Exercise 2: Topology - BE Coursescourses.be.washington.edu/udp/422/exercises/exc2.doc  · Web viewLearn how to use the new ArcGIS 10 object oriented topology rules to create a ...

UDP 422 – Geo-Spatial AnalysisExercise 2: Topology

Exercise 2: Topology

Learn how to use the new ArcGIS 10 object oriented topology rules to create a topology feature set.

To gain a better understanding of what topology is Create new rules Inspect errors Fix errors.

This time you will type in the answers to the questions below, and put the final word document in the submittals folder as: yourname_exc2.doc (please remember to include your name in the title, getting 20 assignments all called exc2 is confusing!) In addition, please type in your name at the top of the document. You do not need to hand in any images of the project. You can work on this exercise in teams of two, but each person must hand in their own final document. All “required fields” to be answered are highlighted throughout this document.

Your answers are due in CollectIt by January 16, 2014.

Go to the website’s exercise page and click on data under Exercise 2 to save the zip file onto your personal space. When you extract this zip file you should notice it has 4 features:

1. Northwest (a polygon shapefile with one feature with the boundary of the Northwest Neighborhood in Seattle)

2. Roads (a line shapefile with all the roads within the northwest neighborhood)3. Parcels (a polygon shapefile with parcel boundaries for the northwest

neighborhood)4. Footprints (a polygon shapefile with building footprints for all buildings within the

Northwest Neighborhood)

As practice, you should check to make sure the projections are defined. Remind yourself which is the projection, and which is the coordinate system?

There are a few topological problems with this dataset; your mission is to tell ArcGIS what constitutes a problem (a broken rule), to find those errors, and to fix (some of) them. In order to speed up processing we are using clipped datasets for just the northwest neighborhood of Seattle.

Below are descriptions of how the data should be, look at the list of rules in the attached handout (topology rules), and determine which rules the following

- 1 -

Objectives

Setup

Deliverable

Directions

UDP 422 – Geo-Spatial AnalysisExercise 2: Topology

descriptions correspond to. Write down each rule below, and specify which feature it corresponds to (sometimes you need to indicate a relationship between more than one feature)

1. No area should be covered by more than one parcel, which would result in confusion as to whom owns what.Q1. RULE & FEATURE(S):

2. Each building must be inside a single parcel, we shouldn’t have houses outside of legal boundaries.Q2. RULE & FEATURE(S):

3. All of the parcels for this dataset should be inside of the northwest neighborhood.Q3. RULE & FEATURE(S):

In order to tell ArcGIS what rules it must follow, you have to first create a geo-database, within which will be a feature set (in which all of the files have the same spatial reference). For this feature dataset you will create a topological feature, which will explicate the rules. The following steps explain how to create a feature dataset; afterwards you will validate the topology and fix some of the errors.

Create a geo-databaseOpen ArcCatalog, navigate to the location where you extracted the four files.Right click on the folder and click on New>Personal Geodatabase (rename it “northwest.mdb”).

Create a feature datasetNow right click on the new Geodatabase and click on New>Feature DatasetType in a new name (like “northwest”), Click on <Next> to specify a spatial reference, select Import… and choose the roads shapefile to import the information from (the spatial reference should be NAD_1983_StatePlane_Washington_North_FIPS_4601_Feet).

- 2 -

UDP 422 – Geo-Spatial AnalysisExercise 2: Topology

Choose North America > NAD83 as the Vertical Coordinate System.

Click <Modify> to see the parameters such as semimajor axis and flattening. Click <Ok> to exit the general window, and click <Next>.

The next window shows the tolerances for the data. Make note of each of the tolerances and uncheck ‘Accept default resolution…’ The next window displays the default resolution for the data—this is where you would change the resolutions if needed (only in extreme cases). For now we will accept the defaults and finish creating the feature dataset. Click <Finish> to wrap up creation of the feature dataset.

Add features into the feature datasetRight click on the feature dataset and click on Import> Feature Class (multiple)Select the four shapefiles you extracted (roads, parcels, footprint, and northwest)Drag each one separately into the input features box. When all four are included, press <Ok>.

(This may take a few moments, please be patient, if Arc is working, there will be scrolling text at the bottom right of the screen.)

You are now ready to create topology.

