EXERCISE 9– DIGITIZING:
DOING IT YOUR OWN WAY

Sometimes the data layer you need isn't already available and it's up to you to create it. In this lab, you'll learn how to take a base map such as an aerial photograph (preferably a digital orthophotograph that has been corrected for spatial distortion) and make a new geodatabase using this photograph as a backdrop. The process of creating data in this manner is called heads-up digitizing, and it's a whole lot better than the old way of digitizing using a digitizing tablet.

Why a geodatabase? Well, we want to create a layer that has topology, so shapefiles are out and the current version of ArcGIS doesn't allow us to edit coverages in ArcMap. Geodatabases are the future of ArcGIS, so gaining a little (and we do mean little) experience with them is a good idea.

Creating a new geodatabase:
We are going to digitize the roads in a new housing development in Hopewell Township, Mercer County, NJ. First, create a new personal geodatabase using ArcCatalog (be sure it is in your folder) and call it hopewell. Geodatabases are complicated structures, so to say that we are just scratching the surface is a gross understatement. What we've just created is a container that can hold any number of related data sets. Right-click on your new geodatabase. Notice under the New menu we can create a number of different elements. We're going to create one feature data set that will eventually contain two feature classes. Feature data sets are used to hold feature classes that have the same spatial reference information (extent, coordinate system, etc.). A feature data set can contain feature classes of different geometries (i.e. polygons, points and lines). The only thing those feature classes must have in common is that they share the same spatial reference information.

Create a new feature data set in the hopewell geodatabase, called update, and import a coordinate system from Y:/intgeo/rowan/arc8class/hopewell/hopegeol. Now create a new feature class in update. Name it newroads, then hit the Next button twice. This brings you to the window where you can add fields and set the geometry (point, line polygon) of the feature class. Click on field under Data Type that says Geometry. Notice the bottom half of the window changes. Click on Polygon in Geometry Type and select Line from the drop down menu. This will allow us to create line data in this feature class. Now go back to the table in the top half of the window and click on the empty record below Shape. Here you can add fields to the feature class. Add a field called Name. Make it type Text with a width of 30. Add another field RoadType (such as street, blvd or lane) with widths of 10. What do you think the type of this field should be?

Digitizing:
Bring your newroads feature class into ArcMap. Also add Y:/intgeo/rowan/arc8class/hopewell/772.jpg, which is a digital orthophoto quad, or DOQ. Zoom into a development in the upper right hand corner. We are going to digitize the roads in this development.

Editing:
In ArcGIS, all digitizing occurs in within edit sessions. To start an edit session, click on the Editor button on the Editor toolbar and select Start Editing (you may need to add the Editor toolbar). If a data frame contains data from more than one workspace (think directory), you have to select the workspace you're interested in editing. Explore the options (especially the 3-t's: tool, task and target) associated with the Editor toolbar - you will need to be sure you understand what they are before you proceed with your first digitizing endeavor.

For this task, we need to Create a New Feature using the icon that looks like a pencil (there are other choices there you can explore later). Use your mouse to draw in the roads. Click once to create a vertex (starting point) and double click to end a particular line. ArcGIS refers to your current edits to a feature as sketch. Double-clicking while drawing a sketch completes it and turns it into a feature. Make a mistake? Use the undo command, or right click to delete the last vertex (or the entire sketch) if you haven't finished the sketch already.

We can make our lives a little easier if we make our new lines snap to an existing line. Snapping means that a new line will automatically connect with an existing line if it falls within the user-defined snapping tolerance. Find the Snapping command on the Editor menu, choose the file you are digitizing and turn on all three snapping options. Set the snapping tolerance to 15 map units (Editor/Options). Notice that when digitizing with the snapping tool turned on, the cursor changes as you approach a road you have already digitized (if this doesn't happen, increase your tolerance or zoom in). Continue to digitize all the roads in the development. You don't have to keep each road separate (as a separate arc) since in a geodatabase (how is this different than a shapefile?) each arc gets split wherever it intersects another arc. This doesn't happen automatically, though. We have to explicitly create the topology. When you've digitized all the roads in this development stop your editing and save your edits.

Cleaning up and creating topology:
Even with greatest care, you are likely not to get all the nodes lined up - some of your lines will not be quite long enough and others will overshoot. Not only are over- and undershoots bad, they also mean that the topology of the geodatabase will not correctly reflect reality. These problems will (mostly) be corrected when we create the topology. Any time you add features to a geodatabase you should create the topology. To do this, we first need access to the Topology toolbar. It can be found under More Editing Tools on the Editor menu. Now, select all of the features in the roads layers. To create the topology click on the Construct Features button on the Topology toolbar. Your cluster tolerance controls how wide your window is for eliminate overshoots and undershoots. You want to make it large enough to catch the errors but not so large that it eliminates correct features. For this purpose, 20 feet should be adequate.

How did the cleaning process go? If you are not completely satisfied, you can do some additional editing (remember to check the snapping settings for the new layer you are about to edit). For overshoots, click on the offending arc(with the select tool in edit mode) that overshoots and delete it. Undershoots are a bit of a different beast. Set the task to modify feature and click on the offending arc with the edit tool. You will now be able pick up the vertices and endpoints and move them where you want them. The arc you just extended should intersect another arc, but the receiving arc doesn't 'know' that it is now suppose to be two arcs and therefore the topology will not be correct. This means we need to Construct Features again once we finish our manual editing (don't forget to select all of the features first).

Editing the Attribute Table:
Remember the fields we added to the blank feature class when we started? Let's see about filling in some of the information. Add the mertiger layer from the Hopewell directory and use it to identify some of the roads (notice the fit is not so good, hence one motivation to re-digitize these roads). If you didn't have a layer containing street names, how might you go about finding them? Select an arc in your new and improved road layer. Look at the attributes of this arc and enter the road name and road type. Remember that a single road may consist of several arcs, and you'll need to add the name to each arc.

Assignment:
1. Now create an updated landuse map for a part of the Hopewell image. Start by creating a polygon feature class inside of the update feature dataset in the hopewell geodatabase (we do told you geodatabases are complicated). Be sure to add fields for the land use Type and the LUCode (see table below). It will take forever if you do the whole image so confine your efforts to the upper right hand corner of the map surrounding the area where you digitized the roads. For consistency, use the prominent north-south feature (a railroad) as the left (west) boundary. Use the small pond and southern edge of the golf course as the southern boundary and the image edge as the east and north boundaries. In lab you worked with arcs but now you are going to work with polygons so be sure to choose the appropriate menu options. Landuse types you should use are:

Suggestions:

Don't get too worked up about capturing the finest level of detail possible or you will drive yourself crazy.

Remember to be careful about your tasks and targets! Don't forget to set your snapping tolerance.

Explore other tasks and commands besides 'create a new feature'. 'Auto polygon complete' is particularly useful for creating new polygons adjacent to existing polygons. With this as the chosen task, start by clicking inside an existing polygon, draw your vertices and end up inside the same or another existing polygon and double click. If something doesn't work right or you make a mistake, don't forget about the undo command.

Note that you can also merge adjacent polygons using the Editor menu if you have two adjacent polygons selected.

Create an appropriate BLACK/WHITE/GRAYSCALE map. Include a summary graph on the map that shows the area included in each landuse type. (Look at the Options button on the summary table for graphing tools.)