GIS Fundamentals: Digital Data, Basic Tables Lab 6 1 Lab 6: DEMs, National Wetlands Inventory and Basic Table Manipulations in ArcMap What You’ll Learn: This lab introduces some commonly available digital data sets, and introduces a number of rudimentary table operations. Data: Unless noted, data are in UTM Zone 15, NAD83, meters, coordinates, and the data files are found in the \L6 subdirectory, including: US_cities, a point layers of cities and towns, in NAD83 geographic coordinates, cen_Stx_U83, a subset of U.S. census bureau block group data, for a small area spanning the Minnesota/Wisconsin border, nlcd_border, a National Landcover data set, in an Alber’s conic projection, USGS_sheds, watershed boundaries at the hydrologic unit code level 8 (HUC-8), NED_lstx30, a National Elevation Dataset for the lower St. Croix HUC-8 watershed NHD_LStx_high.mdb, a National Hydrologic Dataset geodatabase for the lower St. Croix HUC-8 watershed Stil_wetU83, wetlands data for the Stillwater, Minnesota USGS 7.5 minute quadrangle What You’ll Produce: four maps: 1) a census data set, with subset cities from a US data set 2) shaded relief/hydrography map, 3) summary of wetlands by size, and 4) summary of wetlands by type. Background: This exercise introduces table queries. Most GIS store attribute data in tables. Each feature in a data layers is associated with a row in a table. A table query is a selection of rows (also called table records) based on a set of criteria. For example, you might select all polygons that are more than 100 hectares in size. We typically then further process the selected set, for example, assign values for a specific item, or export the records. You’ll be doing this sort of table manipulation in most of the remaining labs, so we’ll keep these first selections and subsequent operations rather simple. First, we’ll compose two maps including some of the wide range of data that are available for download from web sources. Next, we will introduce table operations using National Wetlands Inventory (NWI) data. We will select subsets of these data based on the size of the wetland polygons, and on wetland type (is it an upland or wetland, is it in Wisconsin or Minnesota), and produce corresponding maps. Digital Data Much data is available for download from the worldwide web, as described in Chapter 7 and appendix B of the GIS Fundamentals textbook. We will examine a few of these,GIS Fundamentals: Digital Data, Basic Tables Lab 6 2 and compose maps using downloaded data. Start ArcMap and add the raster data set named nlcd_border. (Video: Layers) Notice the table of contents lists a set of numbers for the NLCD dataset. Each code corresponds to a defined landcover type, e.g., 11 = open water, 43 = mixed forest. These data are available for the entire U.S. for approximately 1990, and 2000, although the numbering system changes slightly between dates. It is inefficient to re-enter a pleasing color scheme every time we load the data (i.e., blue water, green trees), and even more burdensome to retype the labels. Try the following: Open the symbology tab for the NLCD data set (remember, right click on the name in the TOC, Properties > Symbology). Make sure you have selected Unique values in the “Show” window to the left, and then, left click on the Import button in the Layer Properties window and the Import Symbology window that opens. Browse to the \L6 folder and left click on the nlcd.lyr file in the Browse window Then click Add to close the Browse OK to close the Import Symbology And Apply-OK in Layer Properties.GIS Fundamentals: Digital Data, Basic Tables Lab 6 3 Notice that this now applies a “standard” color scheme and lists the names for the NLCD data set? Any time you have the “standard” data set to display, you can apply a saved layer file. How do you create this layer file? By first producing in manually, that is, assigning the colors one at a time, and left clicking on each label and typing in your text. After you’ve applied a Symbology, you may save it by: Right clicking on a file name in the table of contents, and then left-clicking on Save As Layer File…. Next time you display a data set with the same categories, you can import this layer file via Symbology > Import, as described above. Census Data Add a new data frame, and delete the nlcd_boarder data frame. Add the census data, cen_StX_U83.shp to your new empty data frame. This is a subset of the census attributes for a portion of the Minnesota/Wisconsin border. Each polygon is a block group, a unit of aggregation for population census data. Open the attribute table (right click on cen_StX_U83 in TOC, then click Open Attribute Table), and inspect the values for the column labeled pden_psqkm. This is the population density for the census block groups in this area. Next, right click on the column name, and select Statistics (see right). This should display a column summary, and a histogram (see below).GIS Fundamentals: Digital Data, Basic Tables Lab 6 4GIS Fundamentals: Digital Data, Basic Tables Lab 6 5 Note that the data has quite a few small values, and a few very large values. This “long-tailed” distribution is common in some types of data, and often displays better with a non-uniform set of symbol ranges. We’ll demonstrate. Close the table, and open the Symbology Properties menu for the census data layer. Symbolize this as Quantities-Graduated Colors, and display the population density data (pden_psqkm) in the Value field, with no normalization. Keep the default number of classes, and select a gradient color ramp (in the example to the right, I selected the orange-purple-blue color ramp). Notice the rather useless symbology, in that the lines obscure the colors for the small polygons, and there is little range. (Video: MoreSymbols) We can get a more useful symbology by opening the Layer Properties menu (remember, right click the file name in TOC, then left click on Properties, then the Symbology tab). Specify a Quantities symbol scheme in the list of the leftmost panel Left click on the Classify button on the right, near the top, opening the classification window (shown at right) Set the Classification Method to Geometric Interval, with Classes to 20 Then OK – Apply – OK Notice this improves the colors somewhat, but the lines still obscure the small polygons.GIS Fundamentals: Digital Data, Basic Tables