Create topologyRight click on the feature dataset (northwest) and click on New> Topology.A wizard dialog box should appear, click Next. Enter a name for your new topology (“northwest_Topology” is already filled in for you) and leave the cluster tolerance at the default value. Press next. You should now see four unchecked features, select everything but the roads (we will not create any topological rules about roads in this exercise). Click Next.

Ignore the ranks and click Next again. You are now ready to add a rule. We will start with just adding one, and will add other rules in the following steps. Add Rule 1 from the questions above. Click OK, click next to view the Topology summary and click Finish to build the topology. When it asks if you want to validate it now, say NO.

Now click on the topology feature (northwest_Topology) the preview should be a rectangle with a diagonal hatching.

- 3 -

UDP 422 – Geo-Spatial AnalysisExercise 2: Topology

We will validate the topology in ArcMap. Open ArcMap, add the entire feature dataset into it (you can drag the northwest featureset directly into the map table of contents or add as you would a normal data layer with add data).

Viewing ErrorsYou should see a new feature called “northwest_Topology” which has three elements in it, one for areas, lines and point errors. However, if you turn everything else off except it, you will see that it is blank. This is because you haven’t validated the topology yet.

Before you do that, turn the other layers back on, look through the data (hint: maybe look for building footprints that overlap parcel boundaries) and think about all three rules you have identified, can you find any errors on your own? Note, finding where the first rule is violated is tricky, but do look for violations of the other rules?

Q4. Explain which errors you find, and what they look like. What do you think is the problem with checking errors this way?

Make sure that both the Topology and the Editor toolbars are visible, if not go to Customize>Toolbars and select each one separately.

In the editor toolbar click on Editor>Start Editing

Ensure that the map extent encompasses the entire study area.

In the Topology toolbar click on the “Validate Topology in Current Extent” button (this might take a few moments)

When it is done, turn everything but the northwest_Topology feature off, and zoom to the extent.Q5. How many red areas do you see?

Click the Error Inspector button in the topology toolbar, and click on “Search Now”.Q6. How many errors did it find? What type of error(s) did it find?

You will now fix the problem, and then continue to add more rules.First zoom into the error such that it takes up about half of the screen space, then turn the parcel layer on, and the topology layer off, and change the polygon fill to “hollow”. Q7. Can you see what caused the problem? What caused the problem?

Turn the northwest_Topology layer back on, and click on Fix Topology Error Tool in the topology toolbar.

- 4 -

UDP 422 – Geo-Spatial AnalysisExercise 2: Topology

Right click over the error; you should see the options as shown in figure 2.Specifically, Subtract, Merge and Create feature are three “automatic fix” options for how to resolve the error. Subtract creates a hole where the overlap exists (it subtracts the polygon whose extents are demarked by the error polygon)Merge dissolves the overlap with one of the adjacent polygons. In this case you can merge the error polygon with either the top (7646) or bottom (7713) polygon.Create feature makes a new polygon out of the error polygon.

Q8. Choose one of these options; explain which one you chose and why you chose it. Explain what happened when you applied the fix.

If you zoom to the extent of the topology feature and search for topological errors again you will see that none show up. Another way to check this is to right click on the topology feature, click on <Properties>, go to the Error tab and click <Generate summary>. You should see no errors.

However, we know that there are still a lot of rules being broken; in order to find them we must tell ArcGIS the rest of our rules.

Editing the RulesStop editing by going to Editor>Stop Editor and save the changes. Save the map as exc2.mxd in your personal file directory. Close ArcMap and open ArcCatalog.

Navigate to the northwest_Topology feature dataset and right click it. Go to Properties, select the Rules tab and click on <Add Rule…>. Add rules 2 and 3 from above.

Go ahead and let it validate the entire topology (still in ArcCatalog), right click on the

northwest_Topology feature and validate it .

Close ArcCatalog and open Exc2.mxd in ArcMap.

In ArcMap you can use the Error inspector to let you individually check each error; we will fix 3 errors in different ways. (Remember you must restart editing in order to use the topology toolbar).

Q9. How many errors did you find this time?

Q10. What is the predominant error type and associated features? (Hint, use the error inspector to tell you)

You can sort the errors by type by choosing a specific error type under the Show: pulldown menu .

- 5 -

(Also available is the option to mark as an exception, marking an error as an exception is completely reversible, and every time you revalidate the topology it will reoccur. Marking it as an error is the reverse of marking something as an exception.)

