Intro to Atlas GIS 2.0
Mackenzie, Tanjuakio and Sparco
Intro to Atlas GIS 2.0
A thematic (or "choropleth") map conveys information about specific characteristics of geographic features via different colors, line styles, hatch patterns, etc. This chapter covers the thematic mapping capabilities of AtlasGIS.
Thematic maps are developed using three work screens under the Thematic menu. First, in the Thematic-Variable screen, you identify one or perhaps two geographic layers which will be assigned thematic coloring or styles, the database expression defining the attribute or function of attributes by which the layer will be mapped, and the general type of thematic map you want. Next, you complete a Thematic-Settings screen for the thematic layer, specifying value ranges and display styles; a second Thematic-Settings screen controls ranges and display of a second thematic layer, if any. Finally (and optionally), you adjust the look of the thematic legend(s) with the Thematic-Legend screen(s).
For example, suppose you want a map of population densities by county in the Northeast U.S. Activate your Northeast U.S. geographic file (File-Geographic-Use) and its associated county attribute file (File-Attribute-Use). Now access the Thematic-Variable work screen and fill in the Thematic Variable Settings window entries. Use the down-arrow key to highlight the "Layer" entry field under "Variable 1:" Hit SPACEBAR to see the pick-list of geographic layers, and select "Counties." (You will be mapping population density by county.)
Now move (down-arrow) to the Data Expression field, and hit the spacebar once to edit the field, then F2 or spacebar again to see a pick-list of attribute fields. (Note that The internal attributes of the active geographic file have field names beginning with underscores; field names from the active attribute file don't.) Select POP90 from the pick-list. Type in the /, then hit F2 to see the pick list of field names again and select _AREA. The final expression should read "POP90/_AREA" (if you type this in directly, don't forget the underscore: _AREA is an embedded attribute field in the geographic file). Leave Thematic Type as "Ranged Fill" for now. Hit the F10 (<<Done>>) key to go to the Thematic-Settings window.
After AtlasGIS has calculated data expression values for each feature in the specified thematic layer, the right side of the Thematic-Settings window will show summary statistics including value ranges, the number of geographic features with missing values, etc. On the left side of the Range Settings window you select the Ranging Method (default is quantiles), number of ranges (default is 4) and range of values to be mapped (default is the entire range of values). You can come back to this menu later to change any of these. Hit the F10 key to return to the map; then View-Redraw to repaint it.
Atlas's default colors aren't very good for range-fill thematic maps with more than 4 ranges. You can return to the Range Settings window via Thematic-Settings-1 and edit the Range Styles. The "Edit Range Styles" window lets you adjust fill style, percent fill, color and background for each range. You can choose a cool-to-hot color sequence, faint-to-solid fill percents or whatever to better convey the relative population densities in your map.
While AtlasGIS has a somewhat limited set of colors to work with, you can mix background and foreground colors in the "Edit Range Styles" screen within the Thematic-Settings window to achieve virtually any color you want. For example, you might display one range as orange by overlaying red with a 40% fill on a yellow background with a 100% fill. This will generally print as a uniform color, even if your monitor doesn't show it as uniform.
Familiarize yourself with the different
In creating ranged-fill thematic maps of region features, you can set the region line style to "None" under the Display-Layer-Settings so that region boundaries are merely indicated by color contrasts.
Thematic maps of line or point features can be Ranged, showing each feature's data category by style or color or Proportional, showing each features data value by line width or symbol size. (You can't "fill" lines or points.) You can change the standard line widths and styles of line features, or symbols and symbol sizes of point features under Display-Layer-Settings.
You can use any of 9 methods for determining data ranges in ranged maps: quantiles contain equal numbers of observations; equal size ranges have equal-sized increments; std. dev. divides the data into standard deviations around the mean; optimal uses a data-clustering algorithm; counts lets you specify numbers of observations per range; percentages lets you specify percent of observations per cell; continuous lets you specify maximum values for each range; discontinuous lets you specify both minimum and maximum values for each range; list of values lets you specify exact values as ranges (best for character data).
An easy way to get a reasonable distribution of features into ranges with nice round boundary values is to use a quantile range method first, then switch to a continous range method and round off the boundary values.
Unless you will be printing to a color hardcopy device, you should consider how your thematic map will look in black-and-white. Light colors like yellow will print as lighter grays than dark colors. You can work with differing shadings (percent fills), line widths, etc. just in black if you want.
Thematic-Legend lets you specify the legend title, type sizes, fonts, colors, etc. in the variable legend for your thematic map.
You can create a thematic map of 2 variables in the same layer or even in different layers simultaneously. "Two--Same Layer" lets you specify which variables control fill or symbol style, fill percent or symbol size, and color; the second variable can only be ranged or proportional based on the first variable. "Two--Different Layers" option is more flexible (and works fine even when your two variables are actually in the same layer).
In the Thematic-Variable control window, specify a 2-variable map, identify the variables and display method; then fill out a settings window for each variable. You can edit these with Thematic-Settings-[1/2]. Note that you will have to use Display-Page-Settings to turn the second variable's thematic legend on; the first variable's thematic legend is on by default. Your map will now have two variable legends as well as the geographic legend. Use Display-Page-[Move/Resize] to position page elements as desired; and Thematic-Legend-[1/2] to change legend layouts, title text, fonts, etc.
