Massachusetts Institute of Technology Department of Urban Studies and Planning 11 520 A Workshop on Geographic Information Systems 11 188 Urban Planning and Social Science Laboratory Lab Exercise 7 Raster Spatial Analysis Distributed Lab 7 Due Lab 8 Overview The purpose of this lab exercise is to introduce spatial analysis methods using raster models of geospatial phenomena Thus far we have represented spatial phenomena as discrete features modeled in the GIS as points lines or polygons i e so called vector models of geospatial features Sometimes it is useful to think of spatial phenomena as fields such as temperature wind velocity or elevation The spatial variation of these fields can be modeled in various ways including contour lines and raster grid cells In this lab exercise we shall focus on raster models and examine ArcGIS s Spatial Analyst extension We shall use raster models to create a housing value surface for Cambridge A housing value surface for Cambridge would show the high and low value neighborhoods much like an elevation map shows height To create the surface we will explore ArcGIS s tools for converting vector data sets into raster data sets in particular we will rasterize the 1989 housing sales data for Cambridge and the 1990 Census data for Cambridge block groups The block group census data and the sales data contain relevant information about housing values but the block group data may be too coarse and the sales data may be too sparse One way to generate a smoother housing value surface is to interpolate the housing value at any particular location based on some combination of values observed for proximate housing sales or block groups To experiment with such methods we will use a so called raster data model and some of the ArcGIS Spatial Analyst s capabilities The computation needed to do such interpolations involve lots of proximity dependent calculations that are much easier using a so called raster data model instead of the vector model that we have been using Thus far we have represented spatial features such as Cambridge block group polygons by the sequence of boundary points that need to be connected to enclose the border of each spatial object for example the contiguous collection of city blocks that make up each Census block group A raster model would overlay a grid of fixed cell size over all of Cambridge and then assign a numeric value such as the block group median housing value to each grid cell depending upon say which block group contained the center of the grid cell Depending upon the grid cell size that is chosen such a raster model can be convenient but coarse grained with jagged boundaries or fine grained but overwhelming in the number of cells that must be encoded In this exercise we only have time for a few of the many types of spatial analyses that are possible using rasterize data sets Remember that our immediate goal is to use the cmbbgrp and sales89 data to generate a housing value surface for the city of Cambridge We ll do this by rasterizing the block group and sales data and then taking advantage of the regular grid structure in the raster model so that we can easily do the computations that let us smooth out and interpolate the housing values I Setting Up Your Work Environment 1 Launch the ArcGIS and add five data layers listed below M data cam lu99 shp Cambridge land use in 1999 per MassGIS M data cambbgrp point shp Census 1990 block group centroids for Cambridge M data cambbgrp shp Census 1990 block group polygons for Cambridge M data cambtigr coverage U S Census 1990 TIGER file for Cambridge M data sales89 Cambridge Housing Sales Data M data camborder Cambridge polygon 2 Set Display unit meter In this exercise you will use Meter instead of using Mile II Spatial Analyst Setup ArcGIS s raster manipulation tools are bundled with its Spatial Analyst extension It s a big bundle so lets open ArcGIS s help system first to find out more about the tools Open the ArcGIS help page by clicking Help ArcGIS Desktop help from the menu bar Click the index tab and type Spatial analyst During the exercise you ll find these online help pages helpful in clarifying the choices and reasoning behind a number of the steps that we will explore Be sure at some point to take a look at the Overview section The Spatial Analyst module is an extension so it must be loaded into ArcGIS separately To load the Spatial Analyst extension o o o Click the Tools menu Click Extensions and check Spatial Analyst Click Close Fig 1 Add Extension Although you just activated the Spatial Analyst extension you have to add the Spatial Analyst tool bar on the menu manually to use the extension quite inconvenient To add Spatial Analyst tool bar go to View Tool bars from the menu bar and click Spatial Analyst Fig 2 Add Spatial Analyst toolbar Once the Spatial Analyst tool bar loaded a new main menu heading called Spatial Analysis will be available whenever you launch the ArcGIS Setting Analysis Properties Before building and using raster data sets we should set the grid cell sizes the spatial extent of our grid and the no data regions that we wish to mask off Let s begin by specifying a grid cell size of 100 meters and an analysis extent covering all of Cambridge To do this click Spatial Analyst Option When the Options window pops up In the General tab set your working directory select None for the Analysis mask We will set the mask later and select the first option for the Analysis Coordinate System o In the Extent tab select Same as Layer camborder polygon for the Analyst extent o In the Cell Size tab select As Specified Below then specify Cell size 100 Number of rows 57 Number of columns 79 o Number of rows and Number of columns will be automatically computed Now that we ve set the analysis properties we are ready to cut up Cambridge into 100 meter raster grid cells Convert the camborder to a grid layer using these steps and parameter settings o Select Spatial Analyst Convert Feature to Raster Features to Raster window will show up Choose camborder polygon for the Input features Choose COUNTY for the Field We just want a single value entered into every grid cell at this point Using the County field will do this since it is the same across Cambridge Output cell size should be 100 Set the saving location your working directory and the name of the grid file cambordergd and click OK If successful the CAMBORDERGD layer will be added to the data frame window Turn it on and notice