ArcGIS Raster Analysis and Spatial Analyst Study Guide

Raster Data Characteristics and Storage

Recap of Previous Concepts: The lecture builds upon previous discussions regarding raster characteristics and storage considerations.
Storage Trade-offs: * Compression techniques are used to make files smaller, saving hard drive space. * A disadvantage of compression is a slight increase in processing time due to the decompression required during file use.
Upcoming Topics: The discussion will cover the advantages of using vector versus raster models, surface models, and an introduction to the Spatial Analyst extension in ArcGIS.

Comparing Raster and Vector Data Models

Choice of Model: Modeling the real world requires selecting between vector or raster data. The choice depends on the specific use case, data display needs, or the type of analysis required.
Key Advantages of Raster Data: * Simple Data Structure: Raster data structures are significantly less complex than vector data. * Header Information: Essentially, only a header row is stored. This contains: * The real-world coordinates of the raster origin. * Information on the number of rows and columns. * The rest of the file simply holds values for each individual cell. * Implicit Geographic Location: The location of each cell is implied by its position within the cell matrix relative to the origin point. No additional geographic coordinates are stored for each cell. * Comparison to Vector Complexity: Vector data requires many coordinates and may include associated topological information (nodes and vertices), adding complexity to storage.
Analysis and Overlay Capabilities: * Map Algebra: The simple structure allows for easy overlaying. Multiple layers can be combined to create new layers using mathematical operations. * Mathematical Modeling: Raster maps (one value per cell per layer) are ideally suited for quantitative analysis and mathematical modeling. * Example Calculations: If a cell in layer A has a value of $1$ and the corresponding cell in layer B has a value of $2$ , the new output layer cell will have a value of $3$ . * Remote Sensing Integration: Rasters integrate seamlessly with satellite imagery, provided the user understands what the values in each layer represent. * Efficiency: Data analysis is generally quick and easy to perform due to the storage technique.
Disadvantages of Raster Data: * Data Redundancy: Can lead to large file sizes (though compression mitigates this). * Resolution and Accuracy: Spatial accuracy is determined by the size of the cell; the location is only as accurate as the resolution allows. * Topological Limitations: Raster models do not well support spatial analyses based on topological relationships. * Resolution Changes: Changing data resolution can introduce various processing problems.

Surface Models and Continuous Data

Matching Models to Phenomena: * Vector Models: Best for distinct, discrete landscape features such as rivers, roads, buildings, or political boundaries. * Raster Models: Best for continuous phenomena that occur across a landscape, such as elevation or temperature.
Examples of Raster-Friendly Data: * Digital Elevation Models (DEM): Grids containing spot elevations. * Derived Layers from DEMs: These include the angle and aspect of slope, terrain slope, and topographic moisture potential. * Climate Data: Examples include average annual precipitation and average annual mean temperature.
Topographic Moisture Potential: A conceptual model where ridges and upper slopes typically have less soil moisture than lower slopes or valley bottoms.
Land Cover Data: * Can be expressed in both vector and raster forms. * Often available as raster data because it is generated by classifying remotely sensed imagery. * Manual Delineation: In small project areas, users may manually trace land cover as vector polygons; however, this is labor-intensive compared to automated remote sensing software.

Photogrammetry and DEM Construction

Surfaces Defined: Geographic phenomena with values at every point across an extent (e.g., chemical concentrations, temperature, elevation).
Mount Rainier Case Study: A DEM of Mount Rainier in Washington State demonstrates elevation variation across the $14,000\,foot$ mountain. Light areas indicate high elevation (the peak), while dark areas indicate lower elevations (stream and river bottoms).
Applications of DEMs: * Calculating slope angle and aspect. * Determining drainage basins, watersheds, drainage networks, and channel locations. * Determining cost surfaces, travel times, pollution levels, and groundwater depth. * Path Optimization: Determining the optimal path from point A to point B (e.g., traveling over a mountain takes longer than going around it).
The US Geological Survey (USGS): Most DEMs in the US are produced by the USGS using Photogrammetry.
Photogrammetry Process: * Definition: The science of making measurements from photographs. * Method: An airplane with a mounted camera takes images at regular time intervals. * Overlap Requirements: * Approximately $60\%$ overlap along the flight lines. * Approximately $30\%$ overlap between flight lines. * Stereo Pairs: Two consecutive overlapping images form a "stereo pair." In the past, a photogrammetrist used a stereoscope to see terrain in 3D and used trigonometry to derive elevations and plot contour lines. * Computational Evolution: Today, the process is handled by high-speed computers, making it less labor-intensive.
Orthorectification: * Overlapping photos allow for the removal of distortions caused by varying ground elevations and camera tilt. * Controlled by identifying points with known real-world coordinates. * Digital Orthophoto Quadraquads (DOQQ): Resulting images that are orthorectified to represent the earth's surface accurately.

