Lab 6: Raster Modeling with ArcView's Spatial Analyst Overview The purpose of this lab exercise is to introduce spatial analysis methods using raster models of geospatial phenomena. Thus far this semester 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 varous ways including contour lines and raster grid cells. In this lab exercise, we shall focus on raster models and examine ArcView's 'Spatial Analyst' extension. We shall use raster models to create a housing value 'surface' for a portion of Boston. To create the 'surface' we will explore ArcView's tools for converting vector data sets into raster data sets—in particular, we will 'rasterize' the 1986-1995 housing sales data for Boston and the 1990 Census data for Boston block groups. (For those of you who took 11.520 in the Fall, this lab is very similar to what we did to interpolate housing values for Cambridge. The main difference is that, this time, the sales data are in Oracle and they involve multiple-sale and other complications that require data manipulation before being pulled into ArcView.) A housing value 'surface' for Boston would show the high and low value neighborhoods much like an elevation map shows height. 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 Boston 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 Boston and then assign a numeric value (such as the block group median housing value or the block group ID) 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 rasterized datasets. Remember that our immediate goal is to use the Boston sales data to generate a housing-value 'surface' for Boston. We'll do this by 'rasterizing' the 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 reported in the housing sales datasets.(1) Spatial Analyst Setup Start with a brand new ArcView project, create a new view, and add several Boston themes (stored in the /mit/crlmaps/meters83/massmaps/boston directory) as indicated below. If you have a CRLspace directory, you should also set your working directory to be a subdirectory of your CRL space. Hence, you should: • Activate the Project window • Select 'Properties' from the Project menu • Set the working directory to /mit/crlspace1/YourAthenaID/lab6 (Note: you may need to 'attach' the locker and/or create the sub-directory first.) • Activate a View window or open a new one • Add the following themes to the view (all from the /mit/crlmaps/meters83/massmaps/boston directory which, on the PC is also know as: \\crypt\crlmaps\meters83\massmaps\boston). Note that some of these are shapefiles and some are ArcInfo 'coverages' where you can click the coverage icon in the dialogue box to get the expanded list of coverage types to be sure that you load the 'polygon' layer. o bostnbrd (polygon) o msa_town (polygon) o msa_water.shp o wardbound (polygon) • Also add a polygon and a point theme for Boston's Ward 12 parcels and Boston's 1990 Census block-groups: o /mit/crlclass/11.521/data/boston_bg (polygon) o /mit/crlclass/11.521/data/ward12bnd.shp • Order the themes, top to bottom, as follows: o Ward12bnd.shp o Msa_water.shp o Wardbound o Bostnbrd o Boston_bg o Msa_town Highlight the Wardbound theme and press the 'Zoom to Active Theme(s)' button so that the map shows all of Boston. Adjust the colors and transparency of their symbols so that the ocean is blue with no border, the town boundaries around Boston are visible and you can see Boston's Ward 12 as the area from Franklin part up through Lower Roxbury to Dudley Square. Create a thematic map of median housing value by shading the Boston_bg theme based on the 1990 medhhval values. (See the data dictionary for thislayer on the MassGIS site for the meaning of all the variables in the Boston_bg attribute table—but you will only need medhhval). Be sure to exclude block groups with medhhval = 0 by setting '0' to be the null value. At this point, your ArcView window should look something like this: Setting 'Spatial Analyst' Properties ArcView's raster manipulation tools are bundled with its Spatial Analyst extension. It’s a big bundle so lets open Arcview's help system first to find out more about the tools. Follow this sequence of clicks to find the relevant spatial analyst topics: Help Topics > Contents > Extensions > 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 and performing analysis sections. The Spatial Analyst module is an extension, so it must be loaded into ArcView separately. (Note: this is because raster modeling capabilities do not come standard with ArcView. The Spatial Analyst extension must be purchased separately, and actually costs more than ArcView alone.) • Select 'Extensions' from the 'File' menu • Checkmark 'Spatial Analyst' and OK the selection (Note: if you only highlight 'Spatial Analyst' without checking the box, then clicking 'OK' will not load the extension.)Once the Spatial Analyst extension is loaded, a new main-menu heading called Analysis will be available whenever a View window is active. Make sure that you go to View > Properties and set the map units