of spatial feature that varies continuously (usually elevation) Terrain analysis: Surface analysis based on elevation values Used in: Landscape architecture Ecological modeling Engineering/site design

    19. 19 Slope analysis Definition of slope: Rate of change of elevation Purpose of slope analysis: Grade Steepness of terrain Show limitations to buildings, other development Calculated as: Degree Percent

    20. 20 Aspect analysis Definition of aspect: Direction of maximum slope Example: South-facing aspect Aspect is important for: Vegetation/habitat Microclimate Site characteristics Measured in compass directions (degrees)

    21. 21 Contours Definition of contours: Lines connecting equal values Uses of contours: Common on topographic maps Site planning Show rate of change in elevation Show ridges, valleys

    22. 22 Hillshade Definition of a hillshade: Shaded relief model, using simulated light source and shadows Often used in layer display and presentations

    23. 23 Hillshades, continued Hillshades in layer display (use two layers and transparency property)

    24. 24 Viewshed analysis Definition of a viewshed: Area visible from a specified location or point Purpose of viewshed analysis: Useful in site selection or evaluation Modeling tool

    25. 25 Raster Analysis in ArcGIS Spatial Analyst extension Included in student evaluation software Performs all functions described here Spatial Analyst Toolbar ArcToolbox: Spatial Analyst Tools Working with Grids in ArcGIS Only use ArcCatalog to copy/paste/move/delete Grid datasets! Many results of Spatial Analyst operations are temporary grids until you make them permanent

    26. 26 Using Spatial Analyst in ArcGIS Load extension (Tools menu > Extensions) Load toolbar (View menu > Toolbars)

    27. 27 Review Raster analyses: Clipping, resampling, reclassifying grids Querying grids Terrain analyses: Slope, aspect, contour Hillshade, viewsheds Coming up: Lab 7: Raster analysis Lectures next week: Modeling, suitability analysis, advanced analysis

    28. Raster cell values Cell values represent geographic features Types of cell values: The Raster data model stores data in individual cells. The values in cells will vary depending on the type of data the raster represents. A binary raster may be composed of 0 and 1 values, and may be used to indicate presence and absence of some geographic feature (for example, roads or slope values greater than 10%). Integer values in raster cells are used in both continuous and categorical rasters. Integers are whole numbers with no decimal places. Integer based GRID datasets are generally smaller in file size than floating point GRIDs, even if the cell count is the same. Floating point values are values with decimal places. Floating point value GRIDs may be used to store exact values, such as precipitation values in hundredths of an inch, or very precise values. Floating point GRIDs take up more memory to store.The Raster data model stores data in individual cells. The values in cells will vary depending on the type of data the raster represents. A binary raster may be composed of 0 and 1 values, and may be used to indicate presence and absence of some geographic feature (for example, roads or slope values greater than 10%). Integer values in raster cells are used in both continuous and categorical rasters. Integers are whole numbers with no decimal places. Integer based GRID datasets are generally smaller in file size than floating point GRIDs, even if the cell count is the same. Floating point values are values with decimal places. Floating point value GRIDs may be used to store exact values, such as precipitation values in hundredths of an inch, or very precise values. Floating point GRIDs take up more memory to store.

    29. The Raster Calculator Use with Analysis mask, analysis extent to “clip” rasters Use to base new raster on existing raster Convert m to ft The Raster Calculator is an interface for performing queries and map algebra (addition, subtraction, etc) on raster layers in ArcGIS. The Raster Calculator is activated through the corresponding Spatial Analyst menu choice. Previously, we used the Raster Calculator to make a clipped copy of an input raster dataset (by setting a smaller output mask and/or extent). The Raster Calculator can also be used to query individual cell values and find cells that fit a user-specified query expression. Query expressions may be entered into the calculator’s expression builder window by using the list of layers and other buttons found on this dialog. Examples of queries include querying an elevation raster to find all cells with elevation greater than 1000 m. Note that raster layer names need to be enclosed in square brackets in query expressions. It is best if you use the interface provided (i.e. click on the And, Or, Not buttons) to ensure that your query expressions avoid the dreaded Syntax Error.The Raster Calculator is an interface for performing queries and map algebra (addition, subtraction, etc) on raster layers in ArcGIS. The Raster Calculator is activated through the corresponding Spatial Analyst menu choice. Previously, we used the Raster Calculator to make a clipped copy of an input raster dataset (by setting a smaller output mask and/or extent). The Raster Calculator can also be used to query individual cell values and find cells that fit a user-specified query expression. Query expressions may be entered into the calculator’s expression builder window by using the list of layers and other buttons found on this dialog. Examples of queries include querying an elevation raster to find all cells with elevation greater than 1000 m. Note that raster layer names need to be enclosed in square brackets in query expressions. It is best if you use the interface provided (i.e. click on the And, Or, Not buttons) to ensure that your query expressions avoid the dreaded Syntax Error.

    30. Map algebra Raster calculator can also perform map algebra (mathematical functions on raster layers) Examples: [Elevation] * 10 [Grid 1] * [Grid 2] Log [Grid 1] Many other ARC/INFO GRID functions – see help for details The Raster Calculator can also be used to perform mathematical operations on raster layers, known as Map Algebra. There are numerous mathematical and logical functions that can be performed on raster layers. For a complete reference, consult the ArcGIS online help. You can also run many ARC/INFO GRID functions (such as FOCALSUM, REGIONGROUP, ISNULL etc.) directly within ArcGIS by using the Raster Calculator.The Raster Calculator can also be used to perform mathematical operations on raster layers, known as Map Algebra. There are numerous mathematical and logical functions that can be performed on raster layers. For a complete reference, consult the ArcGIS online help. You can also run many ARC/INFO GRID functions (such as FOCALSUM, REGIONGROUP, ISNULL etc.) directly within ArcGIS by using the Raster Calculator.

    31. NODATA cells Special cell value Used to indicate: No value at location No data at location (missing) Data outside study boundary NODATA is not zero! Can affect calculations in Spatial Analyst NODATA is a special cell value used to indicate that while a cell is within a GRID’s extent, the cell has no value. This may occur for a variety of reasons. NODATA values are not the same as zero values, since zero is a valid cell value. NODATA is ignored when computing statistics. NODATA cells are very influential in raster analysis, and it is important that you realize the extent of NODATA values in your input raster datasets. There are many different ways to control how NODATA values may affect your raster output from different functions. See the ArcGIS Help and page 101 of the book Using ArcGIS Spatial Analyst for more details.NODATA is a special cell value used to indicate that while a cell is within a GRID’s extent, the cell has no value. This may occur for a variety of reasons. NODATA values are not the same as zero values, since zero is a valid cell value. NODATA is ignored when computing statistics. NODATA cells are very influential in raster analysis, and it is important that you realize the extent of NODATA values in your input raster datasets. There are many different ways to control how NODATA values may affect your raster output from different functions. See the ArcGIS Help and page 101 of the book Using ArcGIS Spatial Analyst for more details.

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