Geog 280: Basic Geographic Techniques

Exercise 5: On-Screen Digitizing

Creating Your Own Data

If you can't find appropriate data for your GIS project, you may need to create it yourself.  The process of converting data to digital, computer format is called digitizing.  Currently three major approaches are available for digitizing:

1. Keyboard Entry:  here you manually type in data, either as raster cell values (using a spreadsheet or similar software) or as vector lines.  These days mostly only used to enter property parcel lines in a technique called coordinate geometry (COGO).
2. Graphics Tablet: also called a digitizing tablet.  This is what many people in the GIS industry think of if you say "digitizing."  Here a flat, rectangular surface senses and records the position of a specialized mouse-like cursor (or puck).  You can tape a paper map to the tablet and trace features so they become vector features in the GIS.  This used to be the most common way of converting a paper map to digital format.
3. On-Screen Digitizing:  also called "heads-up digitizing," since your head is looking up at the screen rather than down at a tablet.  Here the map is first scanned into a raster format.  Then a person traces features using a mouse while looking at the screen display of the scanned map.

On-screen digitizing is becoming popular for at least three reasons.

• the scanned map can be useful in and of itself -- as you saw in the previous page with DRG files.
• you can digitize the scanned image as you need to.  For instance, you may only need to have the streams in vector format at first and only as you have time do you digitize the rest of the map.
• software is available that automatically traces features as vector lines.  Typically you tell the computer where to start and it traces along a line until it finds a gap or intersection, at which point you must tell it what to do -- stop or jump to another line.  These automatic vectorizing programs aren't perfect yet, but they are useful for some types of maps.  We won't use them here, but we have two types of them in the GIS Lab if you want to experiment at some point (one is ArcScan, an Arc/Info utility; the other is Corel Trace, a simple bitmap tracing utility).

You will use on-screen digitizing to create a new theme in ArcView.  You will trace a few features on the scanned photo of the SSU campus.

Create a New Theme

To digitize some new information, you need to make a theme available to be edited (changed).  You could add data to an existing theme, but let's create a brand new theme to add features to.  Follow this procedure:

• Close the Available Databases view, and use the Project Window to open the Raster/Vector Data view.
• Choose View-New Theme from the menu.  ArcView opens a dialog box asking the type of data that the new theme will be -- point, line or polygon.
• In the drop-down list of the dialog box, choose Polygon and click OK. 
• Another dialog box opens for the theme name and directory.  Change if necessary the drive to U:, then change the directory to your personal folder.  Type Bldg2 as the file name and then click OK.

ArcView then adds your new theme to the legend of the view.  Click the check-box to draw the theme if it's not already checked (of course, nothing will draw until you digitize, as you'll do below).

Notice also that a dotted line is around the check-box of Bldg2.  This is an important signal -- it means that you can currently edit (change) features in the theme.  You can add, change or delete features while the dotted line is showing.  You should only have editing turned on while you are changing a theme, since you could inadvertently change something you didn't want changed.  You will next add some features to the theme, then turn the editing off.

Add Building Features to the Theme

You will digitize (trace) a few more of the buildings in the photo.

• If necessary, zoom out to see the whole photo with View-Full Extent, then use the zoom tool to zoom in to see just one of the buildings that isn't already digitized -- there are several on the west side of campus you can digitize.
• To digitize a feature, you must use the appropriate drawing tool.  Several tools are available for drawing points, lines or polygons.  To see them, click and hold down the left mouse button on the far right icon in the tool bar -- it probably looks like a single dot  at first.  If you hold down the mouse on it, you'll see icons of several types of features.  Slide down until the polygon tool  is highlighted, then release.  The toolbar icon should change to this polygon tool.
• Move the mouse cursor to one corner of a building and click once.  Move along the edge of the building to the next corner and click again.  Continue moving around the building and clicking on each corner -- try to keep the edges straight and accurate.  The polygon shape of the building will become apparent as you work.  When you get to the last corner before the one you started on, double-click the mouse.  This will complete the polygon and shade it in.
• If you find the resolution of the image too coarse (that is, the pixels are too big to be able to see the buildings clearly), you can add a larger version of the photo and use it instead -- if you're in the GIS Lab, that is.  The larger version is called SSU-big and it's in the folder H:\GeoData\Sonoma\SSU.  You would use View-Add Theme, change the file type to Image Data Source and add it from this folder.  Turn off the smaller photo theme to make the view draw faster.
• Repeat the digitizing step above for at least a total of four buildings.  You can zoom out a little to see another building, then zoom back in.  You can also right-click on the view and choose to "pan" over to find another building.  The buildings may turn yellow after you finish them rather than the color indicated in the legend.  Don't worry -- this just means they are selected.  You can choose the Select None button  to deselect any buildings.
• If you want to delete any of your buildings and redo it, just select it using the Select Feature tool -- the building should turn yellow after you click on it with this tool.  Then choose Edit-Delete Graphics to delete it.  Redo the feature using the procedure above
• When you're satisfied with your work, choose Theme-Stop Editing and choose to save your edits.

Export View

To show you've accomplished the tasks in this part, create a graphics file of your view.  Do this as you have done before, i.e.:

• Zoom the view out enough to see at least the buildings you digitized and at least one of each of the other vector themes you added (trees, roads and buildings).
• Choose File-Export from the ArcView menu.
• Click the Options button and change the Resolution (DPI) to 144. This will improve the appearance of your exported graphic. Then close the Options box to return to the main Export dialog.
• Make sure the Type box in the lower left is Windows Metafile (.wmf).
• Change the drive/directory to your personal folder if necessary (U:\username); if you're not in the GIS Lab, save it to a folder of your choice.
• Type Exer5 as the file name and then click OK.

That's It! (Just submit your assignment)

Submit in an e-mail message the answers to the four questions in the previous pages, and attach the graphics file you just created.  Please add Exercise 5 as the subject before you send your assignment.  Refer to tips on submitting assignments if you need help.

Bryan Baker, Sonoma State University, bryan.baker@sonoma.edu
Updated 17 February 1999