Making visually intelligible 2-variable thematic maps requires creativity and experimentation. AtlasGIS supports far too many options and tools to review fully here, but you are encouraged to try these out on your own later on.
There is a natural temptation to try and pack as much information into a map as possible, but be careful not to overwhelm your maps. Don't get carried away by the glitzy tools: keep things as simple as possible. Get colleagues to critique your maps: their comments will let you know if your map effectively conveys the information you want it to or not.
Load your Northeast counties geographic file and associate demographic attribute file.
1. Construct a ranged-fill thematic map showing population density (county population divided by county area) by county in a cool-to-hot color sequence. Make the thematic legend display integers only. Maximize the size of the map area and move and resize the other map elements around it. Add an appropriate title. Change all fonts to Times Roman. When you get the map exactly the way you like it, you can save the mapfile so you can access everything for printing hardcopy later on.
2. Now try a dot-density map of population, experimenting with different values (numbers of people) per dot.
3. Now try creating a thematic map showing both population density and percent elderly (age 65 or older), by county. Try out various options: e.g., displaying population as dot-density; percent elderly as a color range. Try showing both variables in proportional symbols, one as an open circle, the other as a dot (filled circle) inside it.
Printing maps created by AtlasGIS is generally pretty straightforward. The Print-Page command brings up a list of installed device drivers. (If the driver you want isn't listed, use Configure-Output-Add to install it, and Configure-Output-Edit to change its defaults.) After you select the appropriate driver, AtlasGIS displays its default configuration; you can change resolution, paper size, orientation, margins, etc. as needed. When you are <<Done>> with this screen, confirm the index master colors (hit "Enter"), and the map will print. (Note: printing complex maps can take quite a few minutes.)
Before you print a map, make sure that its Display-Page-Pagesize settings are consistent with the settings in the active printer driver. Also, if you are creating large-size hardcopy, consider how your titles, legends, annotations and other page elements will look. You may have to scale down font sizes, line widths, etc. in order to get decent-looking output.
The only hardcopy devices actually located in the Spatial Analysis Lab are an HP4 black-and-white laser printer available to all machines, and an older 6-pen color plotter (which is always running out of pens) connected to one machine. Fancier color hardcopy can be output at Smith Hall via the campus network. To obtain fancier color hardcopy, select the generic Postscript driver, change the default configurations (page size and margins, etc.) as needed, and print the map to a file on your disk. Now you can ftp the file to the central UNIX system (see the Appendix at the back of this manual for more information on how to use ftp) and print it from there.
CNS has two color hardcopy devices, both handling Postscript format only. The QMS color laser printer produces standard 8.5 x 11 output at 300 dots per inch resolution; its name is smicolps, and it is accessed via the printcolor command at the UNIX prompt, e.g.:
strauss% printcolor mymap.cps
Note that you will have to pay 50 cents per page for color hardcopy output from the QMS printer.
The other CNS color hardcopy device is a new HP 650 DesignJet printer which produces large-scale output (up to E-paper size) at up to 400 dpi resolution. Its name is smicoljet, and it is accessed via the qpr at the UNIX prompt, e.g.:
strauss% qpr -q smicoljet mymap.cps
While there is presently no charge for using this machine, keep in mind that screw-ups can waste a lot of expensive paper and ink. Before you try using this machine, call the CNS I/O desk for advice. They are likely to have you direct your print job to a special queue so that they can make necessary adjustments on the machine console so that your job prints correctly. This machine should only be used for final output.
Another option is to export your map as a Computer Graphics Metafile (.CGM) which you can then incorporate into a document or presentation graphic using almost any word-processor, slide-maker or presentation manager package.
A GIS provides dynamic spatial analysis capabilities; hardcopy provides only static views of such analyses. Hardcopy does have several advantages, however. First, it is portable (although laptop computers are now making GIS's themselves almost as portable). Second, lay people are more familiar and comfortable with paper maps than with computer screens.
As you get more involved with GIS work, generating eye-popping hardcopy becomes less and less important. Hardcopy does make nice wallpaper as long as you have wall space to fill. It is also useful for impressing people who control your funding but know little or nothing about GIS.
While color maps are visually appealing, you should avoid making maps which depend on color to convey their information. Since color copying is expensive, you can expect that people will be likelier to make black-and-white xerox copies of your maps. Will they still convey the information you want them to convey?
Load the mapfile containing your thematic map of Northeast population density by county.
1. To print a color version of your map, print a color Postscript file to your disk. Then ftp (see appendix) a copy of this file up to your account on Strauss and print it to the QMS color laser printer with the printcolor command. If you want to get an idea what this map will look like before you print it, log in to Strauss on one of the X-terminals and view it with ghostscript. Ghostscript may not replicate your true map colors very well, but it will reassure you about map orientation, size and feature scaling.
2. To print to the black-and-white laser printer in the lab, change the color scheme in the thematic range styles setting to ascending light shades of gray (e.g., 0, 4, 8, 15, 25, 40 and 70 percent black). Try to keep your county and state boundaries clearly visible. Other color map elements will print as black or dark gray.