UNIT 3 – MODULE 6: Data Analysis

TERMINOLOGY There are several terms that are important to know when discussing data analysis: – Entity – an individual point, line or area in a GIS database. – Attribute – data about an entity. – Feature – an object in the real world to be encoded into a GIS database. – Data Layer – a data set for the area of interest in a GIS. – Image – a data layer in a raster GIS. – Cell – an individual pixel in a raster image.

MEASUREMENTS IN GIS A standard component to a GIS is the ability to calculate lengths, perimeters & areas. All GIS measurements are an approximation. There are three raster GIS measurements available for calculating distance: – Pythagorean distance – Manhattan distance – Proximity For raster data, perimeter & area calculations can be impacted by: 1) cell size, 2) origin, and 3) grid orientation.

MEASUREMENTS IN GIS (Continued) For a vector GIS, distances are measured via the Pythagorean theorem. To calculate perimeters & areas, geometry is used. Credit: NASA

QUERIES A way to retrieve specific data within a GIS database. Allows you to ask questions by selecting a specific criteria. A GIS database then highlights the data that fits your query. Queries are useful throughout the GIS project, particularly for checking data quality. There are two types of queries: – Spatial – Aspatial

ASPATIAL QUERIES Questions about a features’ attributes. Example #1: “How many luxury homes are there?” This is an aspatial query because location is absent from the question. Example #2: “Where are the luxury homes?” This is a spatial query because location (where) is being asked. Spatial and aspatial queries can be asked in conjunction with one-another. Example: “Where are the luxury homes that have more than five bedrooms?”

RECLASSIFICATION As previously discussed, a raster GIS involves a collection of cells that have values assigned. Values can be reclassified. This can result in a new image if working with a raster land use image. It could also be used to assign new values to different land uses based on ecological importance. It all depends on what you’re trying to accomplish/show. Reclassification allows for a greater analysis beyond the initial analysis.

EXAMPLE Top-left image shows four different land covers. Top-right image shows two different land covers after reclassification. Bottom image shows reclassification of thematic values (leads to a new image). Credit: Credit:

BUFFERING A zone around an object or map feature. Measured in distance, but can also be measured in time. Very useful for proximity analysis. Credit: *

VECTOR & RASTER BUFFER Credit:

MAP OVERLAY Allows the GIS user to analyze two or more different data sources. Has many applications. Example: which hotels are located 500 meters or less of the main road? Can apply a buffer zone, then overlay hotel data. Two types of map overlay: vector & raster. Credit: Penn State University

VECTOR OVERLAY Also called a polygon overlay. Relies heavily on geometry & topology. There are three types of vector overlay:  Point-In-Polygon  Line-In-Polygon  Polygon-On-Polygon Credit: Penn State University

TYPES OF VECTOR OVERLAY Credit: ESRI

RASTER OVERLAY Also called a grid overlay. Everything represented by cells:  Point – Single Cell  Line – String of Cells  Area – Group of Cells With raster overlay, layers can be added, subtracted, multiplied or divided. Two issues need to be considered during this process: resolution & measurements scales. Credit: Penn State University

NETWORK ANALYSIS Network – a set of interconnected lines making up a set of features through which resources can flow. Examples: rivers, road, pipelines & cables. Different network analysis methods available to solve problems: – Shortest Path Method – Traveling Salesperson – Location-Allocation Modeling – Route Tracing Credit:

ROUTE TRACING The ability to trace flows of goods, people, services & information through a network. One of several methods for conducting network analysis. Can be useful for finding residents impacted by a broken cable, or customers served by a specific sewer main. Credit: ESRI