GIS Concepts and Analyses


The bulk of the analysis conducted for this project was directed at conducting a suitability analysis of wolf habitat. Suitability analysis is the process of defining relative suitability of various inputs, in our case land cover and densities of roads, prey, and human populations, and assigning them a unitless score in order to compare across factors.

Our analysis consisted of several stages of spatial and data analysis. Each spatial data set, whether raster or vector, required preparation before it could be compared to other layers.



Terminology and Underlying Concepts*


Layer: In ArcGIS, a reference to a data source, such as a shapefile, coverage, geodatabase feature class, or raster, that defines how the data should be symbolized on a map. Layers can also define additional properties, such as which features from the data source are included.

Projection: A method by which the curved surface of the earth is portrayed on a flat surface. This generally requires a systematic mathematical transformation of the earth's graticule of lines of longitude and latitude onto a plane.

Projection transformation: The mathematical conversion of a map from one projected coordinate system to another, generally used to integrate maps from two or more projected coordinate systems into a GIS.

Geodatabase: A database or file structure used primarily to store, query, and manipulate spatial data. Geodatabases store geometry, a spatial reference system, attributes, and behavioral rules for data. Various types of geographic datasets can be collected within a geodatabase, including feature classes, attribute tables, raster datasets, network datasets, topologies, and many others.

Raster: A spatial data model that defines space as an array of equally sized cells arranged in rows and columns, and composed of single or multiple bands. Each cell contains an attribute value and location coordinates. Unlike a vector structure, which stores coordinates explicitly, raster coordinates are contained in the ordering of the matrix. Groups of cells that share the same value represent the same type of geographic feature.

Vector: A coordinate-based data model that represents geographic features as points, lines, and polygons. Each point feature is represented as a single coordinate pair, while line and polygon features are represented as ordered lists of vertices. Attributes are associated with each vector feature, as opposed to a raster data model, which associates attributes with grid cells.



Data preparation stage


Reclassification: The process of taking input cell values and replacing them with new output cell values. Reclassification is often used to simplify or change the interpretation of raster data by changing a single value to a new value, or grouping ranges of values into single values—for example, assigning a value of 1 to cells that have values of 1 to 50, 2 to cells that range from 51 to 100, and so on.

Density: In ArcGIS Spatial Analyst, a function that distributes the quantity or magnitude of point or line observations over a unit of area to create a continuous raster—for example, population per square kilometer. In our project we used line density for the road density layer and we used point density for human population density. Prey density was not generated using Spatial Analyst.

Join: Appending the fields of one table to those of another through an attribute or field common to both tables. A join is usually used to attach more attributes to the attribute table of a geographic layer.

Clip: an operation that uses one input layer to change the feature geometry of a second layer. Works like a cookie cutter, and creates a new layer by clipping the first layer.

Majority: A technique for resampling raster data in which the value of each cell in an output is calculated. Majority resampling does not create any new cell values, so it is useful for resampling categorical or integer data, such as land use, soil, or forest type. Majority resampling acts as a type of low-pass filter for discrete data, generalizing the data and filtering out anomalous data values.

Merge: Combining features from multiple data sources of the same data type into a single, new dataset.



Analysis stage


Overlay: A spatial operation in which two or more maps or layers registered to a common coordinate system are superimposed, either digitally or on a transparent material, for the purpose of showing the relationships between features that occupy the same geographic space.

Raster Calculator: An ArcGIS Spatial Analyst tool for performing mathematical calculations with operators and functions, setting up selection queries, or typing Map Algebra syntax. Inputs to the Raster Calculator can be raster datasets, raster layers, coverages, shapefiles, tables, constants and numbers.

Map Algebra: A language that defines a syntax for combining map themes by applying mathematical operations and analytical functions to create new map themes. In this project, we used map calculator to multiply the suitability scores of each raster data set to those of the other raster datasets to derive overall suitability.

Error Propagation: In the context of this project, the persistence of an error into new datasets calculated or created using datasets that originally contained errors. The study of error propagation is concerned with the effects of combined and accumulated errors throughout a series of data processing operations. Data manipulation such as simplification and reclassification adds to the propagation of error in our project.



* All definitions were derived from ArcGIS Help and the ESRI ArcGIS Resource Center Glossary accessed at http://resources.arcgis.com/glossary on December 8, 2010