Spatial Interpolation and Drainage

Defining Interpolation: The process of predicting values for cells in a raster using a limited number of known sample points.
Use Cases: Can estimate elevation, rainfall, chemical concentrations, and noise levels.
Accuracy: The quality of the surface depends on the number and accuracy of the known data points. Having few points results in an inaccurate surface.
Drainage Modeling: A raster DEM can determine drainage patterns. Water is assumed to flow from a cell to the lowest of its eight neighboring cells.

Triangulated Irregular Networks (TIN)

History: Developed in the early 1970s as a model for elevation.
Structure: Uses irregularly spaced points connected by lines to form triangles. Each triangular "facet" is a tilted plane defined by the elevation values of its three nodes.
Point Distribution: Points are concentrated in areas of high relief (mountainous/hilly areas) and are sparse in flat areas.
Relief Defined: The difference in height or elevation between the high point and the low point on a landscape.
Advantages of TINs: * Captures horizontal linear features like ridge lines or stream courses effectively. * Stores $x$ , $y$ , and $z$ (elevation) coordinates for nodes and information on edges forming the facets.
ArcGIS Usage: TINs can be used to create contour lines, determine steepest paths, and perform line-of-sight analysis.
Disadvantages of TINs: * Gathering field data (via ground surveying or GPS) is time-consuming and labor-intensive. * Modern aerial/satellite methods for DEMs are more efficient. * Inaccuracy occurs if insufficient points are collected. * TIN analysis is less efficient compared to raster grid DEMs.

Contour Lines and Topographic Maps

Isolines/Isoplets: Generic names for lines on a map connecting points of equal value.
Contour Lines: Specifically refer to lines of equal elevation.
Rules of Contours: * Contour lines cannot cross each other. * The distance between lines is the contour interval. * Large intervals are used in mountainous areas (high relief); small intervals (e.g., $1$ or $2\,feet$ ) are used in flat coastal areas.
Reading Topo Maps: * Circular/Closed Polygons: Represent mountain peaks or hilltops. * V-shaped Bends: Lines bending downhill represent ridges; lines bending uphill (upstream) represent valleys or draws. * Shading: Green usually represents forest; white represents open areas.
Hill Shading: A method of representing relief by depicting shadows cast by high ground relative to a specific light source.

Introduction to Spatial Analyst Extension

Extension Overview: Spatial Analyst is a versatile extension used in ArcGIS Pro to manipulate and analyze raster data.
Licensing: * Requires a separate, expensive license for private entities, though educational institutions usually include it. * Check status in ArcGIS Pro via Settings > Licensing > Esri Extensions.
Capabilities: Tools are grouped in the ArcToolbox (e.g., Surface toolset, Interpolation tools).
Map Algebra: Features the Raster Calculator tool for writing expressions and functions to process layers.

Key Surface Analysis Tools

Hillshade Tool: * Creates shaded relief by simulating an illumination source. * Sun Location Defaults: * Azimuth: $350^\circ$ (Northwest, where North is $0^\circ$ and East is $90^\circ$ ). * Altitude: $45^\circ$ from the horizon. * Application: Enhances visualization when paired with elevation raster transparency. Useful for identifying areas in shadow (e.g., for building ski slopes to prevent snowmelt).
Contour Tool: * Located in the Surface Toolset. * Creates lines from a raster dataset. Can use a Z factor to convert units (e.g., meters to feet). If the Z factor is the default ( $1$ ), the output matches the input units. * To use with point data, points must first be interpolated into a raster.
Slope Tool: * Measures the steepness of incline (Rise over Run). * Two Expression Units: 1. Degree of Slope: Arc tangent of rise/run. 2. Percent Slope: $\frac{\text{Rise}}{\text{Run}} \times 100\%$ . * Examples: A $45^\circ$ angle equals a $100\%$ slope. A vertical rock face is a $90^\circ$ slope with a percent slope of infinity. * Method: Fits a plane to the $z$ values of a $3 \times 3$ cell neighborhood.
Aspect Tool: * Identifies the compass direction (downslope) that a slope faces. * Physical Effects: Influences temperature and moisture due to the angle of the sun in the Northern Hemisphere. * Temperature/Moisture Regimes: * South-facing slopes: Tilted toward the sun; warmer and drier (higher evapotranspiration). * North-facing slopes: Often shaded; cooler and moister. * Southwest slopes: Warmest and driest (sun is in the west during the hottest part of the day). * Measurement: Clockwise in degrees from $0$ to $360$ . Flat areas are assigned a value of $-1$ . * Use Cases: Finding optimal ski runs (north-facing), predicting snowmelt runoff (southerly slopes), or finding emergency landing zones (flat areas).
Viewshed Analysis: * Identifies geographic areas visible from a specific location (line-of-sight). * Excludes areas obstructed by terrain or the horizon. * Applications: National park scenic views, locating water towers, and real estate development. * Tools: ViewShed, Visibility, and Observer Points.