Figure 2

UDP 422 – Geo-Spatial AnalysisExercise 2: Topology

‘Must be larger than cluster tolerance’ is a default error type that is used when a topology is created. Basically if a polygon is below a set tolerance it will be picked up by this. Q11. Did our data contain any small polygon(s) that are below the cluster tolerance?

Can you find the parcel that overhangs the neighborhood boundary? Zoom to it. (In the error inspector, right click on the error found then choose “Zoom To” from the menu)We will trim the parcel so that its exterior boundary is the same as the neighborhood boundary.

It helps if you make the parcel layer hollow, with an outline width of 2 (in a contrasting color like orange). Make sure that your parcel layer is displayed above the northwest polygon layer.

To trim the parcel polygon, first zoom in to the problem area.

Use the Select Feature tool to select the parcel (make sure that no other features are selected by right clicking on parcels and selecting make this the only selectable layer first or uncheck all the other layers under the Table of Contents > Selection tab).

In the Editor Taskbar, click on the Cut Polygon Tool.

Then click on the Trace Tool, also in the Editor Taskbar.

Starting with the bottom intersection between the parcel and the boundary and trace the boundary edge of the neighborhood within the parcel. When you get to the other side, double click on the last vertex. The parcel polygon should now be split into two polygons; it should look something like figure 3.

Select the extra sliver and press Delete. Save your edits.

Zoom to the full extent of your data and click Validate Topology in Current Extent.

To see if the error has been fixed, search for those types or errors in with error inspector. Q12. Do any appear?

Lastly, we will fix the error caused by footprints, which span more than one parcel, these should represent the rest of your errors. Zoom in to the very northeast corner (first navigate to the

- 6 -

Figure 3

Figure 4

UDP 422 – Geo-Spatial AnalysisExercise 2: Topology

peninsula looking segment in the northeast, than zoom further to the northeast corner of it). Make sure the northwest_Topology feature is turned on. You should see something somewhat similar (the footprints may look different) to figure 4 on your screen.

Zoom into the very small footprint in the northeast corner; zoom in such that the polygon takes up most of the screen.

For these errors there are no easy automatic fixes; the easiest solution is to simply move the footprint slightly to the southeast. Before you begin, reselect footprints in the Selection tab of the Table of Contents, and turn off the snapping aids in the editor toolbar.

Use the Edit tool in the Editor toolbar to drag the footprint into the southeastern parcel. When you are done click on <validate topology in current extent> within the Topology toolbar. With the topology feature turned on, can you see how this error has been fixed?

Now, select another footprint that slightly overhangs a parcel (any will do). Select it and use the <Reshape Feature> task within the Editor toolbar to trim it back. Click on the Trace Tool and click once on the parcel line outside of the building polygon on one side, extend a line to the other side such that the overextending edge is split along the parcel border (see figure 5). Double click on the second point outside of the building polygon. Select and delete the section that is not covered by the parcel.

Another quick and dirty way of editing and fixing errors is to right-click in one of the rows with an error in the error inspector. Zoom to one of the features. If the error is one that hangs outside of the parcel boundary, you can use the <Cut Polygons> task to delete the overhang part of the polygon. Try it out.

More rules….There are too many errors to fix, so we will call it a day. Go to the Editor Toolbar and click on <Stop Editing> under the editor button. Save the project. However, before you close ArcMap look carefully at the data, and the type of errors that were detected.

- 7 -

Figure 5

UDP 422 – Geo-Spatial AnalysisExercise 2: Topology

Q13. Can you think of other rules that could be useful to add? Which ones?

Look at the road and parcels features specifically. Note how for the most part the roads never overlap parcels.

Q14. Is there a rule that would help you find errors where road segments overlap parcels? This question is harder than it may seem, so think hard!)

Q15. Can you think of any limitations to this new object based method of defining rules?

This time you will type in the answers to the questions below, and put the final word document in the submittals folder as: yourname_exc2.doc (please remember to include your name in the title, getting 20 assignments all called exc2 is confusing!) In addition, please type in your name at the top of the document. You do not need to hand in any images of the project. You can work on this exercise in teams of two, but each person must hand in their own final document. All “required fields” to be answered are highlighted throughout this document. Make sure you answer all 15 questions.

- 8 -

Deliverable