geog387_lab5

geog387_lab5 - Geography 387 Fall 2011 Lab 5 Data Model...

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Geography 387 – Fall 2011 Lab 5 Data Model Transformations 1 Lab 5: Data Model Transformations 1.0 Overview Although modern GIS systems can manage both raster and vector data, it is still often necessary to convert from one data model to the other. The primary reasons are that 1) the vector format stores discrete features more efficiently than does the raster format and 2) it is often necessary to have all data in one format for analytical operations. ArcGIS for Desktop provides simple-to-use tools for converting between data models. We will experiment with these tools in this lab. Along the way, we will also learn how to store symbology in a layer file, how to measure map distances with an ArcMap tool, and how to compute summary statistics of a numeric attribute. The overall steps of this lab are: Convert vector polygon, line, and point features to a raster data model Create a map comparing the results of rasterized features at different raster cell sizes Measure the offset distance of vector point features converted to raster Convert raster features to vector polygon, line, and point features Summarize and compare area of vector polygon features to area of the same polygon features that were rasterized at different raster cell sizes, then converted back to vector Summarize and compare length of vector line features to length of the same line features that were rasterized at different raster cell sizes, then converted back to vector ** Before you begin this lab, review the "General" section of the ArcGIS Tips and Tricks on the course webpage. Don't skip this step. ..it will ultimately save you time!
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Geography 387 – Fall 2011 Lab 5 Data Model Transformations 2 2.0 Rasterization “Rasterization” is the process of converting features in the vector data model to a raster data model. There are three possible types of conversions: Polygon features to raster: When polygons are converted, all grid cells lying within the vector polygon are assigned the attribute value of the polygon. Edges are assigned cell values by taking the vector feature that is most dominant. Line features to raster: Cells are given the value of the line that intersects each cell. Cells that are not intersected by a line are given a value of NoData. If more than one line is found in a cell, the cell is arbitrarily given the value of the first line it encounters when processing. Using a smaller cell size during conversion will alleviate this problem, but could considerably increase storage requirements. Point features to raster: When converting points, cells are given the value of the point found within each cell. Cells that do not contain a point are given the value of NoData. If more than one point lies within a cell, the cell is arbitrarily assigned the value of the first point encountered when processing. Using a smaller cell size will also alleviate this problem. The cell size you choose when converting from vector to raster should be based on several factors,
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This note was uploaded on 03/02/2012 for the course GEOG 387 taught by Professor Staff during the Fall '08 term at Sonoma.

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geog387_lab5 - Geography 387 Fall 2011 Lab 5 Data Model...

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